Class 10 Science · Chapter 13

Our Environment

141 Class 10 Science questions on Our Environment from the NCERT textbook, NCERT Exemplar and CBSE board papers 2024–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 5 of 8

1. Q81 CBSE 2026
   1. (i)Chlorofluorocarbons (CFCs) have ​been linked to depletion of ozone layer. In 1987, ​the ​United Nations Environment ​Programme ​(UNEP) succeeded in making an agreement. State this agreement.
   2. (ii)​What is the ​role of ​ozone in our atmosphere ?
   Show answer

   UNEP's 1987 Montreal Protocol froze CFC production at 1986 levels; ozone shields the Earth from harmful UV radiation.

   (i) In 1987 the United Nations Environment Programme (UNEP) forged an agreement to freeze the production of CFCs at 1986 levels; it is known as the Montreal Protocol. (ii) Ozone (${\text{O}}_3$) at the higher levels of the atmosphere shields the surface of the Earth from the harmful ultraviolet (UV) radiation of the Sun, which is highly damaging to organisms and causes skin cancer in human beings; the ozone layer thus protects life on Earth.

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2. Q82 CBSE 2026
   How is ozone layer ​formed in our atmosphere ​? State ​one important role of ozone in our ​atmosphere. ​What is responsible ​for ​its depletion ?
   Show answer

   UV splits oxygen into atoms that combine with oxygen to form ozone; the layer absorbs harmful UV radiation, and CFCs deplete it.

   Formation: high-energy ultraviolet radiation from the Sun splits molecular oxygen at the higher levels of the atmosphere into free oxygen atoms, ${\text{O}}_2\to \text{O}+\text{O}$, and each free atom then combines with a molecule of oxygen, $\text{O}+{\text{O}}_2\to {\text{O}}_3$; this builds up the ozone layer. Role: ozone absorbs the harmful ultraviolet radiation of the Sun and stops it from reaching the Earth's surface, where it would damage living cells and cause skin cancer. Depletion: chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers, rise to the layer and break ozone down.

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3. Q83 CBSE 2026
   Give any two ​ways in which non-biodegradable ​substances ​would ​effect ​the environment ​adversely.
   Show answer

   They persist because decomposers cannot break them down, and they pollute soil, water and air.

   (1) Non-biodegradable substances are not broken down by decomposers, so they persist in the environment for a long time and damage it: plastic and polythene waste, for example, litters the land, chokes drains and harms animals that swallow it. (2) They cause soil, air and water pollution: they pollute the soil and the water bodies where they are dumped or washed, and burning them releases harmful gases into the air.

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4. Q84 CBSE 2026
   Explain why food ​chains ​generally consist ​of three ​or four ​steps.
   Show answer

   With only about 10% of energy passing to each next level, too little remains beyond three or four steps.

   Because of the ten per cent law of energy transfer. Only about $10\%$ of the energy held at one trophic level is passed on to the next; the other $90\%$ is spent in the organism's own life processes and lost to the surroundings as heat, or leaves undigested. The loss repeats at every step, so $1000$ J trapped by the grass leaves $100$ J for the herbivore, $10$ J for the carnivore and only $1$ J for the top carnivore. A fifth level would have about $0.1$ J, one ten-thousandth of the original energy, which cannot support a population of organisms. Since energy flow is also unidirectional, this loss can never be made up. Food chains therefore generally consist of only three or four steps.

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5. Q85 CBSE 2026
   What will ​be the impact on an ​ecosystem ​if all the bacteria, fungi or microorganisms are ​removed ​from the ​environment ?
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   Without decomposers, dead matter and wastes would pile up and nutrients would not return to the soil, so plants and food chains would fail.

   The bacteria, fungi and other microorganisms are the decomposers of an ecosystem, so removing them would stop decomposition altogether. (i) The dead bodies of plants and animals and all their waste products would no longer be broken down and would go on accumulating in the environment. (ii) The simple inorganic nutrients locked inside that dead matter would never be released back into the soil and the air, so the cycling of nutrients would stop; the plants would be left without the raw materials they absorb, and with the producers failing the food chains and the energy flow of the whole ecosystem would break down.

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6. Q86 CBSE 2026
   Why ​do food ​chains ​in ​an ecosystem generally consist of only three or ​four ​steps ?
   Show answer

   Only about $10\%$ of energy passes to the next level, so after three or four steps too little energy remains for another level.

   Because energy is transferred between trophic levels according to the ten per cent law: only about $10\%$ of the energy available at a level reaches the level above it, while the remaining $90\%$ is spent in the organism's own life processes and lost as heat, or stays in parts that are not eaten or not digested. The energy therefore falls very steeply along the chain — of $10000$ J captured by the producers about $1000$ J reaches the herbivores, $100$ J the next level and $10$ J the one after — so after three or four steps too little energy is left to support a further trophic level.

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7. Q87 CBSE 2026
   1. (a)Explain the role ​of UV radiation in the formation ​of ​ozone layer.
   2. (b)​Why ​is UV ​radiation ​considered ​as highly damaging to organisms ?
   Show answer

   UV splits oxygen into atoms that combine with oxygen to make ozone; its high energy breaks bonds in DNA and proteins, damaging organisms.

   (a) Ultraviolet radiation supplies the energy that makes ozone. High-energy UV radiation from the Sun splits some molecules of oxygen high in the atmosphere into free oxygen atoms, ${\mathrm{O}}_2\stackrel{\text{UV}}{\to }\mathrm{O}+\mathrm{O}$, and each free atom then combines with another oxygen molecule to give ozone, $\mathrm{O}+{\mathrm{O}}_2\to {\mathrm{O}}_3$. Repeated throughout the upper atmosphere, these two steps build up the ozone layer. (b) Ultraviolet radiation is of much shorter wavelength and higher energy than visible light, so it carries enough energy to break chemical bonds. It is absorbed by the DNA and proteins of living cells and damages them, causing mutations; in human beings it is known to cause skin cancer, as well as cataract and a weakened immune system, and in plants it damages chlorophyll and lowers the rate of photosynthesis.

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8. Q88 CBSE 2026
   Suggest ​one best ​way to dispose of the following wastes ​:

   1. (i)​Kitchen waste
   2. (ii)Metallic waste
   3. (iii)Plastic bottles
   4. (iv)​Sanitary waste
   Show answer

   Compost kitchen waste, recycle or reuse metal and plastic bottles, and incinerate sanitary waste.

   (i) Kitchen waste: Composting. It is biodegradable, so decomposers in a compost pit turn it into manure. (ii) Metallic waste: Recycling (or reuse). Metals do not decompose, but they can be melted and made into new articles. (iii) Plastic bottles: Recycling (or reuse). Plastic is non-biodegradable, and recycling turns the bottles into new articles instead of leaving them to pollute soil and water. (iv) Sanitary waste: Incineration. It is wrapped, collected separately and burnt at a high temperature in an incinerator, which kills the disease-causing microbes.

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9. Q89 CBSE 2026
   Suggest ​two ways ​of dealing ​with ​waste ​generated at ​home.
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   Segregate waste and compost the biodegradable part; reduce, reuse and recycle the non-biodegradable waste.

   (1) Segregate the waste at home into biodegradable waste (peels, leftover food, tea leaves) and non-biodegradable waste (plastic, glass, metal) in separate bins, and turn the biodegradable waste into compost in a compost pit; the compost is used as manure. (2) Reduce, reuse and recycle the non-biodegradable waste: use cloth bags instead of polythene bags, reuse bottles and containers, and send plastic, glass and metal for recycling. Other correct ways, such as vermicomposting or giving dry waste to a kabadiwala, also earn credit.

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10. Q90 CBSE 2026
    Suggest one precaution to ​be taken ​before disposing of the ​following ​:

    1. (a)Sewage ​from homes
    2. (b)Industrial ​wastes
    Show answer

    Fit a sieve at the drain opening to keep solid waste out of sewage; give industrial wastes chemical treatment before disposal.

    (a) Sewage from homes: Sewage should not carry solid waste, so a sieve should be installed at the opening of the drain. Treating the sewage in a sewage treatment plant before it is discharged into a river is another valid precaution. (b) Industrial wastes: They should be given chemical treatment before disposal, to remove or neutralise their harmful chemicals (for example, neutralise acids and alkalis and remove toxic metals).

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11. Q91 CBSE 2026
    In the following ​food ​chain, 5 J of energy is available ​to man. Calculate, how much energy was ​available at the ​producer level. ​Why ​? Plant $⟶$ Goat $⟶$ Man
    Show answer

    The producer level had $500\text{J}$, because only about 10% of the energy passes to each next level.

    $500$ J of energy was available at the producer (plant) level. Why: by the ten per cent law, only about $10\%$ of the energy at one trophic level passes on to the next, the rest being used for life processes and lost as heat. So the goat level had $5\times 10=50$ J and the plant level $50\times 10=500$ J.

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12. Q92 CBSE 2026
    Describe why ​do food ​chains generally ​consist of ​a ​few ​steps.
    Show answer

    Only about $10\%$ of energy passes to each next level, so after three or four steps too little remains to support another level.

    Food chains are short because the transfer of energy from one trophic level to the next is very wasteful. Only about $10\%$ of the energy present at a level is stored in the bodies of its organisms and so passed on; the other $90\%$ is used up in respiration, movement and maintaining body temperature, or leaves as undigested and excreted matter, and is finally lost as heat. So if the producers capture $1000$ kJ, the herbivores get about $100$ kJ, the small carnivores about $10$ kJ and the large carnivores only about $1$ kJ. After three or four such transfers the energy left is too little to support a population at a further level, and since the flow of energy is unidirectional it can never be replaced. Food chains therefore generally consist of only three or four steps.

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13. Q93 CBSE 2026
    Given below is a pyramid showing various trophic ​levels in an ecosystem ​:

    1. (a)From the ​organisms listed below, identify which one is to ​be placed ​at which trophic level ? Deer, Grass, Lion, Snake, Rabbit
    2. (b)Discuss the reason why primary ​consumers will have more energy ​as compared to secondary consumers ?
    3. (c)Why is the base of the ​pyramid broad ?

    Show answer

    Grass is the producer, deer and rabbit are primary consumers, snake secondary, lion tertiary; each level passes on only about 10% of its energy.

    (a) (i) Producers: Grass; (ii) Primary consumers: Deer and Rabbit; (iii) Secondary consumers: Snake (the lion also feeds at this level when it eats deer); (iv) Tertiary consumers: Lion. (b) At each trophic level most of the food energy is used up in life processes and lost as heat; only about $10\%$ is passed on to the next level. Primary consumers get energy directly from producers, whereas secondary consumers get only about $10\%$ of the energy of the primary consumers, so primary consumers have more energy. (c) The base is formed by the producers, which trap solar energy directly and so hold the maximum energy (and, in a grassland, the largest number of organisms); because only about $10\%$ of the energy passes to each higher level, every level above receives less energy and supports less life, so the base is the broadest.

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14. Q94 CBSE 2026
    What role does ​UV radiation play in the formation of ​the ozone ​layer ​? Explain ​the process with relevant chemical reactions. Why is the excessive use of CFCs (Chlorofluoro Carbons) a ​cause ​of concern for organisms ​?
    Show answer

    UV splits ${\text{O}}_2$ into atoms that form ${\text{O}}_3$; CFCs release chlorine that destroys ozone, letting in UV that harms organisms.

    Role of UV radiation and the process : Ozone at the higher levels of the atmosphere is a product of ultraviolet radiation acting on oxygen. The high-energy ultraviolet radiation of the Sun splits molecules of oxygen into free oxygen atoms, and each free atom then combines with a molecule of oxygen to form ozone. The reactions are ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$ and $\text{O}+{\text{O}}_2\to {\text{O}}_3$. Ultraviolet radiation is thus the source of the energy that breaks the oxygen molecule, without which the ozone layer could not form; the ozone so formed then absorbs the harmful ultraviolet radiation and shields the surface of the Earth. Why CFCs are a cause of concern : Chlorofluorocarbons, used as refrigerants and in fire extinguishers, are very stable and are not destroyed in the lower atmosphere, so they drift up to the ozone layer. There ultraviolet radiation frees chlorine atoms from them, and each chlorine atom destroys many molecules of ozone, so the layer becomes thinner. More ultraviolet radiation then reaches the Earth, and it is highly damaging to organisms, causing skin cancer and cataract and weakening the immune system in human beings, damaging plants and lowering crop yields, and killing plankton on which aquatic food chains depend.

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15. Q95 CBSE 2026
    1. (a)Mention any one harmful effect of using plastic bags on the environment. Suggest ​better alternatives to ​the usage of plastic bags.
    2. (b)Paddy fields require a large ​amount of water. The pesticides and chemical fertilizers used are washed down into the soil or waterbodies. How ​do ​these ​chemicals reach our bodies ? ​What is this phenomenon ​known as ?
    Show answer

    Plastic bags persist for years, so use cloth, jute or paper bags; chemicals reach us through the food chain by biological magnification.

    (a) Harmful effect (any one): plastic bags are non-biodegradable, so they persist in the environment for years; they choke drains and sewers and cause water-logging, they stop water and air from entering the soil they cover, and animals that swallow them may die. Better alternatives: cloth or jute bags, which can be used again and again, and paper bags, which are biodegradable. (b) The chemicals are washed into the soil and water bodies; from the soil the paddy and other plants absorb them along with water and minerals, and in water bodies aquatic plants and animals take them up, so they enter the food chain. Being non-degradable they are not broken down or excreted, so they get accumulated at each trophic level, and human beings, who are at the top of the food chain, get the maximum concentration through food such as rice, vegetables and fish. This phenomenon is known as biological magnification.

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16. Q96 CBSE 2026
    1. (a)​Improper waste disposal is a curse to the environment. Explain.
    2. (b)Why are crop ​fields considered ​as artificial ​ecosystems ?
    3. (c)Farmers are using pesticides on the crop ​plants, but the chances ​of accumulation ​of harmful chemicals is ​greatest in human beings. Why ? Name the phenomenon.
    Show answer

    Waste pollutes and spreads disease; crop fields are made and maintained by people; pesticides build up by biological magnification, most in humans.

    (a) When waste is not segregated at source, it simply accumulates, and much of it is non-biodegradable, such as plastics, glass, metals, chemicals and pesticides. Decomposers cannot break these down, so they remain in the environment for years: they pollute the soil and water, choke drains and water bodies and kill animals that swallow them. Biodegradable waste left in the open rots, gives off foul smells and harmful gases and breeds flies, mosquitoes and micro-organisms that spread disease, while harmful chemicals seep into the soil and groundwater and enter food chains. Burning the waste instead releases poisonous smoke and gases into the air. (b) A crop field is a man-made ecosystem. It is created and maintained by human beings: people decide which crop is grown, plough the land, sow selected seeds, irrigate the field, add manure, fertilisers and pesticides and weed out all other plants, so very few species live there and the field could not sustain itself without this care, unlike a natural ecosystem such as a forest, a lake or a pond. It is still an ecosystem, because the living organisms in it interact with each other and with the abiotic components such as soil, water, air and sunlight. Being made and kept going by people, it is called an artificial ecosystem. (c) Pesticides are non-biodegradable. They are washed into the soil and water and are absorbed by the crop plants along with water and minerals, so they enter the food chain. No organism can break them down or excrete them, so the whole amount taken in with the food stays in the body, and since each consumer eats many organisms of the level below, the concentration increases at every trophic level. Human beings are at the top of the food chain and feed from several food chains, so the greatest concentration collects in their bodies. The phenomenon is called biological magnification (biomagnification).

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17. Q97 CBSE 2026
    Consider a ​simple aquatic ecosystem like a lake which consists of algae, small fish and large fishes. It was found that the lake ​water was contaminated with a low concentration of a non-biodegradable pesticide, DDT. Based on this ecosystem, answer the following questions :

    1. (a)In which organism will the concentration of the pesticide DDT be the highest ? ​Name and explain the phenomenon.
    2. (b)​Why is the flow ​of energy in ​a food ​chain always unidirectional ?
    Show answer

    Large fishes have the most DDT through biological magnification; energy flow is one-way as it enters only at producers and is lost as heat.

    (a) The concentration of DDT will be highest in the large fishes. The lake's food chain is algae, then small fish, then large fish, and the large fish stand at the highest trophic level. The phenomenon is biological magnification (biomagnification): DDT is non-biodegradable, so it is neither broken down nor excreted and stays stored in the body. The algae absorb it from the water and retain it; each small fish eats a great many algae and keeps the DDT of all of them; each large fish eats many small fish and keeps all of theirs. Since the amount of pesticide taken in grows far faster than body mass, its concentration increases at every successive trophic level and is greatest at the top. (b) The flow of energy in a food chain is unidirectional for two reasons. First, the energy enters the chain only once — sunlight is captured only by the green plants, the producers — and thereafter it passes from producer to herbivore to carnivore and never back; a carnivore returns no energy to the herbivore it ate. Second, at each transfer only about ten per cent of the energy passes on, while about ninety per cent is spent in the life processes of that organism and escapes into the surroundings as heat. No organism can take that heat back and convert it into food, so the energy leaves the chain permanently at every step and cannot be reused or returned.

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18. Q98 CBSE 2026
    (A) If ​no decomposers in an ecosystem, what will happen to the garbage and dead animal and ​plant wastes. (B) As a student, what are ​the four changes in your ​daily habits ​you would adopt to reduce ​generation of non-biodegradable wastes ?
    Show answer

    Without decomposers, dead matter piles up and nutrient cycles break down; use cloth bags, steel bottles, refills and segregation to cut non-biodegradable waste.

    (A) Nothing would be broken down. The garbage and the dead bodies of animals and plants would go on accumulating, so the land and the water would be choked with dead matter, which would also become a source of foul smell and of disease. More seriously, the nutrients locked inside that dead matter - nitrogen, phosphorus, carbon and the minerals - would never be released back into the soil, the water and the air. The nutrient cycles would break down, the producers would be starved of the raw materials they need to make food, and the whole ecosystem would in time collapse. (B) Four changes in daily habits: (i) Carry a cloth or jute bag while shopping and refuse plastic carry bags. (ii) Use a steel or glass water bottle and a steel tiffin box instead of disposable plastic bottles, cups, straws and cutlery. (iii) Buy goods in refill or larger packs rather than in many small plastic-wrapped ones, and choose things that can be repaired or refilled, such as a pen with a refill or a rechargeable cell, over things meant to be thrown away. (iv) Separate the waste at home so that plastic, glass, metal and electronic items are handed over for recycling instead of being mixed with the rest.

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19. Q99 CBSE 2025
    Which of the following groups do ​not constitute ​a food chain ?

    1. (i)​Wolf, ​rabbit, grass, lion
    2. (ii)Plankton, man, grasshopper, fish
    3. (iii)​Hawk, ​grass, ​snake, ​grasshopper, frog
    4. (iv)Grass, snake, wolf, tiger

    1. (a)(i) and (iv)
    2. (b)(i) and (iii)
    3. (c)(ii) and (iii)
    4. (d)(ii) and (iv)
    Show answer

    (d) (ii) and (iv)

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20. Q100 CBSE 2025
    The ​organism ​occupying ​the ​fourth trophic ​level in ​a food chain operating in ​grassland is :

    1. (a)Green plants
    2. (b)Frog
    3. (c)Grasshopper
    4. (d)Snake
    Show answer

    (d) Snake

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