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 1 of 8

1. Q1 NCERT · Exercises
   Which ​of ​the ​following ​groups ​contain ​only ​biodegradable items?

   1. (a)Grass, flowers and leather
   2. (b)Grass, wood and plastic
   3. (c)Fruit-peels, cake and lime-juice
   4. (d)Cake, wood and grass
   Show answer

   (a) Grass, flowers and leather

   (c) Fruit-peels, cake and lime-juice

   (d) Cake, wood and grass

   Step-by-step solution →
2. Q2 NCERT · Intext Questions (Page 212)
   What are trophic ​levels? Give ​an example ​of ​a food chain and ​state the different trophic levels ​in it.
   Show answer

   A trophic level is a step of a food chain; in Grass → Grasshopper → Frog → Snake the four levels are producer, herbivore and two carnivores.

   Each step or level of a food chain at which food, and the energy in it, is transferred from one organism to the next is called a trophic level. The organisms of a food chain are placed at these levels according to how they get their food: the autotrophs or producers are at the first trophic level, the herbivores or primary consumers at the second, the small carnivores or secondary consumers at the third, and the larger carnivores or tertiary consumers at the fourth. Example of a food chain in a grassland: Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake. Its trophic levels are: first trophic level, grass, the producer, which fixes solar energy by photosynthesis; second trophic level, grasshopper, a herbivore and primary consumer; third trophic level, frog, a small carnivore and secondary consumer; fourth trophic level, snake, a larger carnivore and tertiary consumer. Only about ten per cent of the energy at each level reaches the next, so a chain of this kind rarely runs beyond three or four trophic levels.

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3. Q3 NCERT · Exercises
   Which of the ​following ​constitute ​a ​food-chain?

   1. (a)Grass, wheat and mango
   2. (b)Grass, goat and human
   3. (c)Goat, cow and elephant
   4. (d)Grass, fish and goat
   Show answer

   (b) Grass, goat and human

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4. Q4 NCERT · Intext Questions (Page 212)
   What ​is the role ​of ​decomposers ​in the ecosystem?
   Show answer

   Decomposers break dead remains and waste into simple inorganic substances that return to the soil, so nutrients are recycled and the environment stays clean.

   Decomposers are microorganisms, mainly bacteria and fungi, that live on the dead remains and waste products of other organisms, and they have three linked roles in an ecosystem. First, they break down the complex organic substances of dead plants, dead animals and excreta into simple inorganic substances, a job no producer or consumer performs. Second, these simple substances go back into the soil, water and air, where the producers absorb them once more through their roots and build them into new organic matter, so the nutrients of the ecosystem are recycled and the fertility of the soil is naturally replenished; energy flows through an ecosystem in one direction and is lost as heat, but matter is used over and over, and it is the decomposers who close that loop. Third, by clearing away dead bodies, dead plant parts and waste, they keep the environment clean and leave room for new growth. If they were absent, dead matter would pile up and the nutrients locked inside it would never return to the soil, so the producers would run short of raw material and the whole ecosystem would run down.

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5. Q5 NCERT · Exercises
   Which ​of the ​following ​are ​environment-friendly ​practices?

   1. (a)Carrying cloth-bags to put purchases in while shopping
   2. (b)Switching off unnecessary lights and fans
   3. (c)Walking to school instead of getting your mother to drop you on her scooter
   4. (d)All of the above
   Show answer

   (d) All of the above

   Step-by-step solution →
6. Q6 NCERT · Exercises
   What ​will happen if ​we kill all ​the ​organisms in ​one trophic level?
   Show answer

   The food chain breaks: the level above starves, the level below multiplies unchecked, and the ecosystem loses its balance.

   The food chain is broken and the balance of the ecosystem is destroyed, and the damage travels in both directions from the gap. Above the gap: the organisms of the level immediately above ate nothing but the organisms that have been killed, so they lose their food supply, starve and die; as they die, the level above them loses its food in turn, and the shortage passes on up to the top of the chain, because energy can reach those levels by no other route. Below the gap: the organisms of the level immediately below were being kept in check by being eaten, so with that check removed their numbers grow very rapidly; they overrun the ecosystem and consume the level beneath them faster than it can recover, until they have exhausted their own food and starve as well. The gap cannot be filled, because an organism belongs to the trophic level that its food habits give it and those habits cannot be changed, so the organisms of a trophic level cannot be replaced. If the level removed is that of the producers the effect is worst of all, since energy then has no way of entering the ecosystem at all and every level above dies out.

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7. Q7 NCERT · Exercises
   Will the ​impact of removing all ​the organisms in a trophic level ​be ​different for different ​trophic levels? Can the organisms of any trophic level be removed without causing ​any ​damage to the ecosystem?
   Show answer

   Yes, the impact differs, being most severe for producers; no, no trophic level can be removed without damaging the ecosystem.

   Yes to the first question and No to the second. The impact is different at different trophic levels, because each level occupies a different position in the chain. If the producers are removed, energy has no way of entering the ecosystem at all — no food is made, every level above starves in turn, and the whole ecosystem collapses; this is the most severe case. If a middle level such as the herbivores is removed, the damage runs in both directions: the carnivores above lose their only food and starve, while the plants below are no longer grazed and grow unchecked until they exhaust the space, water and minerals available. If the top carnivores are removed, nothing starves for want of them, but the level below is released from the check that being eaten placed on it, multiplies without limit, eats out the level beneath it and then collapses from starvation. The general rule is that the lower the level removed, the more levels are cut off from their supply of energy and the more immediate and complete the damage; this is also why so few organisms occupy the higher levels, since only about ten per cent of the energy passes from one level to the next. But no trophic level can be removed without damaging the ecosystem. Every level does two things — it feeds the level above it and it keeps the numbers of the level below it in check — so removing any level destroys at least one of them, and because an organism belongs to the trophic level that its food habits give it, nothing can move in to fill the gap. What differs from level to level is the severity, the speed and the direction of the damage, never whether damage is caused at all.

   Step-by-step solution →
8. Q8 NCERT · Exercises
   What ​is biological ​magnification? Will ​the ​levels of this magnification be different ​at different levels of ​the ​ecosystem?
   Show answer

   Biological magnification is the progressive rise in concentration of harmful chemicals at each trophic level; yes, it differs, being greatest at the top.

   Biological magnification is the progressive increase in the concentration of a harmful chemical in the bodies of organisms at each successive trophic level of a food chain. The chemicals responsible are non-biodegradable ones such as the pesticides used to protect crops. Sprayed on the fields, they are washed into the soil and into water, and from there they are taken up by plants and by aquatic plankton, so they enter the food chain at the first trophic level. Because they cannot be broken down and are not excreted, they are stored in the body, largely in the fatty tissues; every animal therefore keeps all of the chemical it has ever taken in and hands it on complete to whatever eats it. A consumer eats a very large number of organisms of the level below, yet only about a tenth of what it eats becomes its own body, so the chemical collected from a great mass of food ends up in a far smaller mass of tissue and its concentration rises. Yes, the levels of magnification are different at different levels of the ecosystem: the concentration is least in the producers, greater in the herbivores, greater still in the small carnivores and greatest of all at the top of the chain. Since human beings occupy the top of food chains, the maximum concentration of these chemicals accumulates in our own bodies.

   Step-by-step solution →
9. Q9 NCERT · Exercises
   What are ​the problems ​caused by ​the ​non-biodegradable ​wastes that we ​generate?
   Show answer

   They are not decomposed, so they accumulate, harming land, animals and food chains; pesticides build up in humans and burning causes air pollution.

   Non-biodegradable wastes such as plastics, polythene, glass, metals and pesticides are not broken down by decomposers, because no organism has the enzymes to act on them. Everything we throw away therefore stays in the environment, in some cases for hundreds of years, and the quantity present only grows. On land the waste piles up: rubbish heaps and landfill sites take up land and make the surroundings dirty and unhygienic, buried plastic makes the soil impermeable so that rainwater cannot seep down to recharge the groundwater, and litter chokes drains and sewers, where the stagnant water that collects breeds mosquitoes and spreads disease. Animals are killed by it: stray cattle swallow polythene bags that then lodge undigested in the stomach, and turtles and fish in the sea are killed by floating plastic. In food chains the harm is chemical: pesticides washed into the soil and water are taken up by plants and plankton, and because they cannot be broken down or excreted their concentration rises at each successive trophic level, so that the maximum concentration collects in us at the top of the chain; these chemicals also kill useful insects and soil organisms that were never their target. The materials locked into such waste are lost to the cycles of nature, since nothing returns them to the soil. And burning the waste is no remedy, because plastics give off poisonous gases when burnt and land pollution is merely exchanged for air pollution. The only real answers are to generate less of it, and to reuse and recycle what is generated.

   Step-by-step solution →
10. Q10 NCERT · Exercises
    If all the ​waste we generate ​is biodegradable, ​will ​this ​have no ​impact on ​the environment?
    Show answer

    No: biodegradable waste still harms the environment, causing disease, oxygen loss in water and methane, because decomposition is slow.

    No. Biodegradable waste will still have an impact on the environment, because 'biodegradable' means only that decomposers are able to break the waste down in the end; it does not mean that the waste is harmless while they are doing so, and it says nothing about how fast they can work. Decomposition is slow and needs a large enough population of decomposers together with warmth, moisture and air, so if waste is generated faster than it can be broken down the excess accumulates. Heaps of rotting waste give off foul-smelling gases, make the surroundings dirty and unhygienic and occupy land; they breed flies, mosquitoes and rats and harbour disease-causing microorganisms, so diseases such as cholera, dysentery, typhoid, malaria and dengue spread. The most serious effect is in water: decomposers respire while they work, and in a pond or river they take the oxygen they need from the water itself, so a large quantity of organic waste supports so many decomposers that the dissolved oxygen is used up and the fish and other aquatic animals suffocate and die, although nothing poisonous was ever added. Decomposition also releases nutrients such as nitrates and phosphates, which make algae and water plants grow so thickly that the water body is choked, and that growth in turn has to be decomposed and takes still more oxygen; and waste buried deep in a dump, where there is no air, decomposes anaerobically and gives off methane, which is inflammable and is a greenhouse gas. The impact is different in kind from that of non-biodegradable waste, which persists indefinitely and is magnified along food chains, but it is a real impact, and it grows with the quantity we generate. The remedy is to generate less waste, to compost what we do generate, and to keep it out of our water bodies.

    Step-by-step solution →
11. Q11 NCERT · Exercises
    Why is damage to ​the ​ozone ​layer a cause ​for concern? ​What steps are being taken to ​limit this damage?
    Show answer

    Ozone damage lets harmful ultraviolet radiation reach the earth; CFC production was frozen at 1986 levels in 1987 and CFC-free refrigerators are mandatory.

    Ozone is a molecule of three atoms of oxygen, and high up in the atmosphere it forms a layer. It is made there when ultraviolet radiation from the Sun splits an oxygen molecule into two free atoms, each of which then joins another oxygen molecule. This layer absorbs the ultraviolet radiation coming from the Sun and prevents it from reaching the surface of the earth, and it is the only shield we have against it. Damage to the layer is a cause for concern because ultraviolet radiation is highly damaging to organisms: a thinner layer lets more of it through, and in human beings that means skin cancer, damage to the eyes leading to cataract and a weakening of the immune system, while plants are damaged and crop yields fall, and the plankton of the surface waters of the sea are killed, so the harm is passed up every marine food chain. The amount of ozone began to drop sharply in the 1980s, and the cause was traced to synthetic chemicals, chiefly the chlorofluorocarbons or CFCs, used as refrigerants in refrigerators and air-conditioners and in fire extinguishers; released at the ground they reach the layer and destroy the ozone there, and because one molecule destroys many, a small quantity does damage out of all proportion to its amount. As to the steps being taken: in 1987 the United Nations Environment Programme (UNEP) succeeded in forging an agreement to freeze CFC production at its 1986 levels, and it is now mandatory for all manufacturing companies throughout the world to make CFC-free refrigerators. CFCs have been replaced in refrigerators, air-conditioners, aerosol sprays and fire extinguishers by chemicals that do not attack ozone, products are labelled CFC-free, and old appliances are disposed of in ways that recover the gas instead of releasing it.

    Step-by-step solution →
12. Q12 NCERT · Intext Questions (Page 214)
    Why are some ​substances ​biodegradable ​and ​some ​non-biodegradable?
    Show answer

    Decomposer enzymes are specific: natural substances have matching enzymes and are biodegradable, whereas man-made materials such as plastics have none and are non-biodegradable.

    The difference lies in the decomposers and in the specificity of their enzymes. Bacteria, fungi and other saprophytes break substances down using enzymes, and enzymes are specific in their action: a particular enzyme can act only on a particular substance, which is why the same enzyme does not break down everything we eat and why we get no energy if we try to eat coal. Substances of plant and animal origin, such as leaves, wood, paper, cotton, leather, food waste and cow-dung, have been part of nature for as long as decomposers have existed, so those decomposers already possess enzymes that fit them; these substances are broken down by biological processes and are therefore biodegradable. Human-made materials such as plastics, polythene, synthetic fibres and pesticides like DDT are new to nature, so no organism has ever needed to digest them and no enzyme that can attack them exists. They are not broken down by bacteria or other saprophytes; they will be acted upon by physical processes like heat and pressure, but under the ambient conditions found in our environment they persist for a long time, and this makes them non-biodegradable. The test is therefore biological, not physical: what decides the group is whether a matching enzyme exists, and not how hard, heavy or durable the material is.

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13. Q13 NCERT · Intext Questions (Page 214)
    Give any ​two ​ways ​in ​which ​biodegradable ​substances would ​affect the environment.
    Show answer

    Rotting waste on land smells and spreads disease, and in water it uses up dissolved oxygen so aquatic animals die.

    Biodegradable substances are broken down by decomposers in the end, but they still affect the environment while that is happening, because we generate them much faster than the decomposers can work. Two ways are these. (i) On land, the accumulated waste rots and gives off foul-smelling gases, so the surroundings become dirty and unhygienic; the heaps breed flies, mosquitoes, cockroaches and rats and let disease-causing microorganisms multiply, so diseases such as cholera, dysentery, typhoid, malaria and dengue spread from them, and the heaps occupy land as well. (ii) In water, when such waste is thrown into a pond, river or lake it becomes food for the decomposers there; their numbers rise, they respire using the oxygen dissolved in the water, and the dissolved oxygen falls so low that fish and other aquatic animals suffocate and die although nothing poisonous was ever added; the nutrients released also make algae grow thickly and choke the water body. Two further effects may be added: waste buried without air gives off methane, a greenhouse gas, and, on the useful side, biodegradable waste broken down completely enriches the soil, which is why it can be composted into good manure.

    Step-by-step solution →
14. Q14 NCERT · Intext Questions (Page 214)
    Give any two ways ​in ​which non-biodegradable substances would ​affect the ​environment.
    Show answer

    They accumulate because no decomposer breaks them down, and they are magnified along food chains, reaching the highest concentration in humans.

    No decomposer has an enzyme that can act on non-biodegradable substances such as plastics, polythene, glass, metals and pesticides, so nothing removes them and everything thrown away stays in the environment, in some cases for hundreds of years. Two ways in which they affect the environment are these. (i) They accumulate: heaps of plastic and glass occupy land and make the surroundings dirty, polythene in the soil makes it impermeable so that rainwater cannot seep down to recharge the groundwater and the soil grows less fertile, discarded bags choke drains and sewers where the stagnant water that collects breeds mosquitoes, and animals are killed by the litter, stray cattle swallowing polythene bags that lodge undigested in the stomach and sea turtles and fish being killed by floating plastic. (ii) They enter food chains and are biologically magnified: pesticides washed into the soil and water are taken up by plants and by plankton at the first trophic level, and because they can neither be broken down nor excreted their concentration rises at each successive trophic level, so that the maximum concentration collects in human beings at the top of the chain, which is why our food grains, vegetables, fruits and even meat carry pesticide residues that washing cannot always remove. In addition they kill useful insects, earthworms and soil microorganisms, and burning them releases poisonous gases into the air.

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15. Q15 NCERT · Intext Questions (Page 216)
    What is ozone and ​how ​does ​it affect any ​ecosystem?
    Show answer

    Ozone is ${\text{O}}_3$; its layer shields ecosystems from ultraviolet radiation, but CFC thinning harms humans, plants and plankton, passing the damage up food chains.

    Ozone is a molecule formed by three atoms of oxygen, written ${\mathrm{O}}_3$. It is not the oxygen we breathe, which is ${\mathrm{O}}_2$ and is essential for all aerobic forms of life. At the higher levels of the atmosphere ozone is a product of ultraviolet radiation acting on oxygen: the higher energy ultraviolet radiations split molecular oxygen into free oxygen atoms, ${\mathrm{O}}_2\to \mathrm{O}+\mathrm{O}$, and these atoms then combine with molecular oxygen to give ozone, $\mathrm{O}+{\mathrm{O}}_2\to {\mathrm{O}}_3$. Its effect on an ecosystem depends entirely on where it is. Near the ground ozone is a deadly poison and harms living organisms directly. High in the atmosphere it performs an essential function: it forms a layer that shields the surface of the earth from the ultraviolet radiation of the Sun, which is highly damaging to organisms and is known to cause skin cancer in human beings, so this layer protects every ecosystem on the earth. Since the 1980s the amount of ozone has been dropping sharply because of synthetic chemicals such as chlorofluorocarbons, the CFCs used as refrigerants and in fire extinguishers. A thinner layer lets more ultraviolet radiation reach the ground, where it causes skin cancer, cataract and weakened immunity in human beings and other animals, reduces the growth and yield of green plants and kills the plankton of the sea; and since green plants and plankton are the producers at the first trophic level, the damage passes up to every level above them. In 1987 the United Nations Environment Programme forged an agreement to freeze CFC production at 1986 levels, and CFC-free refrigerators are now mandatory the world over.

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16. Q16 NCERT · Intext Questions (Page 216)
    How ​can you ​help in reducing the ​problem ​of waste disposal? ​Give any two ​methods.
    Show answer

    Segregate waste and compost the biodegradable part; reduce and reuse so that less waste is created.

    Any two genuine methods are acceptable; the two strongest are these. (i) Segregate the waste at home and compost the biodegradable part. Keep two bins and put the waste into the right one as it is thrown, kitchen waste, peels, leftover food, dead leaves and paper in one, and plastic, polythene, glass and metal in the other. Put the biodegradable waste into a compost pit or bin, keep it moist, and the decomposers turn it into manure in a few weeks, so that this waste never reaches a dump and its nutrients return to the soil; the other bin, kept clean and dry, can be given to a recycler, because mixed waste can be neither composted nor recycled. (ii) Reduce and reuse, so that less waste is created at all. Carry a cloth or jute bag for shopping instead of taking a plastic bag, refuse disposable cups, plates, spoons and straws and use steel or china that can be washed, use both sides of every sheet of paper, refill and reuse bottles and jars, and repair or pass on clothes, books and toys instead of throwing them away. Waste that is never generated needs no disposal at all. Other acceptable methods are giving newspapers, bottles, tins and old metal to a recycler, returning batteries and old electronic items to a proper collection point instead of the dustbin, never littering, and persuading others at home and at school to do the same.

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17. Q17 Exemplar · Multiple Choice Questions
    Which one ​of the ​following ​is an ​artificial ecosystem?

    1. (a)Pond
    2. (b)Crop field
    3. (c)Lake
    4. (d)Forest
    Show answer

    (b) Crop field

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18. Q18 Exemplar · Multiple Choice Questions
    In a food ​chain, ​the ​third ​trophic level ​is always ​occupied by

    1. (a)carnivores
    2. (b)herbivores
    3. (c)decomposers
    4. (d)producers
    Show answer

    (a) carnivores

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19. Q19 Exemplar · Multiple Choice Questions
    An ecosystem includes

    1. (a)all living organisms
    2. (b)non-living objects
    3. (c)both living organisms and non-living objects
    4. (d)sometimes living organisms and sometimes non-living objects
    Show answer

    (c) both living organisms and non-living objects

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20. Q20 Exemplar · Multiple Choice Questions
    In ​the ​given food chain, ​suppose the amount of ​energy at fourth trophic level is 5 kJ, what will be the energy ​available at ​the producer level? Grass → ​Grasshopper → Frog → Snake → Hawk

    1. (a)$5$ kJ
    2. (b)$50$ kJ
    3. (c)$500$ kJ
    4. (d)$5000$ kJ
    Show answer

    (d) $5000$ kJ

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