NCERT Solutions for Class 8 Science Curiosity provide students with a clear, step-by-step approach to understanding complex scientific concepts in an easy and engaging manner. Designed according to the latest CBSE curriculum, these solutions cover all chapters in detail, ensuring that learners not only find accurate answers to textbook questions but also develop strong problem-solving skills. Whether it’s Physics, Chemistry, or Biology, the explanations are presented in a way that nurtures curiosity and encourages deeper learning. This guide is a valuable resource for exam preparation, homework help, and building a solid foundation in science.
Presenting the comprehensive table of contents for NCERT Solutions for Class 8 Science Curiosity, designed to enhance your understanding and spark your curiosity in the subject!
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Chapter 1 of the NCERT Solutions for Class 8 Science Curiosity confidently ushers readers into the fascinating realm of scientific exploration. It effectively ignites curiosity and provides a strong foundation for understanding the investigative principles that drive the world of science.
Q1: What is the main goal of science education in Grade 8 according to the chapter?
A1: The main goal is to teach students how to investigate by asking focused questions, designing simple experiments, and using observations to deepen understanding, turning them into young scientists who explore real-world puzzles (Page 2).
Q2: How does the chapter describe the progression of science learning from Grades 6 to 8?
A2: In Grade 6, science begins with wonder and simple “Why?” and “How?” questions. In Grade 7, students learn that science evolves, with each answer leading to new questions. In Grade 8, students combine wonder and evolution to become investigators, asking focused questions, experimenting, and explaining observations (Page 2).
Q3: What do the symbols of the root and kite represent in the textbook design?
A3: The root on left-hand pages symbolizes a deep, solid foundation of knowledge connected to the environment, traditions, and heritage. The kite on right-hand pages represents curiosity soaring to explore the unknown. Together, they encourage balancing careful observation with creative thinking (Page 2).
Q4: Why is the kitchen described as a place for scientific investigation?
A4: The kitchen is a place for scientific investigation because everyday phenomena, like a puri puffing up in hot oil, can spark curiosity. Students can observe, ask questions, and conduct simple experiments without needing a laboratory, making science accessible (Page 6).
Q5: What are some examples of scientific questions or phenomena mentioned in the chapter?
A5: Examples include: Why is one side of a puri thinner than the other? Why does a puri puff up like a balloon? Are there more grains of sand on Earth or stars in the galaxy? Why does nature have such diverse plants and animals? (Pages 1, 6).
Q6: How does the chapter suggest conducting a scientific investigation, using the puri example?
A6: To investigate why a puri puffs up or has a thinner side, students should:
Q7: What are the different types of materials mentioned, and how are they classified?
A7: Materials are classified as:
Q8: How does the chapter connect light to everyday phenomena?
A8: Light’s reflection off flat or curved mirrors and bending through lenses explains phenomena like images in a shiny spoon or how corrective glasses work. Rough surfaces and the Moon also reflect light, causing lunar phases based on the positions of Earth, Moon, and Sun (Page 4).
Q9: How are calendars linked to astronomical observations?
A9: Calendars were developed by observing periodic cycles of the Moon’s phases, sunrises, and sunsets. These astronomical patterns helped humans create systems to track time, connecting earthly routines to celestial motions (Page 4).
Q10: What makes Earth suitable for life, according to the chapter?
A10: Earth’s suitability for life comes from its position in the habitable zone, where liquid water exists, and its atmosphere, which provides oxygen and shields against harmful ultraviolet rays. These conditions support diverse ecosystems (Page 5).
Q11: What is the triple planetary crisis, and why is it significant?
A11: The triple planetary crisis involves climate change, biodiversity loss, and pollution, caused by human activities like burning fossil fuels and habit
at destruction. These disrupt Earth’s delicate balance, threatening life. Scientific principles (observing, measuring, experimenting) are key to understanding and addressing these challenges (Page 5).
Q12: How do ecosystems function, and why are they important?
A12: Ecosystems involve complex relationships between living organisms (insects, whales, grasses, trees) and their environment (air, water, sunlight). These interactions form systems that support life, with each component depending on others to maintain balance (Page 5).
Q13: How can students contribute to solving environmental challenges?
A13: Students can use scientific methods (observing, experimenting) to understand climate change and guide actions. By staying curious and applying scientific principles, they can help protect Earth’s balance through informed decisions and solutions (Page 5).
Q1: Why is one side of a puri thinner than the other?
A1: One side of a puri may be thinner due to uneven rolling of the dough or the way it is dropped into hot oil. When fried, the dough puffs up as water inside turns to steam, pushing the dough outward. If one side is thinner, it expands more, creating a thinner layer compared to the thicker side. Experimenting with dough thickness or dropping methods (e.g., vertical vs. angled) can test this (Page 6).
Q2: Are there more grains of sand on all the beaches and deserts of the world, or more stars in our galaxy?
A2: Estimating this involves comparing large quantities. A single beach may contain billions of sand grains (e.g., a cubic meter of sand has ~1 billion grains). Earth’s beaches and deserts could have ~10^22 to 10^23 grains. The Milky Way has ~100-400 billion stars (~10^11 to 4×10^11). Sand grains likely outnumber stars due to the vast volume of Earth’s beaches and deserts, but precise calculations require more data (Page 1).
Q3: Why has nature created such a vast variety of plants and animals?
A3: The vast variety of plants and animals results from evolution through sexual reproduction, where genetic mixing creates diverse traits. This diversity allows species to adapt to different environments (e.g., deserts, forests, oceans), enhancing survival. Ecosystems benefit from this variety, as each species plays a role in maintaining balance (Pages 1, 5).
Q4: Is there such a question that makes you curious about the world? Write it here!
A4: Example question: Why do some clouds produce rain while others don’t? This sparks curiosity about weather patterns, water cycles, and atmospheric conditions, encouraging observation and investigation (Page 1).
Q5 (Implied from Discover, design, and debate): Design an experiment to investigate why a puri puffs up and why one side is thinner.
A5:
Chapter 2 of NCERT Solutions for Class 8 Science Curiosity delves into the fascinating invisible world of living organisms that lies just beyond our naked eye, revealing the extraordinary life forms that surround us every day.
Q1: Why can’t the human eye see all living organisms?
A1: The human eye can only see objects above a certain size. Many tiny organisms, like microorganisms, are too small to be seen without tools like microscopes, which magnify objects to make them visible (Page 2).
Q2: How did the invention of lenses and microscopes change our understanding of the world?
A2: Lenses, shaped like lentil seeds, magnify small objects. Improved lenses led to microscopes, revealing a hidden world of tiny living creatures like bacteria and cells, expanding our knowledge of life (Pages 2-3).
Q3: What did Robert Hooke discover using a microscope, and why was it significant?
A3: In 1655, Robert Hooke observed a thin slice of cork under a microscope and saw small, empty spaces resembling a honeycomb. He called these “cells,” introducing the term as the basic unit of life in his book Micrographia (Page 3).
Q4: Why is Antonie van Leeuwenhoek called the Father of Microbiology?
A4: In 1655, Antonie van Leeuwenhoek built powerful microscopes with improved lenses, allowing him to be the first to clearly see and describe tiny living organisms like bacteria and blood cells, founding microbiology (Page 3).
Q5: How does a magnifying glass or a water-filled flask help observe small organisms?
A5: A magnifying glass or a water-filled round-bottom flask acts as a lens, bending light to make small objects appear larger. In Activity 2.1, a flask filled with water magnifies book letters, and a magnifying glass reveals small organisms (Page 2).
Q6: What are the main parts of a cell, and what are their functions?
A6: A typical cell has:
Q7: How do plant and animal cells differ?
A7: Plant cells have a cell wall for rigidity, chloroplasts for photosynthesis, and a large vacuole for storage and shape. Animal cells lack a cell wall, have no chloroplasts, and have small or no vacuoles (Pages 5-6, Fig. 2.5).
Q8: Why do cells vary in shape and structure, and how does this relate to their function?
A8: Cells vary in shape (e.g., spindle-shaped muscle cells, branched nerve cells, rectangular plant cells) to suit their functions. Muscle cells contract to move food in the digestive system, nerve cells transmit messages, and tube-like plant cells transport water (Pages 6-7).
Q9: What are the levels of organization in a living organism?
A9: The levels are:
Q10: What are microorganisms, and how do they differ from multicellular organisms?
A10: Microorganisms are tiny organisms, often unicellular (e.g., bacteria, Amoeba) or multicellular (e.g., some fungi, algae), invisible without a microscope. Multicellular organisms (e.g., plants, animals) have many cells with specialized functions (Pages 8, 16).
Q11: How do bacterial cells differ from plant and animal cells?
A11: Bacterial cells lack a well-defined nucleus, having a nucleoid instead, and have a cell wall but no chloroplasts. Plant and animal cells have a defined nucleus and membrane, with plant cells having a cell wall and chloroplasts, and animal cells lacking these (Page 17).
Q12: What are viruses, and how are they different from other microorganisms?
A12: Viruses are microscopic, acellular, and only reproduce inside a host cell, infecting plants, animals, or bacteria, potentially causing diseases. Other microorganisms (bacteria, fungi, protozoa) are cellular and can function independently (Page 10).
Q13: Where are microorganisms found, and how do they show diversity?
A13: Microorganisms are found in water, soil, air, food, and inside organisms (e.g., human gut). They vary in shape (spherical, rod-shaped, irregular), size, and structure, and can survive in extreme conditions like hot springs or snow (Page 11).
Q14: How do microorganisms help clean the environment?
A14: Microorganisms like fungi and bacteria decompose plant and animal waste into nutrient-rich manure, as seen in Activity 2.7, where fruit and vegetable peels turn into manure, enhancing soil fertility (Pages 11-12).
Q15: How do microorganisms contribute to biogas production?
A15: Bacteria in oxygen-free environments decompose waste, releasing a gas mixture of carbon dioxide and methane. Methane is used as a fuel for cooking, heating, electricity, or vehicles (Page 13).
Q16: How do microorganisms like yeast and Lactobacillus affect food production?
A16:
Q17: What role do Rhizobium bacteria play in agriculture?
A17: Rhizobium bacteria live in root nodules of legumes (e.g., beans, peas), trapping nitrogen from the air and converting it into compounds plants use, enhancing soil fertility without chemical fertilizers (Page 15, Fig. 2.12).
Q18: How do microalgae contribute to the environment and human use?
A18: Microalgae produce over half of Earth’s oxygen via photosynthesis, serve as food for aquatic animals, clean water, and are used as health supplements (e.g., Spirulina, Chlorella) and biofuels. Their diversity is threatened by pollution and climate change (Page 15).
Q19: Why is Spirulina considered a superfood, and how can it be cultivated?
A19: Spirulina, a microalga, is a superfood due to its high protein (over 60%), low fat and sugar, and vitamin B12 content. It can be grown in a glass tank with pond water, shaded, stirred twice weekly, and harvested after 3-6 weeks by filtering (Page 16).
Q20: Why is the cell considered the basic unit of life?
A20: Cells are the basic unit of life because they contain components (membrane, cytoplasm, nucleus) that perform essential functions like growth and survival. Unicellular organisms perform all functions in one cell, while multicellular organisms have specialized cells that cooperate (Page 16).
Q1: Various parts of a cell are given below. Write them in the appropriate places in the following diagram.
Common to all three cells: Nucleus, Cytoplasm, Cell membrane
Only in Plant Cell: Cell wall, Chloroplast
Only in Bacterial Cell: Nucleoid
A1:
Q2: Anandi took two test tubes and marked them A and B. She put two spoonfuls of sugar solution in each of the test tubes. In test tube B, she added a spoonful of yeast. Then she attached two incompletely inflated balloons to the mouth of each test tube. She kept the set-up in a warm place, away from sunlight.
(i) What do you predict will happen after 3-4 days? She observed that the balloon attached to test tube B was inflated. What can be a possible explanation for this?
(a) Water evaporated in test tube B and filled the balloon with water vapour.
(b) The warm atmosphere expanded the air inside test tube B, which inflated the balloon.
(c) Yeast produced a gas inside test tube B which inflated the balloon.
(d) Sugar reacted with warm air, which produced gas, eventually inflating the balloon.
A2: Prediction: After 3-4 days, the balloon on test tube B will inflate, while test tube A’s balloon will not.
Observation and Explanation: The balloon on test tube B inflated because yeast, a fungus, ferments sugar, producing carbon dioxide gas that fills the balloon. Test tube A, without yeast, produces no gas, so its balloon remains uninflated (Page 13).
Answer: (c) Yeast produced a gas inside test tube B which inflated the balloon. Options (a), (b), and (d) are incorrect because:
(a) Water evaporation produces minimal vapor, insufficient to inflate a balloon significantly.
(b) Warmth alone cannot expand air enough to inflate the balloon noticeably.
(d) Sugar does not react with warm air to produce gas; fermentation requires a microorganism like yeast.
This is supported by the description of yeast fermentation , where yeast breaks down sugar in warm conditions, releasing carbon dioxide.
(ii) She took another test tube, 1/4 filled with lime water. She removed the balloon from test tube B in such a manner that the gas inside the balloon did not escape. She attached the balloon to the test tube with lime water and shook it well. What do you think she wants to find out?
A2 (ii): Anandi wants to confirm that the gas produced by yeast is carbon dioxide. When carbon dioxide is passed into lime water (calcium hydroxide), it reacts to form calcium carbonate, turning the lime water milky. This test identifies the gas as carbon dioxide (Page 14).
Q3: A farmer was planting wheat crops in his field. He added nitrogen-rich fertiliser to the soil to get a good yield of crops. In the neighbouring field, another farmer was growing bean crops, but she preferred not to add nitrogen fertiliser to get healthy crops. Can you think of the reasons?
A3: The farmer growing beans did not add nitrogen fertilizer because bean crops (legumes) have root nodules containing Rhizobium bacteria. These bacteria fix nitrogen from the air into compounds that enrich the soil, promoting healthy plant growth without additional fertilizers (Page 15).
Q4: Shehald dug two pits, A and B, in her garden. In pit A, she put fruit and vegetable peels and mixed it with dried leaves. In pit B, she dumped the same kind of waste without mixing it with dried leaves. She covered both pits with soil and observed after 3 weeks. What is she trying to test?
A4: Shehald is testing the effect of mixing dried leaves with fruit and vegetable peels on decomposition. Pit A, with mixed leaves, likely decomposes faster due to increased carbon content, aiding microbial activity (fungi, bacteria) to form nutrient-rich manure. Pit B, without leaves, decomposes slower due to less favorable conditions for microbes (Page 12).
Q5: Identify the following microorganisms:
(i) I live in every kind of environment and inside your gut.
(ii) I make bread and cakes soft and fluffy.
(iii) I live in the roots of pulse crops and provide nutrients for their growth.
A5:
Q6: Devise an experiment to test that microorganisms need optimal temperature, air, and moisture for their growth.
A6:
Q7: Take 2 slices of bread. Place one slice in a plate near the sink. Place the other slice in the refrigerator. Compare after three days. Note your observations. Give reasons for your observations.
A7:
Q8: A student observed that when curd is left out for a day, it becomes more sour. What can be two possible explanations for this observation?
A8:
Q9: Observe the set-up given in Fig. 2.15 and answer the following questions.
(i) What happens to the sugar solution in flask A?
(ii) What do you observe in test tube B after four hours? Why do you think this happened?
(iii) What would happen if yeast was not added in flask A?
A9:
(iii) If yeast was not added to flask A, no fermentation would occur, so no carbon dioxide would be produced. Test tube B’s lime water would remain clear, as no gas would enter to react (Page 14).
(i) In flask A, yeast ferments the sugar solution, producing carbon dioxide gas and alcohol, causing bubbles and a change in smell (Pages 13-14).
(ii) After four hours, test tube B (containing lime water) turns milky because carbon dioxide from flask A reacts with lime water (calcium hydroxide) to form calcium carbonate, a milky precipitate (Page 14).
Chapter 3 of the NCERT Solutions for Class 8 Science Curiosity effectively highlights the vital importance of health as our greatest treasure. It encourages students to value and prioritize their well-being for a fulfilling life.
Q1: What is the definition of health according to the World Health Organization (WHO)?
A1: Health is defined as a state of complete physical, mental, and social well-being, not merely the absence of disease.
Q2: What are the key components of being healthy?
A2: Being healthy includes taking care of the body, maintaining a positive mindset, and enjoying strong social relationships.
Q3: How does Ayurveda contribute to health?
A3: Ayurveda emphasizes balancing body, mind, and surroundings through daily (dinacharya) and seasonal (ritucharya) routines, eating fresh food suited to one’s prakriti, regular exercise, cleanliness, restful sleep, and practices like yoga, meditation, and mindfulness.
Q4: What are some ways to stay healthy?
A4: Staying healthy involves eating a balanced diet, maintaining hygiene, staying physically active, getting enough sleep, managing stress, spending time with family and friends, and avoiding harmful substances like tobacco and alcohol.
Q5: Why is a clean environment important for health?
A5: A clean environment prevents diseases caused by pollution, contaminated water, or unhygienic surroundings. Clean air and water reduce health issues like coughing or asthma, and a clean environment supports overall well-being.
Q6: What is the difference between symptoms and signs of a disease?
A6: Symptoms are what a person feels (e.g., pain, tiredness), while signs are observable or measurable indicators (e.g., fever, rash).
Q7: What are the two major types of diseases?
A7: Diseases are classified as:
Q8: How do communicable diseases spread?
A8: Communicable diseases spread through air (coughing/sneezing), direct contact, contaminated food or water, or vectors like mosquitoes and flies.
Q9: What are some causes of non-communicable diseases?
A9: Non-communicable diseases are caused by lifestyle factors (e.g., unhealthy diet, lack of exercise), environmental factors, or poor nutrition (e.g., deficiency diseases like scurvy).
Q10: What is immunity, and how do vaccines help?
A10: Immunity is the body’s natural ability to fight diseases via the immune system. Vaccines train the immune system using weakened, dead, or harmless parts of pathogens to build acquired immunity, preventing diseases like polio or measles.
Q11: What is antibiotic resistance, and how can it be prevented?
A11: Antibiotic resistance occurs when bacteria survive and multiply despite antibiotic treatment. It can be prevented by using antibiotics only when prescribed, in the correct dose, and for the right duration.
Q12: What role does India play in global health?
A12: India is a major vaccine producer, supplying vaccines globally, including during the COVID-19 pandemic, and has contributed significantly to health research and affordable healthcare.
Q1: Group the diseases shown in the images as communicable or non-communicable.
Cold and flu, Typhoid, Diabetes, Asthma, Chickenpox
A1:
Q2: From the options given below, identify the non-communicable diseases.
(i) Typhoid, (ii) Asthma, (iii) Diabetes, (iv) Measles
(a) (i) and (ii), (b) (ii) and (iii), (c) (i) and (iv), (d) (ii) and (iv)
A2: (b) (ii) and (iii)
Asthma and Diabetes are non-communicable diseases, while Typhoid and Measles are communicable.
Q3: There is a flu outbreak in your school. Several classmates are absent, while some are still coming to school coughing and sneezing.
(i) What immediate actions should the school take to prevent further spread?
(ii) If your classmate who shares the bench with you starts showing symptoms of the flu, how can you respond in a considerate way without being rude or hurtful?
(iii) How can you protect yourself and others from getting infected in this situation?
A3:
(i) The school should:
(ii) Respond considerately by:
(iii) To protect yourself and others:
Q4: Your family is planning to travel to another city where malaria is prevalent.
(i) What precautions should you take before, during, and after the trip?
(ii) How can you explain the importance of mosquito nets or repellents to your sibling?
(iii) What could happen if travellers ignore health advisories in such areas?
A4:
(i) Precautions:
(ii) Explain to sibling:
(iii) Consequences of ignoring advisories:
Q5: Your uncle has started smoking just to fit in with his friends, even though it is well known that smoking can seriously harm health and even cause death.
(i) What would you say to him to make him stop, without being rude?
(ii) What would you do if your friend offers you a cigarette at a party?
(iii) How can schools help prevent students from indulging in such harmful habits?
A5:
(i) To uncle:
(ii) If offered a cigarette:
(iii) Schools can help by:
Q6: Saniya claims to her friend Vinita that “Antibiotics can cure any infection, so we don’t need to worry about diseases.” What question(s) can Vinita ask her to help Saniya understand that her statement is incorrect?
A6: Vinita could ask:
Q7: The following table contains information about the number of dengue cases reported in a hospital over a period of one year:
Make a bar graph of the number of cases on the Y-axis and the month on the X-axis. Critically analyse your findings and answer the following:
(i) In which three months were the dengue cases highest?
(ii) In which month(s) were the cases lowest?
(iii) What natural or environmental factors during the peak months might contribute to the increase in dengue cases?
(iv) Suggest a few preventive steps that the community or government can take before the peak season to reduce the spread of dengue.
A7:
Bar Graph:
Below is a textual representation of the bar graph :
(i) Highest three months: July, August, September (65 cases each).
(ii) Lowest month(s): January (10 cases).
(iii) Environmental factors:
Q8: Imagine you are in charge of a school health campaign. What key messages would you use to reduce communicable and non-communicable diseases?
A8: Key messages:
Q9: It is recommended that we should not take an antibiotic for a viral infection like a cold, a cough, or flu. Can you provide the possible reason for this recommendation?
A9: Antibiotics target bacteria, not viruses. Using antibiotics for viral infections is ineffective and can contribute to antibiotic resistance, making bacterial infections harder to treat in the future.
Q10: Which disease(s) among the following may spread if drinking water gets contaminated by the excreta from an infected person?
Hepatitis A, Tuberculosis, Poliomyelitis, Cholera, Chickenpox
A10: Diseases that may spread through contaminated drinking water:
Q11: When our body encounters a pathogen for the first time, the immune response is generally low but on exposure to the same pathogen again, the immune response is much more compared to the first exposure. Why is it so?
A11: The first exposure to a pathogen triggers the immune system to produce antibodies and memory cells, which is a slower process. On subsequent exposures, memory cells recognize the pathogen quickly, leading to a faster and stronger immune response.
Q1: What is the magnetic effect of electric current?
A1: When electric current flows through a conductor (e.g., a wire), it produces a magnetic field around it. This phenomenon, discovered by Hans Christian Oersted in 1800, is called the magnetic effect of electric current. The magnetic field causes a compass needle to deflect when placed near the current-carrying wire.
Q2: How can we detect the magnetic effect of a current-carrying wire?
A2: A magnetic compass placed near a current-carrying wire will show deflection in its needle when the current is on, indicating the presence of a magnetic field. When the current is turned off, the needle returns to its original position.
Q3: What is an electromagnet?
A3: An electromagnet is a coil of wire, often wrapped around an iron core, that behaves like a magnet when electric current flows through it. The magnetic effect stops when the current is turned off. The iron core enhances the magnetic strength.
Q4: How does an electromagnet work, and what are its applications?
A4: When current flows through the coil, it creates a magnetic field, turning the coil (and iron core, if present) into a magnet that can attract magnetic materials like iron. Applications include lifting electromagnets in factories/scrap yards, electric bells, motors, fans, and loudspeakers.
Q5: Do electromagnets have poles like bar magnets?
A5: Yes, electromagnets have two poles (north and south), similar to bar magnets. A magnetic compass can identify the poles by showing which end of the coil attracts the compass’s north or south pole.
Q6: What is the heating effect of electric current?
A6: When electric current flows through a conductor, it encounters resistance, converting some electrical energy into heat energy. This is called the heating effect of electric current. Nichrome wires, with high resistance, heat up significantly.
Q7: What factors affect the amount of heat generated in a conductor?
A7: The heat generated depends on the magnitude of the current, the resistance of the conductor (material, thickness, length), and the duration of current flow. Higher current (e.g., using two cells instead of one) produces more heat.
Q8: What are some applications of the heating effect of electric current?
A8: Household appliances like electric room heaters, stoves, kettles, irons, immersion rods, and hair dryers use the heating effect, where a heating element (e.g., nichrome coil) glows red-hot due to resistance.
Q9: What are the potential issues with the heating effect?
A9: Overheating can cause energy loss in wires, damage to plugs/sockets, melting of plastic parts, or fires. Safety devices in household circuits help minimize these risks.
Q10: How does a Voltaic cell generate electricity?
A10: A Voltaic (or Galvanic) cell consists of two different metal electrodes (e.g., copper and zinc) partially immersed in an electrolyte (e.g., weak acid or salt solution). Chemical reactions between the electrodes and electrolyte produce electric current, flowing from the positive to the negative terminal.
Q11: What is a dry cell, and how does it differ from a Voltaic cell?
A11: A dry cell is a portable electric cell with a paste-like electrolyte (not liquid), consisting of a zinc container (negative terminal) and a carbon rod with a metal cap (positive terminal). Unlike Voltaic cells, dry cells are single-use and more convenient for everyday use.
Q12: What are rechargeable batteries, and what are their advantages?
A12: Rechargeable batteries can be recharged and reused multiple times, reducing waste and cost. They are used in devices like phones, laptops, cameras, inverters, and electric vehicles. Lithium-ion batteries are common, but they wear out after many cycles.
Q13: Why is battery disposal important?
A13: Dead batteries contain harmful materials like acids, lead, cadmium, or lithium, which can cause environmental damage or fires if not disposed of properly. Recycling at e-waste facilities recovers valuable materials and protects the environment.
Q1: Fill in the blanks:
(i) The solution used in a Voltaic cell is called ________.
(ii) A current carrying coil behaves like a ________.
A1:
(i) Electrolyte
(ii) Magnet (or Electromagnet)
Q2: Choose the correct option:
(i) Dry cells are less portable compared to Voltaic cells. (True/False)
(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)
(iii) An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
A2:
(i) False (Dry cells are more portable than Voltaic cells.)
(ii) True
(iii) False (A battery of 2 cells provides more current, increasing the electromagnet’s strength and attracting more clips.)
Q3: An electric current flows through a nichrome wire for a short time.
(i) The wire becomes warm.
(ii) A magnetic compass placed below the wire is deflected.
Choose the correct option:
(a) Only (i) is correct
(b) Only (ii) is correct
(c) Both (i) and (ii) are correct
(d) Both (i) and (ii) are not correct
A3: (c) Both (i) and (ii) are correct
The wire becomes warm due to the heating effect of electric current, and the compass deflects due to the magnetic effect of the current.
Q4: Match the items in Column A with those in Column B.
A4: Here’s the correct matching:
(i) Voltaic cell → (d) chemical reactions
(ii) Electric iron → (c) Works on heating effect of electric current
(iii) Nichrome wire → (a) Best suited for electric heater
(iv) Electromagnet → (b) Works on magnetic effect of electric current
Q5: Nichrome wire is commonly used in electrical heating
devices because it
(i) is a good conductor of electricity.
(ii) generates more heat for a given current.
(iii) is cheaper than copper.
(iv) is an insulator of electricity.
A5: The correct answer is: (ii) generates more heat for a given current.
Nichrome has high electrical resistance and can withstand high temperatures without oxidizing, which makes it ideal for heating elements.
Q6: Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reason(s) to support this statement considering societal impact.
A6: Electric heating devices are considered more convenient than traditional heating methods because:
Q7: Look at the Fig. 4.4a.
If the compass placed near the coil deflects:
(i) Draw an arrow on the diagram to show the path of the electric current.
(ii) Explain why the compass needle moves when current flows.
(iii) Predict what would happen to the deflection if you reverse the battery terminals.
A7: Here’s the solution for each part:
(i) Arrow showing current direction:
In the given diagram, the battery’s positive terminal is at the right and the negative terminal is at the left.
So, the electric current flows from the positive terminal (+) → through the wire to end B → through the coil towards end A → back to the negative terminal (–).
You would draw an arrow along the wire showing this path.
(ii) Why the compass needle moves:
When current flows through the coil, it produces a magnetic field around it.
The compass needle is a small magnet, so the coil’s magnetic field exerts a force on it, causing it to deflect from its usual north–south direction.
(iii) If the battery terminals are reversed:
Reversing the battery changes the direction of current in the coil.
This also reverses the coil’s magnetic polarity, so the compass needle will deflect in the opposite direction compared to before.
Q8: Suppose Sumana forgets to move the switch of her lifting electromagnet model to OFF position. After some time, the iron nail no longer picks up the iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips? Give possible reasons.
A8:
Possible reasons:
Q9: In Fig. 4.11, in which case the LED will glow when the switch is closed?
A9: The LED will glow in case (a) — when the electrodes are dipped in lemon juice.
Reason: In case (b), pure water is a poor conductor because it has very few ions, so the circuit will not complete, and the LED will not glow.
Lemon juice contains acids (citric acid), which produce ions in the solution, making it a good conductor of electricity.
Q10: Neha keeps the coil exactly the same as in activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?
A10: Yes, the coil will still deflect the compass even without the iron nail inside.
Reason: When current flows through the coiled wire, it produces a magnetic field, and this magnetic field can influence the compass needle.
Deflection: The deflection will be less than before because the iron nail acted as a soft iron core, which increased the strength of the magnetic field (by concentrating the magnetic lines of force). Without it, the magnetic field is weaker, so the compass needle will not deflect as much.
Q11: We have four coils, of similar shape and size, made up from iron, copper, aluminium, and nichrome as shown in Fig. 4.12.
When current is passed through the coils, compass needles placed near the coils will show deflection.
(i) Only in circuit (a)
(ii) Only in circuits (a) and (b)
(iii) Only in circuits (a), (b), and (c)
(iv) In all four circuits.
A11: The correct answer is: (iv) In all four circuits.
Reason:
Any wire carrying an electric current produces a magnetic field, regardless of the material (iron, copper, aluminium, or nichrome). This magnetic field will cause a nearby compass needle to deflect. The difference between materials only affects the strength of the field — ferromagnetic materials like iron make the field stronger when used as a core, but the effect is present for all conducting coils.
Q1: What is a force?
A1: A force is a push or pull on an object resulting from its interaction with another object. It is measured in newtons (N) in the SI system.
Q2: What effects can a force have on an object?
A2: A force can:
Q3: Does a change in speed, direction, or shape always indicate a force?
A3: Yes, changes in speed, direction, or shape of an object occur only when a force is applied.
Q4: Are forces always associated with interactions?
A4: Yes, forces result from the interaction of at least two objects. For example, pushing a table involves interaction between the hand and the table.
Q5: What happens when two objects interact?
A5: When two objects interact, each experiences a force from the other. The force ceases when the interaction stops.
Q6: What are contact forces?
A6: Contact forces act only when there is physical contact between objects. Examples include muscular force and frictional force.
Q7: What is muscular force?
A7: Muscular force is the force produced by the contraction and elongation of muscles during activities like walking, running, lifting, or pushing. It is used by humans, animals, birds, fish, and insects for movement and survival, and internally for functions like digestion and blood circulation.
Q8: What is frictional force?
A8: Frictional force (friction) is a contact force that opposes the motion of an object moving or trying to move over another surface. It arises due to surface irregularities and is greater on rough surfaces.
Q9: Does friction act only on solid surfaces?
A9: No, friction also acts on objects moving through liquids (e.g., water) or gases (e.g., air). This is why vehicles like aeroplanes and ships are designed to reduce fluid friction.
Q10: What are non-contact forces?
A10: Non-contact forces act without physical contact between objects. Examples include magnetic force, electrostatic force, and gravitational force.
Q11: What is magnetic force?
A11: Magnetic force is a non-contact force exerted by a magnet on another magnet or magnetic material, causing attraction (unlike poles) or repulsion (like poles).
Q12: What is electrostatic force?
A12: Electrostatic force is a non-contact force exerted by a charged object on another charged or uncharged object. Like charges repel, and unlike charges attract. For example, a charged plastic scale attracts paper pieces.
Q13: What is gravitational force?
A13: Gravitational force is a non-contact, attractive force with which the Earth (or another planet) pulls objects toward itself. It is also called the force of gravity or simply gravity.
Q14: What is the weight of an object?
A14: The weight of an object is the gravitational force with which the Earth pulls it toward itself, measured in newtons (N). It varies slightly by location due to differences in gravitational force.
Q15: How does weight differ from mass?
A15: Mass is the amount of matter in an object, measured in kilograms (kg) or grams (g), and remains constant everywhere. Weight is the gravitational force on the object, measured in newtons (N), and varies by location (e.g., less on the Moon than on Earth).
Q16: How is weight measured?
A16: Weight is measured using a spring balance, which stretches proportionally to the force applied by the Earth. The smallest measurable weight depends on the spring balance’s scale (e.g., 0.2 N for a balance with 5 divisions between 1 N marks).
Q17: What is buoyant force?
A17: Buoyant force (upthrust) is the upward force exerted by a liquid on an object placed in it. It opposes the gravitational force, causing objects to feel lighter in water.
Q18: Why do some objects float while others sink in water?
A18: An object floats if the buoyant force equals its weight, and sinks if the weight exceeds the buoyant force. This depends on the object’s density and the weight of the liquid displaced (Archimedes’ Principle).
Q19: What is Archimedes’ Principle?
A19: Archimedes’ Principle states that an object immersed in a liquid experiences an upward buoyant force equal to the weight of the liquid displaced. If this equals the object’s weight, it floats; if less, it sinks.
Q1: Match items in Column A with the items in Column B.
A1: Here’s the correct matching:
(i) Muscular force → (b) A child lifting a school bag
(ii) Magnetic force → (e) A compass needle pointing North
(iii) Frictional force → (a) A cricket ball stopping on its own just before touching the boundary line
(iv) Gravitational force → (c) A fruit falling from a tree
(v) Electrostatic force → (d) Balloon rubbed on woollen cloth attracting hair strands
Q2: State whether the following statements are True or False.
(i) A force is always required to change the speed of motion of an object.
(ii) Due to friction, the speed of the ball rolling on a flat ground increases.
(iii) There is no force between two charged objects placed at a small distance apart.
A2:
(i) True (A force is needed to change speed, as per the effects of force.)
(ii) False (Friction opposes motion, reducing the speed of a rolling ball.)
(iii) False (Charged objects exert an electrostatic force, attracting or repelling each other.)
Q3: Two balloons rubbed with a woollen cloth are brought near each other. What would happen and why?
A3: The balloons repel each other because they acquire similar charges (e.g., both positive or both negative) when rubbed with the woollen cloth. Like charges repel, as demonstrated in Activity 5.7.
Q4: When you drop a coin in a glass of water, it sinks, but when you place a bigger wooden block in water, it floats. Explain.
A4: The coin sinks because its weight (gravitational force) is greater than the buoyant force exerted by the water. The wooden block floats because its weight is equal to or less than the buoyant force, as the wood is less dense and displaces enough water to balance its weight (Archimedes’ Principle).
Q5: If a ball is thrown upwards, it slows down, stops momentarily, and then falls back to the ground. Name the forces acting on the ball and specify their directions.
(i) During its upward motion
(ii) During its downward motion
(iii) At its topmost position.
A5:
(i) Upward motion: Gravitational force (downward) opposes the ball’s motion, slowing it down. Air friction (downward) also opposes the motion.
(ii) Downward motion: Gravitational force (downward) accelerates the ball toward the Earth. Air friction (upward) slightly opposes the motion.
(iii) Topmost position: Gravitational force (downward) acts on the ball, causing it to begin falling. Air friction is negligible as the ball is momentarily stationary.
Q6: A ball is released from point P and moves along an inclined plane, then along a horizontal surface as shown in Fig. 5.16. It comes to a stop at point A on the horizontal surface. Think of a way so that when the ball is released from the same point P it stops (i) before the point A (ii) after crossing the point A.
A6:
(i) Stop before point A: Increase friction on the horizontal surface by placing a rough material (e.g., sandpaper or cloth) before point A. This increases the frictional force, stopping the ball sooner.
(ii) Stop after crossing point A: Reduce friction by making the horizontal surface smoother (e.g., using a polished or lubricated surface). This decreases the frictional force, allowing the ball to travel farther past point A.
Q7: Why do we sometimes slip on smooth surfaces like ice or polished floors? Explain.
A7: Smooth surfaces like ice or polished floors have fewer surface irregularities, reducing frictional force. This low friction makes it harder to maintain grip, causing slipping when walking or applying force.
Q8: Is any force being applied to an object in non-uniform motion?
A8: Yes, an object in non-uniform motion (changing speed or direction) experiences a net force. This force, such as gravity, friction, or an applied push/pull, causes the acceleration or deceleration.
Q9: The weight of an object on the Moon becomes one-sixth of its weight on the Earth. What causes this change? Does the mass of the object also become one-sixth of its mass on the Earth?
A9: The weight on the Moon is one-sixth that on Earth because the Moon’s gravitational force is one-sixth that of Earth’s. Weight is the gravitational force (W = m × g), and since g is smaller on the Moon, weight decreases. The mass of the object remains the same, as mass is the amount of matter and does not depend on location.
Q10: Three objects 1, 2, and 3 of the same size and shape, but made of different materials are placed in water. They dip to different depths as shown in Fig. 5.17. If the weights of the three objects are w₁, w₂, and w₃, respectively, then
(i) w₁ = w₂ = w₃
(ii) w₁ > w₂ > w₃
(iii) w₁ < w₂ < w₃
(iv) w₁ > w₃ > w₂
A10: (ii) w₁ > w₂ > w₃
Objects dipping to different depths indicate different buoyancies due to their densities. The object that sinks deepest (highest density) has the greatest weight, as it displaces less water and experiences less buoyant force. Assuming object 1 sinks deepest, object 2 is partially submerged, and object 3 floats highest, their weights follow w₁ > w₂ > w₃.
Q1: What is pressure, and how is it defined?
A1: Pressure is the force per unit area. It is defined as:
Pressure = Force / Area. The SI unit of pressure is newton per square meter (N/m²), also called a pascal (Pa).
Q2: Why do broad straps on a bag feel more comfortable than narrow straps?
A2: Broad straps distribute the weight (force) of the bag over a larger area, reducing the pressure on the shoulders compared to narrow straps, which concentrate the force on a smaller area, increasing pressure and discomfort.
Q3: Why is it easier to lift a water-filled bucket with a broad handle than a narrow one?
A3: A broad handle spreads the weight of the bucket over a larger area, reducing the pressure on the hand, making it more comfortable to lift compared to a narrow handle, which increases pressure.
Q4: Why do people place a round piece of cloth under heavy loads on their heads?
A4: The cloth increases the area over which the load’s weight is distributed, reducing the pressure on the head and making it easier to carry the load.
Q5: Why are water tanks placed at a height?
A5: Water tanks are placed at a height to increase the height of the water column, which increases the pressure at the bottom, resulting in a stronger stream of water from taps.
Q6: Do liquids exert pressure, and in which directions?
A6: Yes, liquids exert pressure in all directions—on the bottom and sides of the container. The pressure depends on the height of the liquid column, not its volume.
Q7: Why does water spurt out from holes in a leaking pipe?
A7: Water spurts out due to the pressure exerted by the water on the walls of the pipe, forcing water out through any openings.
Q8: Why is the base of a dam broader than the top?
A8: The base of a dam is broader to withstand the high horizontal water pressure near the bottom, which increases with the height of the water column, and to provide structural stability.
Q9: Does air exert pressure, and how is it demonstrated?
A9: Yes, air exerts pressure, known as atmospheric pressure. This is demonstrated by activities like a rubber sucker sticking to a surface due to higher external air pressure or a balloon inflating due to air pressure inside it.
Q10: Why are we not crushed by atmospheric pressure?
A10: The pressure inside our bodies, caused by the movement of fluids and gases, balances the atmospheric pressure exerted from outside, preventing us from being crushed.
Q11: How do winds form?
A11: Winds form due to the movement of air from a region of high pressure to a region of low pressure, caused by uneven heating of the Earth’s surface. For example, land heats up faster than the sea, creating low pressure over land and causing sea breezes.
Q12: Why do high-speed winds result in lower air pressure?
A12: High-speed winds create a low-pressure area because the fast-moving air reduces the density of air molecules in that region, as demonstrated by balloons moving toward each other when air is blown between them.
Q13: Why are roofs sometimes blown off during storms?
A13: High-speed winds create a low-pressure area above the roof, while higher pressure remains inside the house. This pressure difference can lift weak roofs, causing them to blow off.
Q14: Why is it safer to keep doors and windows open during storms?
A14: Keeping doors and windows open reduces the pressure difference between the inside and outside of the house, preventing roofs from being lifted by the low pressure above caused by high-speed winds.
Q15: What is a storm, and how does it form?
A15: A storm is strong winds accompanied by rain, hail, or snow. It forms when warm, moist air rises due to heating, creating a low-pressure area. Cooler air rushes in, rises, cools, and condenses to form clouds and precipitation.
Q16: What is a thunderstorm, and how does it form?
A16: A thunderstorm is a storm with lightning and thunder. It forms when warm, moist air rises, cools, and condenses into clouds. Strong upward and downward winds cause water droplets and ice particles to rub, generating static electric charges that lead to lightning and thunder.
Q17: How does lightning occur?
A17: Lightning occurs when a large buildup of opposite charges (positive in upper cloud parts, negative in lower parts or on the ground) overcomes air’s insulating property, causing a sudden flow of charges that produces a bright flash.
Q18: What is a lightning conductor, and how does it work?
A18: A lightning conductor is a metallic rod installed on buildings, with one end pointed above the building and the other buried in the ground. It provides a path for electric charges from lightning to safely transfer to the ground, protecting the building.
Q19: What is a cyclone, and how does it form?
A19: A cyclone is a large storm with high-speed winds revolving around a low-pressure center (the eye) over warm ocean waters. It forms when warm, moist air rises, condenses, and releases heat, further lowering pressure. Surrounding air rushes in, spins due to Earth’s rotation, and forms a cyclone.
Q20: What is the eye of a cyclone, and what are its characteristics?
A20: The eye of a cyclone is the calm, low-pressure center of the cyclone, surrounded by strong winds and heavy rainfall. It has relatively clear skies and calm winds.
Q21: Why do cyclones weaken over land?
A21: Cyclones weaken over land because the source of warm, moist air from the ocean is cut off, reducing the energy needed to sustain the storm’s strength.
Q22: What safety precautions should be taken during lightning?
A22: Stay away from tall objects, crouch in a low-lying open area, minimize ground contact, avoid using metallic umbrellas, get out of water, and stay inside a car or bus.
Q1:
(i) Look at Fig. 6.21 carefully. Vessel R is filled with water.
When pouring of water is stopped, the level of water will
be ________.
(a) the highest in vessel P
(b) the highest in vessel Q
(c) the highest in vessel R
(d) equal in all three vessels
(ii) A rubber sucker (M) is pressed on a flat smooth surface and an identical sucker (N) is pressed on a rough surface:
(a) Both M and N will stick to their surfaces.
(b) Both M and N will not stick to their surfaces.
(c) M will stick but N will not stick.
(d) M will not stick but N will stick
(iii) A water tank is placed on the roof of a building at a height ‘H’. To get water with more pressure on the ground floor, one has to
(a) increase the height ‘H’ at which the tank is placed.
(b) decrease the height ‘H’ at which the tank is placed.
(c) replace the tank with another tank of the same height that can hold more water.
(d) replace the tank with another tank of the same height that can hold less water.
(iv)
Two vessels, A and B contain water up to the same level as shown in Fig. 6.22. PA and PB is the pressure at the bottom of the vessels. FA and FB is the force exerted by the water at the bottom of the vessels A and B.
(a) PA = PB, FA = FB
(b) PA = PB, FA < FB
(c) PA < PB, FA = FB
(d) PA > PB, FA > FB
A1: The correct statements are:
(i) (d) equal in all three vessels
Reason: The vessels are interconnected, so water will settle at the same level in P, Q, and R due to the principle of communicating vessels.
(ii) (c) M will stick but N will not stick
Reason: A rubber sucker works only on a smooth surface where an airtight seal can form; on a rough surface, air leaks in, preventing suction.
(iii) The correct answer is: (a) increase the height ‘H’ at which the tank is placed
Reason: Water pressure at the outlet depends on the height of the water column above it (hydrostatic pressure). The greater the height HH, the greater the pressure on the ground floor.
(iv) The correct answer is: (b) PA=PB, FA<FB
Reason:
Force (FF) is pressure × area of the bottom surface. Vessel B has a larger base area, so FB>FA
Pressure (PP) at the bottom depends only on the height of the water column, not the shape or width of the container. Since both have the same water level, PA=PB.
Q2: State whether the following statements are True [T] or False [F].
(i) Air flows from a region of higher pressure to a region of lower pressure. [ ]
(ii) Liquids exert pressure only at the bottom of a container. [ ]
(iii) Weather is stormy at the eye of a cyclone. [ ]
(iv) During a thunderstorm, it is safer to be in a car. [ ]
A2: (i) True (T) ✅ — Air naturally moves from high-pressure regions to low-pressure regions.
(ii) False (F) ❌ — Liquids exert pressure in all directions, not just at the bottom.
(iii) False (F) ❌ — The eye of a cyclone is calm; the stormy weather is around the eye wall.
(iv) True (T) ✅ — A car acts as a Faraday cage, protecting you from lightning during a thunderstorm.
Q3:
Fig. 6.23 a shows a boy lying horizontally, and Fig. 6.23b shows the boy standing vertically on
loose sand bed. In which case does the boy sink more in sand? Give reasons.
A3: Conclusion:
The boy sinks more in the sand when standing vertically (Fig. 6.23b). This is because the pressure is higher due to the smaller contact area, causing the sand to yield more under his weight compared to when he is lying horizontally, where the pressure is lower due to the larger contact area.
The sinking depends on the pressure exerted on the sand. Higher pressure (from standing) compresses the loose sand more, allowing the boy to sink deeper, while lower pressure (from lying down) distributes the weight, reducing the depth of sinking.
Q4: An elephant stands on four feet. If the area covered by one foot is 0.25 m2 calculate the pressure exerted by the elephant on the ground if its weight is 20000N.
A4: Let’s calculate step-by-step:
Given:
Formula: Pressure=Force/Area
Substitute values: P=20000/1.0=20000 Pa
Answer: The pressure exerted = 20000 Pa
Q5: There are two boats, A and B. Boat A has a base area of 7 m², and 5 persons are seated in it. Boat B has a base area of 3.5 m², and 3 persons are seated in it. If each person has a weight of 700 N, find out which boat will experience more pressure on its base and by how much?
A5:
Q6: Would lightning occur if air and clouds were good conductors of electricity? Give reasons for your answer.
A6: No, lightning would not occur if air and clouds were good conductors of electricity. Lightning occurs because air is an insulator, allowing a buildup of opposite charges in clouds and between clouds and the ground. When the charge difference becomes large, the air’s insulating property breaks down, causing a sudden flow of charges (lightning). If air and clouds were good conductors, charges would flow continuously, preventing the buildup needed for lightning.
Q7: What will happen to the two identical balloons A and B as shown in Fig. 6.24 when water is filled into the bottle up to a certain height? Will both the balloons bulge? If yes, will they bulge equally? Explain your answer.
A7: Both balloons will bulge, and they will bulge equally. The pressure exerted by the water depends on the height of the water column, not the volume or diameter of the container. Since the water is filled to the same height in both bottles, the pressure at the bottom is the same, causing both balloons to bulge to the same extent.
Q8: Explain how a storm becomes a cyclone.
A8: A storm becomes a cyclone when it forms over warm ocean waters. Warm, moist air rises, creating a low-pressure area. As the air rises, water vapor condenses, releasing heat that further lowers the pressure. Surrounding air rushes in, begins to spin due to Earth’s rotation (Coriolis effect), and forms a large, rotating system of clouds, winds, and rain. This system, with a calm center (eye) and high-speed winds, is a cyclone.
Q9: Fig. 6.25 shows trees along the sea coast in a summer afternoon. Identify which side is land – A or B. Explain your answer.
A9: Side A is land. In a summer afternoon, the land heats up faster than the sea, creating a low-pressure area over the land. Air moves from the high-pressure sea (side B) to the low-pressure land (side A), causing a sea breeze. The trees lean toward side A due to the wind blowing from the sea (side B) to the land (side A).
Q10: Describe an activity to show that air flows from a region of high pressure to a region of low pressure.
A10:
Q11: What is a thunderstorm? Explain the process of its formation.
A11: A thunderstorm is a storm with lightning and thunder, often accompanied by heavy rain or hail.
Formation: Warm, moist air rises due to heating, creating a low-pressure area. Cooler air flows in, rises, and cools, condensing water vapor into clouds. Strong upward and downward winds cause water droplets and ice particles to rub, generating static electric charges. Positive charges accumulate in the upper cloud, and negative charges in the lower cloud or ground. When the charge difference overcomes air’s insulating property, a sudden charge flow produces lightning, and the rapid air expansion causes thunder.
Q12: Explain the process that causes lightning.
A12: Lightning is caused by a buildup of opposite electric charges in clouds or between clouds and the ground. Warm, moist air rises, cools, and forms clouds. Strong winds cause water droplets and ice particles to rub, generating static charges—positive charges in the upper cloud and negative charges in the lower cloud or ground. Air, normally an insulator, prevents charge flow until the charge difference becomes large, breaking down air’s insulation. This results in a sudden flow of charges, producing a bright flash called lightning.
Q13: Explain why holes are made in banners and hoardings.
A13: Holes are made in banners and hoardings to allow wind to pass through, reducing wind pressure on the surface. Without holes, high-speed winds create a low-pressure area on one side, increasing the pressure difference across the banner, which could tear it or cause it to collapse. Holes minimize this pressure difference, making the banner more stable.
Q1: Why is it possible to pile up stones or sand, but not a liquid like water?
A1: Stones and sand are solids with strong interparticle forces, allowing their particles to stay in fixed positions, enabling piling. Water, a liquid, has weaker interparticle forces, so its particles flow freely, preventing it from being piled.
Q2: Why does water take the shape of folded hands but lose that shape when released?
A2: Water, as a liquid, has particles that move freely within a limited space, taking the shape of the container (folded hands). When released, the particles flow due to weak interparticle forces, losing the shape.
Q3: We cannot see air, so how does it add weight to an inflated balloon?
A3: Air consists of gas particles that have mass. When a balloon is inflated, it contains many air particles, which collectively add weight to the balloon, even though the particles are invisible.
Q4: Is the air we breathe today the same that existed thousands of years ago?
A4: The air we breathe contains the same types of gas particles (e.g., nitrogen, oxygen) as thousands of years ago, but the specific particles have cycled through natural processes like respiration and photosynthesis. Some particles may be the same, but they are constantly mixed and redistributed.
Q5: Where do pebbles, stones, and sand come from?
A5: Pebbles, stones, and sand originate from rocks in mountains that break down due to erosion. Rivers carry these eroded rock pieces, further breaking them into smaller particles like pebbles, stones, and sand, which are transported to plains.
Q6: What is matter composed of?
A6: Matter is composed of extremely small constituent particles, which are the basic building blocks of substances. These particles are held together by interparticle forces of attraction.
Q7: What happens when you grind a stick of chalk into a fine powder?
A7: Grinding chalk reduces it to smaller specks, which are still chalk (a physical change). Each speck consists of many tiny constituent particles, which cannot be broken down further by physical means.
Q8: What happens to sugar when it dissolves in water?
A8: When sugar dissolves in water, its particles break apart into smaller constituent particles, which spread among water particles. These particles are too small to be seen but can be detected by taste, indicating their presence in the solution.
Q9: Why do solids have a fixed shape and volume?
A9: Solids have strong interparticle forces that hold particles tightly in fixed positions, allowing only vibration. This close packing results in a fixed shape and volume.
Q10: Why do liquids have a definite volume but no fixed shape?
A10: Liquids have weaker interparticle forces than solids, allowing particles to move within a limited space. This enables liquids to take the shape of their container while maintaining a fixed volume due to particles staying relatively close.
Q11: Why do gases have no fixed shape or volume?
A11: Gases have negligible interparticle forces, allowing particles to move freely in all directions. This causes gases to spread and fill the entire available space, taking the shape and volume of their container.
Q12: What is the melting point of a solid?
A12: The melting point is the temperature at which a solid turns into a liquid at atmospheric pressure, as particles gain enough thermal energy to overcome interparticle forces and move apart.
Q13: What is the boiling point of a liquid?
A13: The boiling point is the temperature at which a liquid turns into a gas at atmospheric pressure, as particles gain enough energy to overcome interparticle forces and move freely.
Q14: What is evaporation, and how is it different from boiling?
A14: Evaporation is the slow process where liquid particles at the surface gain enough energy to become gas, occurring at any temperature. Boiling is faster, occurring throughout the liquid at a specific temperature (boiling point), with bubble formation.
Q15: How does interparticle spacing differ in solids, liquids, and gases?
A15: In solids, particles are closely packed with minimal spacing. In liquids, particles are slightly farther apart with more spacing. In gases, particles have maximum spacing, being far apart and freely moving.
Q16: Why is water nearly incompressible compared to air?
A16: Water particles in a liquid are closely packed with limited interparticle space, making it nearly incompressible. Air, a gas, has large interparticle spaces, allowing compression when pressure is applied.
Q17: Why does sugar dissolve in water, but sand does not?
A17: Sugar particles have weaker interparticle forces that water particles can overcome, allowing dissolution. Sand has strong interparticle forces that water cannot break, so it remains insoluble and settles.
Q18: How do particles move in different states of matter?
A18: In solids, particles vibrate in fixed positions. In liquids, particles move freely within a limited space. In gases, particles move rapidly in all directions with no restrictions.
Q19: How does temperature affect particle movement?
A19: Higher temperatures increase thermal energy, causing particles to move faster. In solids, vibrations increase; in liquids, particles move more freely; in gases, particles move faster and farther apart.
Q20: How do soap particles help in cleaning oil-stained clothes?
A20: Soap particles have one end that attaches to oil and another that mixes with water. They surround oil particles, lifting them off the fabric and allowing water to wash them away.
Q1: Choose the correct option. The primary difference between solids and liquids is that the constituent particles are:
(i) closely packed in solids, while they are stationary in liquids.
(ii) far apart in solids and have fixed position in liquids.
(iii) always moving in solids and have fixed position in liquids.
(iv) closely packed in solids and move past each other in liquids.
A1: (iv) closely packed in solids and move past each other in liquids.
Explanation: Solids have closely packed particles with strong interparticle forces, restricting movement to vibrations. Liquids have weaker forces, allowing particles to move past each other, giving liquids no fixed shape.
Q2: Which of the following statements are true? Correct the false statements.
(i) Melting ice into water is an example of the transformation of a solid into a liquid.
(ii) Melting process involves a decrease in interparticle attractions during the transformation.
(iii) Solids have a fixed shape and a fixed volume.
(iv) The interparticle interactions in solids are very strong, and the interparticle spaces are very small.
(v) When we heat camphor in one corner of a room, the fragrance reaches all corners of the room.
(vi) On heating, we are adding energy to the camphor, and the energy is released as a smell.
A2:
(i) True: Melting ice into water is a transformation from solid to liquid.
(ii) False: The melting process involves particles gaining enough thermal energy to overcome interparticle attractions, not a decrease in the attractions themselves.
Corrected: The melting process involves particles gaining thermal energy to overcome interparticle attractions, allowing them to move apart.
(iii) True: Solids have a fixed shape and volume due to strong interparticle forces and close packing.
(iv) True: Solids have strong interparticle interactions and minimal interparticle spaces.
(v) True (T) ✅ — Camphor sublimates, and its vapour diffuses through the air, carrying the fragrance to all parts of the room.
(vi) False (F) ❌ — On heating, we add energy to the camphor, causing it to change into vapour; the smell is not energy release, but the vapour molecules reaching our nose.
Q3: Choose the correct answer with justification. If we could remove all the constituent particles from a chair, what would happen?
(i) Nothing will change.
(ii) The chair will weigh less due to lost particles.
(iii) Nothing of the chair will remain.
A3: Correct Answer: (iii) Nothing of the chair will remain.
Justification: A chair, like all objects, is made up of constituent particles such as atoms and molecules, which are held together by various forces (e.g., chemical bonds, intermolecular forces). These particles define the chair’s structure, mass, and physical properties. If we could remove all the constituent particles:
Option (i) Nothing will change: This is incorrect because removing all particles would eliminate the chair’s physical existence, as the chair is fundamentally composed of these particles. Without them, there would be no material or structure left.
Option (ii) The chair will weigh less due to lost particles: This is also incorrect. Weight is a measure of the force exerted by the mass of the chair due to gravity, and mass is determined by the total number of particles. If all particles are removed, the chair would cease to exist, and the concept of weight would no longer apply.
Option (iii) Nothing of the chair will remain: This is correct. Removing all constituent particles would mean disassembling the chair at its most fundamental level, leaving no material or physical form behind. The chair would effectively disappear, as its existence depends entirely on the presence of these particles.
Conclusion– The removal of all constituent particles results in the complete disappearance of the chair, making (iii) the correct answer.
Q4: Why do gases mix easily, while solids do not?
A4: Gases mix easily because the molecules in a gas are far apart and move very quickly in all directions. This high speed and large spacing mean they can spread out and intermingle without much restriction. In solids, the particles are tightly packed in fixed positions and can only vibrate in place, so they cannot move freely to mix with particles of another solid.
Q5: When spilled on the table, milk in a glass tumbler, flows and spreads out, but the glass tumbler stays in the same shape. Justify this statement.
A5: Milk flows and spreads out because it is a liquid, and in liquids, particles are close together but can slide past each other. This allows a liquid to take the shape of its container and flow when poured. The glass tumbler keeps its shape because it is a solid, and in solids, particles are tightly packed in fixed positions, so they can only vibrate but cannot move around. This rigidity makes a solid maintain its own shape.
Q6: Represent diagrammatically the changes in the arrangement of particles as ice melts and transforms into water vapour.
A6:
Q7: Draw a picture representing particles present in the following: (i) Aluminium foil (ii) Glycerin (iii) Methane gas
A7:
Q8: Observe Fig. 7.16a, which shows the image of a candle that was just extinguished after burning for some time. Identify the different states of wax in the figure and match them with Fig. 7.16b showing the arrangement of particles.
A8: In Fig. 7.16a, the candle shows:
Q9: Why does the water in the ocean taste salty, even though the salt is not visible?
A9: Ocean water tastes salty because salt dissolves into its constituent particles, which are too small to be seen. These particles spread among water particles, occupying interparticle spaces, and can be detected by taste.
Q10: Grains of rice and rice flour take the shape of the container when placed in different jars. Are they solids or liquids? Explain.
A10: Grains of rice and rice flour are solids. They appear to take the shape of the container because their small particles can flow and settle, but each grain or particle retains its own fixed shape and volume, characteristic of solids, due to strong interparticle forces.
Q1: What is matter, and what are some examples of it?
A1: Matter is anything that has mass and takes up space. Examples include staircases, air, water, food, clothes, shoes, books, trees, balls, and sticks.
Q2: What is a mixture, and what are its components?
A2: A mixture is formed when two or more substances are mixed together without undergoing a chemical reaction, with each substance retaining its properties. The individual substances in a mixture are called components.
Q3: What is the difference between uniform and non-uniform mixtures?
A3: Uniform mixtures have components evenly distributed and indistinguishable, even under a microscope (e.g., sugar in water, alloys like stainless steel). Non-uniform mixtures have visible components that can be seen with the naked eye or a magnifying device (e.g., sprout salad with green gram, chickpeas, onion, and tomato).
Q4: What are some examples of uniform mixtures?
A4: Examples include sugar dissolved in water, lemonade, soups, seawater, air, and alloys like stainless steel, brass, and bronze.
Q5: What are some examples of non-uniform mixtures?
A5: Examples include sprout salad, poha, sand and gravel, and muddy water.
Q6: What is an alloy, and what are some examples?
A6: An alloy is a uniform mixture of two or more metals, or a metal and a non-metal, appearing the same throughout. Examples include stainless steel (iron, nickel, chromium, carbon), brass (copper, zinc), and bronze (copper, tin).
Q7: Is air a mixture? If so, what type of mixture is it?
A7: Yes, air is a uniform mixture of gases like nitrogen (78%), oxygen, argon, carbon dioxide, and water vapor, with components evenly distributed and indistinguishable.
Q8: How can the presence of carbon dioxide in air be demonstrated?
A8: Mix calcium oxide with water to form calcium hydroxide (lime water). Expose the lime water to air in a petri dish for a few hours, stirring occasionally. The solution turns milky due to the formation of calcium carbonate from the reaction of carbon dioxide in the air with calcium hydroxide, proving the presence of carbon dioxide.
Q9: What are dust particles in the air, and are they part of air’s composition?
A9: Dust particles are tiny solid particles suspended in the air, visible on a black sheet placed near an open window or in a garden. They are not an integral part of air and are considered pollutants.
Q10: What are major air pollutants, and what is the Air Quality Index (AQI)?
A10: Major air pollutants include particulate matter (dust, soot) and gases like carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. The AQI is a tool used to describe air quality, indicating the level of pollution.
Q11: What is a pure substance in science?
A11: A pure substance consists of only one type of particle and cannot be separated into other substances by physical processes. Examples include elements (e.g., oxygen, iron) and compounds (e.g., water, sugar).
Q12: How does the scientific definition of ‘pure’ differ from common usage?
A12: In common usage, ‘pure’ means unadulterated or free from cheaper or poor-quality substances. In science, a pure substance contains only one type of particle, with no other substances present, regardless of quality.
Q13: What are the types of pure substances?
A13: Pure substances are classified as elements (simplest substances, e.g., hydrogen, oxygen) and compounds (two or more elements chemically combined, e.g., water, sugar).
Q14: What happens when electricity is passed through water?
A14: When electricity is passed through water with a few drops of dilute sulfuric acid, water breaks down into hydrogen and oxygen gases. Hydrogen produces a pop sound when tested with a burning candle, and oxygen makes the flame glow brighter.
Q15: What is an element, and what are its characteristics?
A15: An element is a pure substance made of identical atoms that cannot be broken down into simpler substances by chemical reactions. Examples include hydrogen, oxygen, gold, and sulfur. Elements are the building blocks of matter.
Q16: What is a molecule, and how is it related to elements?
A16: A molecule is a stable particle formed when two or more atoms of an element combine. For example, two hydrogen atoms form a hydrogen molecule (H₂), and two oxygen atoms form an oxygen molecule (O₂).
Q17: How are elements classified?
A17: Elements are classified as metals (e.g., gold, silver, iron), non-metals (e.g., carbon, sulfur, oxygen), and metalloids (e.g., silicon, boron), which have properties intermediate between metals and non-metals.
Q18: How many elements are known, and what are their states at room temperature?
A18: There are 118 known elements. Most are solids, 11 are gases (e.g., oxygen, nitrogen, helium), and two are liquids (mercury, bromine) at room temperature. Gallium and cesium become liquids around 30°C.
Q19: What is a compound, and how does it differ from its constituent elements?
A19: A compound is a pure substance formed when two or more elements combine chemically in a fixed ratio, with properties different from its constituent elements. For example, water (H₂O) is a compound of hydrogen and oxygen, with different properties than the gases.
Q20: What happens when sugar is heated, and what does it indicate about its nature?
A20: When heated, sugar turns brown, then blackens, forming charcoal (carbon) and releasing water droplets. This shows sugar is a compound of carbon, hydrogen, and oxygen, not an element, as it decomposes into simpler substances.
Q21: What is the difference between a mixture and a compound, as shown by iron and sulfur?
A21: A mixture of iron and sulfur (Sample A) retains individual properties: iron is magnetic, sulfur is not, and they can be separated. Iron reacts with hydrochloric acid to produce hydrogen gas, while sulfur remains unreacted. When heated, they form iron sulfide (Sample B), a compound with uniform texture, no magnetic properties, and different behavior (produces hydrogen sulfide gas with hydrochloric acid), showing chemical bonding and new properties.
Q22: How are elements, compounds, and mixtures used in everyday life?
A22: Elements like iron and aluminum are used in construction. Compounds like water are essential for life, and compounds like medicines and fertilizers support health and agriculture. Mixtures like air sustain life, and alloys like stainless steel are used in construction and utensils.
Q23: What are minerals, and how are they related to elements and compounds?
A23: Minerals are natural, solid substances with a fixed chemical composition, found in rocks. They are mostly compounds (e.g., quartz, calcite) but can be pure elements (e.g., gold, sulfur). Many everyday items, like cement and talcum powder, are made from minerals.
Q24: How are elements and compounds used in Indian Dhokra art?
A24: Dhokra art uses brass or bronze (alloys, mixtures of metals like copper, zinc, or tin) to create figures. A beeswax model is covered with clay, melted out, and filled with molten metal, showcasing the use of mixtures for durable, shiny art.
Q25: What is not considered matter?
A25: Light, heat, electricity, thoughts, and emotions are not matter, as they lack mass and do not occupy space.
Q26: How could a compound that absorbs carbon dioxide help solve environmental challenges?
A26: A compound that absorbs carbon dioxide could reduce greenhouse gas levels in the atmosphere, mitigating climate change. For example, it could be used in carbon capture technologies to trap CO₂ from industrial emissions or air, reducing global warming and air pollution.
Q1: Consider the following reaction where two substances, A and B, combine to form a product C: A + B → C. Assume that A and B cannot be broken down into simpler substances by chemical reactions. Which of the following statements is correct?
(i) A, B, and C are all compounds and only C has a fixed composition.
(ii) C is a compound, and A and B have a fixed composition.
(iii) A and B are compounds, and C has a fixed composition.
(iv) A and B are elements, C is a compound, and has a fixed composition.
A1: (iv) A and B are elements, C is a compound, and has a fixed composition.
Explanation: Since A and B cannot be broken down into simpler substances, they are elements. When they combine to form C, it is a compound with a fixed composition, as compounds are formed by elements chemically combining in a fixed ratio.
Q2: Assertion: Air is a mixture. Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change.
(i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.
(iii) Assertion is true, but Reason is false.
(iv) Assertion is false, but Reason is true.
A2: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
Explanation: Air is a mixture of gases (nitrogen, oxygen, etc.) that retain their properties and do not undergo chemical changes when mixed. The reason correctly explains why air is a mixture.
Q3: Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.
A3: Water (H₂O) is a compound formed by the chemical combination of hydrogen and oxygen in a fixed ratio (2:1). Hydrogen is a flammable gas, and oxygen supports combustion, but water is a liquid that extinguishes fire, showing distinct properties. This is because the chemical bonding in water creates a new substance with different characteristics than its constituent elements.
Q4: In which of the following cases are all the examples correctly matched? Give reasons in support of your answer:
(i) Elements – water, nitrogen, iron, air.
(ii) Uniform mixtures – minerals, seawater, bronze, air.
(iii) Pure substances – carbon dioxide, iron, oxygen, sugar.
(iv) Non-uniform mixtures – air, sand, brass, muddy water.
A4: (iii) Pure substances – carbon dioxide, iron, oxygen, sugar.
Reasons:
Q5: Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds, or mixtures, with justification.
A5:
Q6: Classify the following as elements, compounds, or mixtures in Table 8.3: Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda.
A6:
Table 8.3
| Elements | Compounds | Mixtures |
|---|---|---|
| Aluminium | Carbon dioxide | Sand |
| Gold | Magnesium oxide | Seawater |
| Oxygen | Rust (iron oxide) | Muddy water |
| Nitrogen | Iron sulfide | Air |
| Sulfur | Glucose | Fruit juice |
| Hydrogen | Water | |
| Sodium chloride | ||
| Baking soda (sodium bicarbonate) |
Pure substances: Carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda, aluminium, gold, oxygen, nitrogen, sulfur, hydrogen.
Explanation: Elements are pure substances with one type of atom. Compounds are pure substances with two or more elements chemically combined. Mixtures contain multiple substances retaining their properties. Sand and muddy water are non-uniform mixtures; seawater, air, and fruit juice are uniform mixtures.
Q7: What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.
A7: When a mixture of iron filings and sulfur powder is heated, iron sulfide is formed.
Differences:
Q8: Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
A8: No, a substance cannot be classified as both an element and a compound. An element is a pure substance made of identical atoms that cannot be broken down further (e.g., oxygen, iron). A compound is a pure substance formed by two or more elements chemically combined in a fixed ratio (e.g., water, iron sulfide). By definition, a substance is either one type of atom (element) or a combination of different atoms (compound), making it impossible to be both.
Q9: How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
A9: If water were not a compound (H₂O) but simply a mixture of hydrogen and oxygen gases, life would be drastically different — and mostly impossible. Here’s why:
In short: without water as a compound, Earth would be a dry, lifeless, and dangerous planet.
Q10:
Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction.
A10: In Fig. 8.24, when dilute hydrochloric acid reacts with iron filings, Gas A is Hydrogen gas (H₂).
Word equation of the reaction:
Iron + Hydrochloric acid ⟶ Iron chloride + Hydrogen
In symbols:
Fe + 2HCl ⟶ FeCl₂ + H₂ ↑
The hydrogen gas is collected in the test tube above.
Q11: Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
A11: Here are two examples:
Carbon dioxide (CO₂)
Use 1: Used in fire extinguishers. Use 2: Used in carbonated drinks.
Sulphur dioxide (SO₂)
Use 1: Used as a preservative in dried fruits. Use 2: Used in the manufacture of sulphuric acid.
Q12: How can gold be classified as both a mineral and a metal?
A12: Gold can be classified as both a mineral and a metal because:
In short, gold in nature is a mineral, and gold as an element is a metal.
Q1: What is a solution, and why does every sip of homemade Oral Rehydration Solution (ORS) taste the same?
A1: A solution is a uniform mixture formed when two or more substances mix, with components evenly distributed. Every sip of homemade ORS tastes the same because sugar and salt are uniformly dissolved in water, creating a consistent composition throughout.
Q2: What is the difference between a uniform and a non-uniform mixture?
A2: A uniform mixture has components evenly distributed and indistinguishable, even under a microscope (e.g., sugar and salt in water). A non-uniform mixture has components that are not evenly distributed and can be seen with the naked eye or a magnifying device (e.g., chalk powder, sand, or sawdust in water).
Q3: What are solutes and solvents in a solution?
A3: In a solution, the solute is the substance present in a smaller amount that dissolves, and the solvent is the substance present in a larger amount that does the dissolving. For example, in sugar syrup, sugar is the solute, and water is the solvent.
Q4: Is air considered a solution? If so, what are its solvent and solutes?
A4: Yes, air is a gaseous solution. Nitrogen, present in the largest amount (78%), is the solvent, while oxygen, argon, carbon dioxide, and other gases are solutes.
Q5: What is a saturated solution?
A5: A saturated solution is one in which the maximum amount of solute has been dissolved at a given temperature, and no more solute can dissolve without changing conditions.
Q6: What is an unsaturated solution?
A6: An unsaturated solution is one in which more solute can be dissolved at a given temperature, as it has not yet reached its maximum solubility.
Q7: What is solubility, and how does temperature affect it?
A7: Solubility is the maximum amount of solute that can be dissolved in a fixed quantity (e.g., 100 ml) of a solvent at a specific temperature. Generally, for solids in liquids, solubility increases with temperature, while for gases in liquids, solubility decreases with increasing temperature.
Q8: How does the dissolution of baking soda in water demonstrate the effect of temperature on solubility?
A8: When baking soda is added to water at 20°C, some remains undissolved. Heating to 50°C dissolves more, and at 70°C, all undissolved baking soda dissolves, showing that solubility increases with temperature.
Q9: How are solvents used in Indian traditional medicine systems like Ayurveda and Siddha?
A9: In Ayurveda and Siddha, water is primarily used as a solvent for medicinal formulations. Hydro-alcoholic extracts, oils, ghee, and milk are also used as solvents to enhance the therapeutic benefits of herbal drugs.
Q10: How did Asima Chatterjee use solvents and solutions in her work?
A10: Asima Chatterjee used solvents and solutions to extract and isolate compounds from medicinal plants, developing anti-epileptic and anti-malarial drugs. Her work earned her the Shanti Swarup Bhatnagar Award and the Padma Bhushan.
Q11: Why do some objects float while others sink in water?
A11: Objects float or sink in water based on their density compared to water. Objects with lower density than water (e.g., wood) float, while those with higher density (e.g., iron) sink. For example, husk floats on water while rice sinks due to their differing densities.
Q12: What is density, and how is it defined scientifically?
A12: Density is the mass per unit volume of a substance, defined as Density = Mass/Volume. It describes how much matter is packed into a given space, independent of the substance’s shape or size.
Q13: What are the SI units of density, and what other units are used for liquids?
A13: The SI unit of density is kilogram per cubic meter (kg/m³). For liquids, common units include gram per milliliter (g/mL) and gram per cubic centimeter (g/cm³).
Q14: How is the density of water used to calculate relative density?
A14: Relative density is the ratio of a substance’s density to the density of water at the same temperature, expressed as a unitless number. For example, if an aluminum block has a density of 2.7 g/cm³ and water’s density is 1 g/cm³, its relative density is 2.7.
Q15: How is the mass of an object measured?
A15: Mass is measured using a balance, such as a digital weighing balance. The object is placed on a watch glass or butter paper, and the balance is tared to zero before recording the mass (e.g., 16.40 g for a stone).
Q16: How is the volume of a liquid measured?
A16: The volume of a liquid is measured using a measuring cylinder, which has markings indicating volume (e.g., 5 mL, 10 mL). The liquid is poured to the desired level, and the reading is taken at the bottom of the meniscus for clear liquids or the top for colored liquids.
Q17: How is the volume of a solid with a regular shape calculated?
A17: The volume of a regular-shaped solid (e.g., a cuboid) is calculated using the formula Volume = length × width × height. For example, a notebook with dimensions 25 cm × 18 cm × 2 cm has a volume of 900 cm³.
Q18: How is the volume of an irregular-shaped solid measured?
A18: The volume of an irregular-shaped solid (e.g., a stone) is measured by the water displacement method. The solid is submerged in a measuring cylinder with a known initial volume of water (e.g., 50 mL). The rise in water level (e.g., to 55 mL) indicates the solid’s volume (55 mL – 50 mL = 5 mL).
Q19: How does temperature affect the density of a substance?
A19: Generally, as temperature increases, the density of a substance decreases because particles spread out, increasing volume while mass remains constant (Density = Mass/Volume). For example, hot air is less dense and rises, as seen in hot air balloons.
Q20: How does pressure affect the density of different states of matter?
A20: For gases, increasing pressure decreases volume, increasing density. For liquids and solids, which are nearly incompressible, pressure has a negligible effect on density.
Q21: Why does ice float on water?
A21: Ice floats on water because it is less dense. Water’s density is highest at 4°C, but when it freezes at 0°C, its particles arrange into a structure that takes up more space, reducing density. This allows ice to float, forming an insulating layer on water bodies.
Q22: How do the Earth’s layers demonstrate density variations?
A22: Earth’s layers (crust, mantle, core) increase in density from the lightest crust to the densest inner core, as pressure and temperature rise deeper within, compacting materials.
Q23: Why were bamboo and wooden logs used in ancient water travel?
A23: Bamboo and wooden logs were used for rafts and boats because they are light, hollow, and less dense than water, allowing them to float easily. They were tied or hollowed out for fishing, trading, and transport.
Q1: State whether the statements given below are True [T] or False [F]. Correct the false statement(s).
(i) Oxygen gas is more soluble in hot water rather than in cold water.
(ii) A mixture of sand and water is a solution.
(iii) The amount of space occupied by any object is called its mass.
(iv) An unsaturated solution has more solute dissolved than a saturated solution.
(v) The mixture of different gases in the atmosphere is also a solution.
A1: Here’s the answer with corrections for the false statements:
(i) False (F) – Oxygen gas is more soluble in cold water rather than in hot water.
(ii) False (F) – A mixture of sand and water is not a solution, it is a suspension.
(iii) False (F) – The amount of space occupied by any object is called its volume, not mass.
(iv) False (F) – An unsaturated solution has less solute dissolved than a saturated solution.
(v) True (T) – The mixture of different gases in the atmosphere is indeed a solution (a gaseous solution).
Q2: Fill in the blanks.
(i) The volume of a solid can be measured by the method of displacement, where the solid is submerged in water and the rise in water level is measured.
(ii) The maximum amount of solute dissolved in a solvent at a particular temperature is called solubility at that temperature.
(iii) Generally, the density decreases with increase in temperature.
(iv) The solution in which glucose has completely dissolved in water, and no more glucose can dissolve at a given temperature, is called a saturated solution of glucose.
Q3: You pour oil into a glass containing some water. The oil floats on top. What does this tell you?
(i) Oil is denser than water.
(ii) Water is denser than oil.
(iii) Oil and water have the same density.
(iv) Oil dissolves in water.
A3: (ii) Water is denser than oil.
Explanation: Oil floats on water because it is less dense than water (e.g., oil’s density is ~0.91 g/cm³, water’s is 1 g/cm³). Oil does not dissolve in water, and their densities is not the same.
Q4: A stone sculpture weighs 25 g and has a volume of 90 cm³. Calculate its density and predict whether it will float or sink in water.
A4:
Density Calculation:
Density = Mass / Volume = 25 g / 90 cm³ = 0.278 g/cm³.
Prediction: The stone will float in water because its density (0.278 g/cm³) is less than water’s density (1 g/cm³). Objects with lower density than water float.
Q5: Which one of the following is the most appropriate statement, and why are the other statements not appropriate?
(i) A saturated solution can still dissolve more solute at a given temperature.
(ii) An unsaturated solution has dissolved the maximum amount of solute possible at a given temperature.
(iii) No more solute can be dissolved into the saturated solution at that temperature.
(iv) A saturated solution forms only at high temperatures.
A5: (iii) No more solute can be dissolved into the saturated solution at that temperature.
Explanation:
Q6: You have a bottle with a volume of 2 liters. You pour 50 mL of water into it. How much more water can the bottle hold?
A6:
Calculation:
2 liters = 2000 mL.
Water poured = 50 mL.
Remaining capacity = 2000 mL – 50 mL = 1950 mL.
Answer: The bottle can hold 1950 mL more water.
Q7: An object has a mass of 400 g and a volume of 40 cm³. What is its density?
A7: Density of the object Density=Mass/Volume
Mass = 400 g, Volume = 40 cm³ Density=400/40=10 g/cm3
Answer: 10 g/cm³
Q8: Analyse Fig. 9.25a and 9.25b. Why does the unpeeled orange float, while the peeled one sinks? Explain.
A8:
Floating and sinking of oranges (Fig. 9.25a & 9.25b)
Unpeeled orange: The peel contains tiny air pockets, making its average density less than water. This helps it float.
Peeled orange: Removing the peel removes the trapped air, increasing the density so it becomes greater than water’s density, and it sinks.
Reason: Floating or sinking depends on density compared to water — less dense objects float, denser objects sink.
Q9: Object A has a mass of 200 g and a volume of 40 cm³. Object B has a mass of 240 g and a volume of 60 cm³. Which object is denser?
A9: Comparing densities of Object A and B
For Object A: Density=200/40=5 g/cm3
For Object B: Density=240/60=4 g/cm3
Answer: Object A is denser because 5 g/cm3>4 g/cm3
Q10: Reema has a piece of modeling clay that weighs 120 g. She first moulds it into a compact cube that has a volume of 60 cm³. Later, she flattens it into a thin sheet. Predict what will happen to its density.
A10: The density of the modeling clay will remain the same.
Explanation: Density = Mass / Volume. The mass (120 g) remains constant, and changing the shape (from cube to thin sheet) does not affect the volume of the material itself. Thus, the density remains 120 g / 60 cm³ = 2 g/cm³.
Q11: A block of iron has a mass of 600 g and a density of 7.9 g/cm³. What is its volume?
A11:
Calculation:
Density = Mass / Volume.
Volume = Mass / Density = 600 g / 7.9 g/cm³ ≈ 75.95 cm³.
Answer: The volume of the iron block is approximately 75.95 cm³.
Q12: You are provided with an experimental setup as shown in Fig. 9.26a and 9.26b. On keeping the test tube (Fig. 9.26b) in a beaker containing hot water (~70 °C), the water level in the glass tube rises. How does it affect the density?
A12:
When the test tube in Fig. 9.26(b) is placed in hot water:
Since Density=Mass/Volume
and volume ↑ while mass stays constant → density decreases.
Final Answer:
The density of the water decreases because heating makes it expand, so the same mass now occupies a larger volume. The rise in water level in the glass tube is due to this expansion.
Q1: What are spherical mirrors, and how do they differ from plane mirrors?
A1: Spherical mirrors are mirrors with a curved reflecting surface, shaped like a part of a hollow sphere. They differ from plane mirrors, which have a flat surface and always produce an erect image of the same size as the object. Spherical mirrors can produce images that are enlarged, diminished, erect, or inverted, depending on the object’s distance and the mirror’s curvature.
Q2: What are the two types of spherical mirrors, and how are they defined?
A2: The two types of spherical mirrors are:
Q3: How are spherical mirrors made?
A3: Spherical mirrors are created by grinding and polishing a flat glass piece into a curved surface. A reflective coating (e.g., aluminum) is applied to the outer curved surface for a concave mirror or the inner curved surface for a convex mirror, rather than slicing a hollow glass sphere.
Q4: What happens to the image in a concave mirror when the object is moved closer or farther away?
A4: In a concave mirror:
Q5: What are the characteristics of images formed by a convex mirror?
A5: In a convex mirror, the image is always erect, diminished (smaller than the object), and virtual, regardless of the object’s distance. The image size slightly decreases as the object moves farther away (Fig. 10.5).
Q6: How do spherical mirrors behave differently from plane mirrors in terms of image formation?
A6: Plane mirrors produce erect images of the same size as the object with lateral inversion. Spherical mirrors (concave and convex) produce images that vary in size (enlarged or diminished) and orientation (erect or inverted for concave; always erect for convex) depending on the object’s distance, unlike the consistent same-size image of plane mirrors.
Q7: What are the practical uses of concave mirrors?
A7: Concave mirrors are used as:
Q8: Why are convex mirrors used in side-view mirrors of vehicles and at road intersections?
A8: Convex mirrors are used because they form erect, diminished images and provide a wider field of view due to their outward-curved surface. This makes them ideal for side-view mirrors on vehicles and road safety mirrors at intersections to prevent collisions by showing traffic from a larger area (Fig. 10.7).
Q9: Why do side-view mirrors on vehicles have the warning “Objects in mirror are closer than they appear”?
A9: The warning is present because convex mirrors, used in side-view mirrors, produce diminished images, making objects appear smaller and thus farther away than they actually are, potentially causing misjudgment of distance.
Q10: What are the laws of reflection, and do they apply to all types of mirrors?
A10: The laws of reflection are:
Q11: How do parallel light beams behave when reflected by different mirrors?
A11:
Q12: How can a concave mirror be used to burn paper, and what does this demonstrate?
A12: A concave mirror can focus sunlight into a sharp, bright spot on a piece of paper by converging parallel rays. Holding the mirror steady for a few minutes concentrates enough heat to ignite the paper, demonstrating the mirror’s ability to converge light and produce heat (Activity 10.7).
Q13: What are solar concentrators, and how are they used?
A13: Solar concentrators are devices that use mirrors or lenses to focus sunlight into a small area to generate heat. They are used to heat liquids for producing steam to generate electricity, for large-scale cooking, or in solar furnaces for melting steel.
Q14: What is a lens, and how does it differ from a mirror?
A14: A lens is a transparent material with curved surfaces that allows light to pass through, bending it to form images. Unlike mirrors, which reflect light, lenses refract (bend) light, and objects are seen through lenses rather than in them.
Q15: What are the two types of lenses, and how are they defined?
A15:
Q16: How do objects appear when viewed through a convex lens?
A16: Through a convex lens:
Q17: How do objects appear when viewed through a concave lens?
A17: Through a concave lens, the object always appears erect, diminished, and virtual, with the size decreasing slightly as the distance from the lens increases (Activity 10.9).
Q18: How do parallel light beams behave when passing through different lenses?
A18:
Q19: Where are lenses used in everyday life?
A19: Lenses are used in:
Q20: Why does a pencil appear bent when placed in a glass of water?
A20: A pencil appears bent in a glass of water due to the refraction of light. Light bends when passing from water (denser medium) to air (less dense medium), causing the submerged part of the pencil to appear at a different angle than the part in air (Fig. 10.28).
Q21: How did ancient Indian astronomers use reflection to observe stars?
A21: Over 800 years ago, during Bhaskara II’s time, astronomers used shallow bowls of water as reflective surfaces. By observing star reflections through tubes at specific angles, they measured celestial positions, indicating a practical understanding of reflection.
Q1: A light ray is incident on a mirror and gets reflected by it (Fig. 10.21). The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
A1: According to the first law of reflection, the angle of incidence (i) equals the angle of reflection (r). If the angle of incidence is 40°, the angle of reflection is also 40°.
Answer: The angle made by the reflected ray with the normal is 40°.
Angle made by the reflected ray with the mirror is 50°.
Q2.
A2: (i) Light ray falls along the normal (Fig. 10.22a)
(ii) Mirror is tilted, but ray still falls along the normal to the tilted surface (Fig. 10.22b)
(iii) Mirror tilted, light ray falls at 20° from the normal (Fig. 10.22c)
Reflected ray will make 20° with the normal, but in the opposite direction to the incident ray.
Angle of incidence = 20°
Angle of reflection = 20° (on the other side of the normal)
Q3:
A3:Here’s the correct matching for the mirrors shown:
Q4: In Fig. 10.24, the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece – all at the same distance. Match each image with the correct optical element.
A4:
Q5: When the light is incident along the normal on the mirror, which of the following statements is true:
(i) Angle of incidence is 90°
(ii) Angle of incidence is 0°
(iii) Angle of reflection is 90°
(iv) No reflection of light takes place in this case
A5:When light is incident along the normal on a mirror, the correct statement is (ii) Angle of incidence is 0°.
Explanation:
“Normal” means perpendicular to the surface. When light hits a mirror along the normal, it has no angle of incidence with the mirror’s surface, resulting in a reflected ray that also travels along the normal.
Q6: Three mirrors — plane, concave and convex are placed in Fig. 10.25. On the basis of the
images of the graph sheet formed in the mirrors, identify the mirrors and write their names above the mirrors.
A6: Let’s carefully observe Fig. 10.25:
Q7: In a museum, a woman walks towards a large convex mirror (Fig. 10.26). She will see that:
(i) her erect image keeps decreasing in size.
(ii) her inverted image keeps decreasing in size.
(iii) her inverted image keeps increasing in size and eventually it becomes erect and magnified.
(iv) her erect image keeps increasing in size.
A7:
The question is about a large convex mirror.
But in the picture (Fig. 10.26), the artist has drawn the image inverted, which actually happens with a concave mirror, not a convex one.
So, based on the text of the question (convex mirror) → Answer is (iv).
Based on the figure shown (which looks like a concave mirror image) → Answer is (iii).
Q8: Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
A8: Observation with a magnifying glass
Q9:
A9: Answer:
(i) Concave mirror → (a)
(ii) Convex mirror → (b)
(iii) Convex lens → (c)
(iv) Concave lens → (d)
Q10: The following question is based on Assertion/Reason.
Assertion: Convex mirrors are preferred for observing the traffic behind us.
Reason: Convex mirrors provide a significantly larger view area than plane mirrors.
Choose the correct option:
(i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(iii) Assertion is correct but Reason is incorrect.
(iv) Both Assertion and Reason are incorrect.
A10: (i) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
Explanation: Convex mirrors form virtual, erect, and diminished images, so they give a wider field of view than plane mirrors. This larger view area (and reduced blind spots) is why convex mirrors are preferred for observing traffic behind us.
Q11:
A11:
(iv) Figure (a) indicates a plane mirror and Figure (b) indicates a convex mirror.
Q12: Place a pencil behind a transparent glass tumbler (Fig. 10.28a). Now fill the tumbler halfway with water (Fig. 10.28b). How does the pencil appear when viewed through the water? Explain why its shape appears changed.
A12:
Appearance: The pencil appears bent at the water’s surface when viewed through the tumbler filled halfway with water.
Explanation: This is due to the refraction of light. When light travels from water (a denser medium) to air (a less dense medium), it bends away from the normal, causing the submerged part of the pencil to appear at a different angle than the part in air. This creates the illusion of a bent pencil at the air-water interface (Fig. 10.28b).
Q1: Why can the Moon sometimes be seen during the day?
A1: The Moon is visible during the day because it reflects sunlight and can be in a part of the sky where it is not outshone by the Sun’s brightness. Its position in its orbit around Earth allows it to be seen at various times, including at sunrise or sunset, depending on its phase (e.g., during the waning phase at sunrise or waxing phase at sunset).
Q2: What causes the Moon’s changing appearance, known as its phases?
A2: The Moon’s phases are caused by the changing angle of sunlight illuminating the Moon as it orbits Earth. Only the illuminated portion facing Earth is visible, and as the Moon moves, different fractions of this illuminated portion are seen, resulting in phases like new Moon, crescent, gibbous, and full Moon (Fig. 11.3, 11.5).
Q3: What are the main phases of the Moon, and how long does a full cycle take?
A3: The main phases are new Moon (invisible), crescent (less than half illuminated), quarter (half illuminated), gibbous (more than half illuminated), and full Moon (fully illuminated). A full cycle from one full Moon to the next takes about 29.5 days, known as a lunar month (Fig. 11.5).
Q4: What are the waxing and waning periods of the Moon?
A4: The waxing period (Shukla Paksha) is when the Moon’s illuminated portion visible from Earth increases, from new Moon to full Moon. The waning period (Krishna Paksha) is when it decreases, from full Moon to new Moon. These cycles repeat every lunar month (Fig. 11.2).
Q5: How does the Moon’s position relative to the Sun change over a month?
A5: The Moon’s position shifts relative to the Sun as it orbits Earth. On full Moon day, it is opposite the Sun (rising as the Sun sets). As it wanes, it appears closer to the Sun in the sky, becoming invisible at new Moon. During waxing, it moves away from the Sun again, visible at sunset (Activity 11.1, Fig. 11.5a).
Q6: Why does the Moon rise about 50 minutes later each day?
A6: The Moon rises later each day because it moves eastward in its orbit around Earth while Earth rotates on its axis. Earth must rotate an additional amount (about 50 minutes’ worth) for the Moon to appear in the same sky position as the previous day (Fig. 11.6).
Q7: How can the Moon’s phases help locate it in the sky?
A7: Knowing the Moon’s phase helps determine its visibility. A waning Moon is best seen at sunrise (westward), while a waxing Moon is best seen at sunset (eastward). The Moon’s position relative to the Sun also shifts daily, aiding in predicting its location (Activity 11.1).
Q8: How does Activity 11.2 demonstrate the Moon’s phases?
A8: In Activity 11.2, a ball (representing the Moon) is held at arm’s length, with a torch or lamp (representing the Sun) illuminating it. As the ball is moved around the observer’s head (Earth), the illuminated portion’s shape changes, mimicking the Moon’s phases from new Moon to full Moon (Fig. 11.4).
Q9: What is a mean solar day, and how is it measured?
A9: A mean solar day is the average time (24 hours) the Sun takes to move from its highest point in the sky (shortest shadow) to the same position the next day, caused by Earth’s rotation. It can be measured by tracking the shortest shadow of a stick over consecutive days (Activity 11.3, Table 11.2).
Q10: How did lunar calendars originate, and what is their limitation?
A10: Lunar calendars originated by tracking the Moon’s phases, with a lunar month of about 29.5 days and a lunar year of 12 months (354 days). Their limitation is that they do not stay synchronized with seasons, as a lunar year is shorter than a solar year (365.25 days) by about 11 days.
Q11: How do solar calendars differ from lunar calendars?
A11: Solar calendars are based on Earth’s revolution around the Sun (365.25 days), aligning with seasons. They adjust month lengths (e.g., 30 or 31 days, 28 for February) and include a leap day every four years. Lunar calendars follow the Moon’s phases and are shorter, causing seasonal drift.
Q12: What are luni-solar calendars, and how do they address the limitations of lunar calendars?
A12: Luni-solar calendars use the Moon’s phases for months but add an intercalary month (Adhika Maasa) every 2-3 years to align with the solar year and seasons. This keeps festivals and agricultural events synchronized with seasonal cycles.
Q13: What is the Indian National Calendar, and when was it adopted?
A13: The Indian National Calendar, based on the Surya Siddhanta, is a luni-solar calendar adopted on March 21, 1956 (1 Chaitra 1878 Saka) for official purposes, following recommendations from the Calendar Reform Committee chaired by Meghnad Saha.
Q14: How are Indian festivals related to astronomical phenomena?
A14: Many Indian festivals are tied to the Moon’s phases in lunar or luni-solar calendars (e.g., Diwali on new Moon of Kartika, Holi on full Moon of Phalguna). Some, like Makar Sankranti, follow a solar sidereal calendar, occurring around the same Gregorian date annually.
Q15: Why do festival dates like Eid-ul-Fitr shift in the Gregorian calendar?
A15: Eid-ul-Fitr, based on a lunar calendar, shifts earlier by about 11 days each year in the Gregorian calendar because a lunar year (354 days) is shorter than a solar year (365 days). Luni-solar festivals like Diwali shift less due to intercalary months.
Q16: What is the significance of Uttarayana and Dakshinayana?
A16: Uttarayana is the Sun’s apparent northward movement from December to June, and Dakshinayana is its southward movement from June to December, linked to solstices and seasons. These were observed in ancient India and recorded in texts like the Taittiriya Samhita.
Q17: What are artificial satellites, and what are their purposes?
A17: Artificial satellites are human-made objects orbiting Earth, typically at 800 km altitude, completing an orbit in about 100 minutes. They are used for communication, navigation, weather monitoring, disaster management, and scientific research (e.g., ISRO’s Cartosat, AstroSat).
Q18: How did ancient Indian astronomers track celestial events?
A18: Ancient Indian astronomers, like those during Bhaskara II’s time, used shallow water bowls to observe star and planet reflections through tubes, measuring their positions. Texts like the Surya Siddhanta noted solstices and equinoxes for calendar-making.
Q19: What is the role of the Rashtriya Panchang?
A19: The Rashtriya Panchang, published by the Positional Astronomy Centre, calculates celestial positions (Sun, Moon) for a central Indian location to standardize festival dates across India, accounting for regional sunrise differences.
Q20: How are tides related to the Moon?
A20: Tides follow a pattern similar to the Moon’s motion, with high or low tides occurring about 50 minutes later each day, corresponding to the Moon’s position and phase, due to its gravitational influence on Earth’s oceans.
Q1: State whether the following statements are True or False.
(i) We can only see that part of the Moon which reflects sunlight towards us.
(ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.
(iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.
(iv) The Moon can only be seen at night.
A1: Here’s the evaluation of the statements:
(i) We can only see that part of the Moon which reflects sunlight towards us.
✅ True
(ii) The shadow of Earth blocks sunlight from reaching the Moon causing phases.
❌ False (Phases are due to changing positions of the Moon and Earth; Earth’s shadow causes lunar eclipses, not phases.)
(iii) Calendars are based on various astronomical cycles which repeat in a predictable manner.
✅ True
(iv) The Moon can only be seen at night.
❌ False (The Moon is often visible during the day also, depending on its phase and position in the sky
Q2: Amol was born on 6th of May on a full Moon day. Does his birthday fall on the full Moon day every year? Explain your answer.
A2: No, Amol’s birthday will not fall on a full Moon day every year. Here’s why:
Sometimes a full Moon may fall close to 6th May, but in most years it will occur on different dates in May (or even late April or early June).
A year (based on Earth’s revolution around the Sun) is about 365 days, while the lunar cycle (time from one full Moon to the next) is about 29.5 days. These two cycles are not in exact sync. That means the date of a full Moon keeps shifting in the calendar every year.
Q3:
A3: Two things shown incorrectly are:
Q4:
A4: (i) Match picture labels with phases of the Moon:
(ii) Phases of the Moon that are never seen from Earth:
So, the hidden half (far side) of the Moon is never seen from Earth.
Q5: Malini saw the Moon overhead in the sky at sunset. (i) Draw the phase of the Moon that Malini saw. (ii) Is the Moon in the waxing or the waning phase?
A5: At sunset, the Sun is in the west. If the Moon is seen overhead at that time, it means:
(i) Phase of the Moon Malini saw
It would look like a half Moon (right half bright, left half dark).
(ii) Waxing or Waning?
Q6: Ravi said, “I saw a crescent Moon, and it was rising in the East, when the Sun was setting.” Kaushalya said, “Once I saw the gibbous Moon during the afternoon in the East.” Who out of the two is telling the truth?
A6:
Q7: Scientific studies show that the Moon is getting farther away from the Earth and slower in its revolution. Will luni-solar calendars need an intercalary month more often or less often?
A7: The Moon’s revolution period increases as it moves farther from Earth, making a lunar month slightly longer over time. A lunar year (12 lunar months) will thus take longer, reducing the gap between a lunar year (354 days) and a solar year (365.25 days). This means the accumulated difference requiring an intercalary month (Adhika Maasa) will take longer to reach a full month. Therefore, luni-solar calendars will need an intercalary month less often.
Answer: Less often.
Q8: A total of 37 full Moons happen during 3 years in a solar calendar. Show that at least two of the 37 full Moons must happen during the same month of the solar calendar.
A8: A solar calendar year has 12 months, so 3 years have 3 × 12 = 36 months. A lunar month is about 29.5 days, and 37 full Moons occur over 3 years (approximately 3 × 365.25 = 1095.75 days). Distributing 37 full Moons across 36 months means at least one month must have more than one full Moon, as 37 events cannot be evenly spread (one per month) over 36 months (37 > 36). By the pigeonhole principle, at least one month will have at least two full Moons.
Answer: Since 37 full Moons exceed the 36 months in 3 years, at least one month must contain two full Moons.
Q9: On a particular night, Vaishhali saw the Moon in the sky from sunset to sunrise. What phase of the Moon would she have noticed?
A9: If the Moon is visible from sunset to sunrise, it is up all night, which occurs when it is nearly opposite the Sun in the sky. This corresponds to the full Moon phase, when the Moon rises at sunset and sets at sunrise (Fig. 11.5a, position A).
Answer: Full Moon.
Q10: If we stopped having leap years, in approximately how many years would the Indian Independence Day happen in winter?
A10: Indian Independence Day is August 15, a summer date in the Northern Hemisphere. Without leap years, the Gregorian calendar would not account for the extra ~0.25 days per year (Earth’s revolution takes 365.25 days). Each year, the calendar would shift forward by ~0.25 days, so seasons would drift backward. To move from mid-August (summer) to winter (e.g., February, ~6 months or ~180 days later), the calendar would need to shift by 180 days. Since the shift is 0.25 days per year, it would take 180 ÷ 0.25 = 720 years.
Answer: Approximately 720 years.
Q11: What is the purpose of launching artificial satellites?
A11: Artificial satellites are launched for communication, navigation, weather monitoring, disaster management, and scientific research, such as mapping (e.g., ISRO’s Cartosat) and studying celestial objects (e.g., AstroSat) (Page 16).
Answer: For communication, navigation, weather monitoring, disaster management, and scientific research.
Q12: On which periodic phenomenon are the following measures of time based: (i) day (ii) month (iii) year?
A12:
Q1: Why do elephants enter human farms and villages?
A1: Elephants enter farms and villages due to loss of forest cover and changes in rainfall patterns, which reduce vegetation and dry up waterholes in their natural habitats. This scarcity forces them to seek food like bananas and sugarcane in human areas, leading to crop damage and potential harm to people and animals (Page 2).
Q2: What is a habitat, and what are its components?
A2: A habitat is the place where an organism lives, providing conditions for survival (e.g., a pond or tree bark). It consists of biotic components (living organisms like plants, animals, microbes) and abiotic components (non-living elements like water, air, sunlight, soil, temperature) (Page 3, Activity 12.1).
Q3: What are the differences between biotic and abiotic components in a habitat?
A3: Biotic components are living organisms (e.g., fish, plants, insects) that interact and depend on each other for food, reproduction, etc. Abiotic components are non-living elements (e.g., water, sunlight, soil) that provide essential conditions for survival. Both interact to sustain the habitat (Page 3, Fig. 12.1).
Q4: What is a population, and how does it differ from a community?
A4: A population is a group of organisms of the same species living in a habitat at a given time (e.g., a group of fish in a pond). A community comprises different populations (e.g., fish, frogs, plants) sharing the same habitat and interacting for survival (Pages 4-5, Activity 12.2).
Q5: Why can’t a habitat have only one type of organism?
A5: A habitat with only one type of organism would lead to competition for the same resources (food, water, space), causing scarcity and potential extinction. Diverse populations in a community ensure balanced resource use and interdependence (Page 5).
Q6: What is pollination, and why is it important?
A6: Pollination is the process where pollen is transferred from stamens (male part) to carpels (female part) of flowers by wind, water, or animals like insects, birds, or bats. It is essential for fruit and seed formation, supporting plant reproduction and ecosystem continuity (Page 5, Fig. 12.2).
Q7: How do fish in a pond affect seed production in nearby plants?
A7: Fish eat dragonfly larvae, reducing dragonfly populations. Fewer dragonflies mean more bees and butterflies (which dragonflies prey on), which pollinate flowers, increasing seed production in plants near ponds with fish compared to those without (Activity 12.3, Fig. 12.4).
Q8: What is an ecosystem, and what are its types?
A8: An ecosystem is formed by interactions between biotic (plants, animals, microbes) and abiotic (air, water, soil) components in a habitat. Types include aquatic ecosystems (ponds, rivers, lakes) and terrestrial ecosystems (forests, grasslands, farmlands). Ecosystems can overlap, like a river within a forest (Page 8, Fig. 12.6).
Q9: How do biotic and abiotic components interact in an ecosystem?
A9: Biotic components depend on abiotic components for food, shelter, and survival (e.g., plants use sunlight and water for photosynthesis; fish need water for oxygen). Abiotic components are influenced by biotic ones (e.g., plants release oxygen, prevent soil erosion) (Activity 12.4, Table 12.3).
Q10: What are producers, consumers, and decomposers in an ecosystem?
A10:
Q11: What is a food chain, and how does it differ from a food web?
A11: A food chain is a linear sequence showing who eats whom in an ecosystem (e.g., Grass → Hare → Fox). A food web is a network of interlinked food chains, showing multiple feeding relationships among organisms (e.g., a frog eaten by both snakes and eagles) (Pages 10-11, Figs. 12.8, 12.11).
Q12: What are trophic levels in a food chain?
A12: Trophic levels are positions in a food chain:
Q13: What is decomposition, and why are decomposers important?
A13: Decomposition is the process where decomposers (fungi, bacteria, insects) break down dead plants, animals, and waste into simpler substances, recycling nutrients into the soil. Decomposers are vital for nutrient cycling, supporting plant growth and ecosystem balance (Page 11, Fig. 12.12).
Q14: What are the types of biotic interactions in an ecosystem?
A14:
Q15: How do ecosystems benefit humans?
A15: Ecosystems provide clean air, water, food, fibers, timber, medicines, and regulate climate. They also offer aesthetic and recreational value, supporting human well-being (Page 14).
Q16: What threatens ecosystems like the Sundarbans?
A16: The Sundarbans face threats from deforestation (mangrove cutting), illegal hunting, overuse of resources, industrial pollution, and sewage, disrupting wildlife and habitat balance. Similar threats affect other Indian ecosystems (Page 15, Fig. 12.17).
Q17: What are protected areas, and why are they important?
A17: Protected areas (e.g., national parks, wildlife sanctuaries, biosphere reserves) conserve habitats and endangered species. Examples include Jim Corbett National Park and Nilgiri Biosphere Reserve. They preserve biodiversity for future generations (Page 15).
Q18: What are human-made ecosystems, and how do they differ from natural ones?
A18: Human-made ecosystems (e.g., farms, fish ponds, parks) are designed to meet human needs and require management. Unlike natural ecosystems, they are less self-sustaining and depend on human intervention (Page 16).
Q19: How did the Green Revolution impact farming, and why is it unsustainable?
A19: The Green Revolution (1950s–1960s) increased food production using tractors, synthetic fertilizers, and pesticides. However, overuse of chemicals, monoculture, and excessive irrigation degrade soil, reduce biodiversity, and harm ecosystems, making it unsustainable (Page 16).
Q20: How can sustainable farming practices help ecosystems?
A20: Sustainable practices like organic farming, crop rotation, and natural pest control (e.g., using predators like beetles) reduce chemical use, maintain soil fertility, support biodiversity, and minimize environmental harm, ensuring long-term food security (Page 17, Fig. 12.18).
Q1: Refer to the given diagram (Fig. 12.19) and select the wrong statement.
(i) A community is larger than a population.
(ii) A community is smaller than an ecosystem.
(iii) An ecosystem is part of a community.
A1:
(i) True: A community includes multiple populations of different species in a habitat.
(ii) True: A community is part of an ecosystem, which includes both biotic (community) and abiotic components.
(iii) False: An ecosystem includes a community and abiotic components, not the other way around.
Answer: (iii) An ecosystem is part of a community.
Q2: A population is part of a community. If all decomposers suddenly disappear from a forest ecosystem, what changes do you think would occur? Explain why decomposers are essential.
A2: If decomposers (e.g., fungi, bacteria) disappear from a forest ecosystem:
Q3: Selvam from Cuddalore district, Tamil Nadu, shared that his village was less affected by the 2004 Tsunami compared to nearby villages due to the presence of mangrove forests. This surprised Sarita, Shabnam, and Shijo. They wondered if mangroves were protecting the village. Can you help them understand this?
A3: Mangrove forests in Selvam’s village acted as a natural barrier during the 2004 Tsunami. Their dense roots and trees slowed down strong waves and winds, reducing the tsunami’s impact on the village. Mangroves also stabilize coastlines, prevent erosion, and absorb storm energy, protecting inland areas. This explains why Selvam’s village was less affected compared to nearby villages without mangroves (Page 15, Fig. 12.17).
Answer: Mangroves protected the village by slowing tsunami waves, stabilizing coastlines, and reducing storm impact.
Q4: Look at this food chain: Grass → Grasshopper → Frog → Snake. If frogs disappear from this ecosystem, what will happen to the population of grasshoppers and snakes? Why?
A4:
Q5: In a school garden, students noticed fewer butterflies the previous season. What could be the possible reasons? What steps can students take to have more butterflies on campus?
A5:
Possible reasons for fewer butterflies:
Q6: Why is it not possible to have an ecosystem with only producers and no consumers or decomposers?
A6: An ecosystem with only producers (plants) is not possible because:
Q7: Observe two different places near your home or school (e.g., a park and a roadside). List the living and non-living components you see. How are the two ecosystems different?
A7:
Example observations:
Q8: Human-made ecosystems like agricultural fields are necessary, but they must be made sustainable. Comment on the statement.
A8: Human-made ecosystems like agricultural fields are essential for food production to support growing populations. However, practices like monoculture, overuse of synthetic fertilizers, and pesticides degrade soil, reduce biodiversity, and harm ecosystems (e.g., killing pollinators, polluting water). Sustainable practices, such as organic farming, crop rotation, and natural pest control, maintain soil health, support biodiversity, and ensure long-term food security with minimal environmental impact (Pages 16-17).
Answer: Agricultural fields are necessary for food but must adopt sustainable practices like organic farming to protect soil, biodiversity, and ecosystems.
Q9: If the Indian hare population (Fig. 12.20) drops because of a disease, how would it affect the number of other organisms?
A9: If the Indian hare population drops:
Q1: Why is Earth considered a unique planet for sustaining life?
A1: Earth is unique because it supports diverse life forms due to its position in the habitable zone, where temperatures allow liquid water to exist. Its size supports an atmosphere with oxygen and an ozone layer, and its magnetic field protects against harmful cosmic rays. The interaction of the atmosphere, hydrosphere, geosphere, and biosphere maintains a balance essential for life (Pages 2, 5-8, 16).
Q2: What is the Earth’s crust, and why is it significant for life?
A2: The Earth’s crust is a thin layer, like an apple’s skin, where all life exists, from mountains to ocean trenches. It provides essential resources like soil, rocks, and minerals that support ecosystems and human activities such as agriculture and construction (Page 2, Fig. 13.1).
Q3: How do satellite images help us understand Earth?
A3: Satellites, like ISRO’s Earth Observation Satellite, capture images that reveal information about plants, ocean organisms, temperature, oil spills, and wind patterns. False-color images highlight specific data, aiding in environmental monitoring and resource management (Page 2).
Q4: How does Earth’s position in the solar system contribute to its habitability?
A4: Earth orbits the Sun in the habitable or Goldilocks zone, where temperatures are ideal for liquid water, essential for life. Its nearly circular orbit ensures stable temperatures throughout the year, preventing extreme heat or cold (Pages 5-6, Fig. 13.4).
Q5: Why is Venus hotter than Mercury despite being farther from the Sun?
A5: Venus is hotter (average 450°C) than Mercury (170°C) due to its thick carbon dioxide atmosphere, which traps heat via the greenhouse effect, making it the hottest planet in the solar system (Page 4, Fig. 13.2).
Q6: What is the greenhouse effect, and how does it differ between Earth and a plant greenhouse?
A6: The greenhouse effect on Earth involves atmospheric gases like carbon dioxide trapping heat radiated from the Earth’s surface, maintaining a life-supporting temperature. In a plant greenhouse, glass walls trap warmed air, preventing heat escape, but the mechanism differs as it’s a physical barrier rather than gas absorption (Page 5, Fig. 13.3).
Q7: How does Earth’s size affect its ability to sustain life?
A7: Earth’s size provides sufficient gravity to retain an atmosphere with oxygen and an ozone layer, which shields life from harmful UV rays. If smaller, gravity would be too weak, losing the atmosphere (like Mars); if too large, excessive gravity could crush organisms (Page 7).
Q8: What role does the ozone layer play in sustaining life?
A8: The ozone layer, formed from three-atom oxygen molecules, absorbs harmful ultraviolet (UV) rays from the Sun, protecting living cells from damage and enabling life to thrive (Page 7).
Q9: How does Earth’s magnetic field contribute to life?
A9: Earth’s magnetic field, generated by molten iron in its core, deflects harmful cosmic rays and solar wind particles, protecting the atmosphere and ozone layer, thus safeguarding life (Page 8).
Q10: What is the hydrosphere, and why is it important for life?
A10: The hydrosphere includes all water on Earth (oceans, lakes, rivers, groundwater), covering 70% of the surface. It supports aquatic life, transports nutrients in plants, regulates body temperature in animals, and provides water for crops and human use. Water vapor forms clouds, influencing rainfall and ecosystems (Page 9, Fig. 13.8).
Q11: What is the geosphere, and how does it support life?
A11: The geosphere comprises Earth’s solid parts (rocks, soil, minerals). Soil provides nutrients like nitrogen and potassium for plant growth, while rocks and minerals supply resources like coal, oil, and metals, supporting ecosystems and human activities (Page 9).
Q12: What is geodiversity, and how does it contribute to ecosystems?
A12: Geodiversity refers to the variety of landforms, rocks, and soils, creating unique habitats for diverse life forms. It shapes ecosystems by providing varied conditions for plants, animals, and microbes to thrive (Page 10, Fig. 13.9).
Q13: What is the biosphere, and how does it interact with other Earth systems?
A13: The biosphere includes all living beings (plants, animals, microbes) and their habitats. It interacts with the atmosphere (oxygen production), hydrosphere (water for life), and geosphere (nutrient cycling), maintaining a balanced system that sustains life (Page 10).
Q14: Why is balance important in Earth’s systems?
A14: Balance between the atmosphere, hydrosphere, geosphere, and biosphere ensures stable conditions for life. Disruptions, like deforestation, affect rainfall, soil, air quality, and biodiversity, threatening ecosystems (Page 10).
Q15: How does reproduction ensure the continuity of life?
A15: Reproduction allows organisms to produce offspring, preventing extinction. Genetic material (genes) passes instructions for development, ensuring species continuity. Small changes in genes enable adaptation to new environments (Page 10).
Q16: What is the difference between asexual and sexual reproduction?
A16:
Q17: How does sexual reproduction in plants and animals work?
A17:
Q18: Why don’t mammals like dogs lay eggs?
A18: Mammals like dogs give birth to live young because their embryos develop inside the mother, receiving nutrition and oxygen directly. This differs from birds, where eggs provide nutrition for external embryo development. Genetic instructions in mammals favor live birth, not egg-laying (Page 14).
Q19: What is the triple planetary crisis, and how does it threaten life?
A19: The triple planetary crisis includes:
Q20: How can we protect Earth’s ecosystems?
A20: Protection involves reducing greenhouse gas emissions (using renewable energy like solar, wind), improving waste management, adopting sustainable farming, and preserving biodiversity through conservation efforts. Individual actions like recycling, saving energy, and raising awareness also help (Pages 15-16).
Q1: What is one major reason Mars cannot currently support life like Earth?
(i) It has too many volcanoes.
(ii) It is too close to the Sun.
(iii) It lacks a thick atmosphere and liquid water.
(iv) Its magnetic field is too strong.
A1: Mars cannot support life like Earth because it lacks a thick atmosphere and liquid water, essential for life as we know it. Its thin atmosphere (100 times thinner than Earth’s) and position at the edge of the habitable zone limit liquid water availability (Pages 6-7).
Answer: (iii) It lacks a thick atmosphere and liquid water.
Q2: Which of these is an example of geodiversity?
(i) Variety of bird chirping in a forest.
(ii) Different landforms like mountains, valleys, and deserts.
(iii) Changing weather during monsoons.
(iv) Number of different types of fish in a pond.
A2: Geodiversity refers to the variety of landforms, rocks, and soils. Different landforms like mountains, valleys, and deserts are examples of geodiversity, as they create unique habitats (Page 10, Fig. 13.9).
Answer: (ii) Different landforms like mountains, valleys, and deserts.
Q3: If the Earth were smaller with the same density, what might happen to its atmosphere?
(i) It would become thicker and hotter.
(ii) It would escape into space due to weaker gravity.
(iii) It would become frozen.
(iv) It would cause stronger winds.
A3: A smaller Earth with the same density would have weaker gravity, unable to retain atmospheric gases, causing them to escape into space, as seen with Mars (Page 7).
Answer: (ii) It would escape into space due to weaker gravity.
Q4: In sexual reproduction, why are offspring different from their parents?
(i) They grow in different climates.
(ii) They eat different food.
(iii) They acquire new instructions after birth.
(iv) They get mixed instructions (genes) from both parents.
A4: Offspring in sexual reproduction differ from parents because they receive a mix of genetic instructions (genes) from both parents via gametes, creating unique combinations of traits (Page 13).
Answer: (iv) They get mixed instructions (genes) from both parents.
Q5: You notice tiny green plants growing on cracks on your school wall after the monsoon. Where do you think the seeds came from? What conditions helped these plants grow there?
A5:
Q6: A city has recently cut down a large patch of forest to build new roads and buildings. Discuss the possible effects this could have on the local climate and biodiversity? How might this affect water availability or quality in the area?
A6:
Q7: A friend says, “The Earth has always had climate changes in the past, so today’s global warming is nothing new.” How would you respond using what you’ve learnt in this and other chapters of your science book?
A7: While Earth has experienced natural climate changes over millions of years (e.g., ice ages), today’s global warming is different because it is primarily driven by human activities, like burning fossil fuels, which release excessive greenhouse gases (carbon dioxide, methane). These gases intensify the greenhouse effect, causing rapid warming, melting ice caps, rising sea levels, and extreme weather, unlike slower natural changes. Deforestation and pollution further exacerbate this, threatening ecosystems and biodiversity (Pages 14-15, Chapter 12). The Paris Agreement (2015) highlights the urgency to limit warming to 1.5°C, but current efforts are insufficient, making human-induced climate change a unique and urgent crisis (Page 15).
Answer: Today’s global warming is human-driven, caused by fossil fuel emissions and deforestation, leading to rapid, severe changes unlike slow natural climate shifts, threatening ecosystems and requiring urgent action.
Q8. Imagine Earth’s magnetic field suddenly disappeared. What kinds of problems could arise for life on Earth? Explain.
A8: The Earth’s magnetic field acts like a protective shield. If it suddenly disappeared, these problems could arise:
1. Radiation from the Sun (Solar Wind)
2. Loss of Atmosphere
3. Disruption of Technology
4. Auroras Everywhere
5. Navigation Problems
Q9. You are tasked with designing a new settlement for humans on Mars. Name three things you would need to recreate from Earth to support human life there. Which of these do you think is the hardest to replicate, and why?
A9: Let’s imagine we are building a human settlement on Mars. To make it habitable, we’d need to recreate some key Earth-like conditions:
Three things needed:
Hardest to Replicate- The hardest to recreate is a protective atmosphere + magnetic shielding.
Reason:
Q10. In a village, the temperature has been increasing and rainfall has become unpredictable over the past few years. What could be causing this change? Suggest two ways the village could adapt to these new conditions.
A10: What could be causing the change?
Two ways the village could adapt:
Q11. If there were no atmosphere on the Earth, would it affect life, temperature, and water on the planet? Explain.
A11: Yes , if Earth had no atmosphere, it would affect life, temperature, and water very severely.
Effect on Life
Effect on Temperature
Effect on Water
Q12. Discuss five examples of vegetative propagation.
A12: Five Examples of Vegetative Propagation:
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Answers are creative but lende so try to write more short because all school teachers are that type they write answers on board that's why
Sir I already learnt the question answer of science ch 1. So is it okay if I write the same answer for the same question . And the new questions that are added , need to learn ? Pls answer me sir
Thank you
Answers are same, wording are different. No issue. Learn whatever you want to learn.
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YES , you can. But you need to send content personally to me. Your content will be verified and then uploaded.
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Sir I have a Question.
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Sir only exercise questions will come in exam or those extra questions will come also