Class 10 Science · Chapter 13

Our Environment

298 Class 10 Science questions on Our Environment from the NCERT textbook, NCERT Exemplar and CBSE board papers 2017–2026, each with its answer and a step-by-step solution.

CBSE Class 10 Science previous year papers, with answers →

Questions · page 10 of 15

1. Q181 CBSE 2023
   What is ​the difference between biodegradable and non-biodegradable substances ? ​List ​two methods of ​safe ​disposal of ​biodegradable domestic ​waste.
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   Biodegradable substances are broken down by micro-organisms, non-biodegradable ones are not; safe disposal methods are composting and vermicomposting.

   Difference: biodegradable substances are broken down by biological processes, that is by micro-organisms such as bacteria and fungi, into simpler, harmless substances (for example vegetable peels, paper, cotton cloth). Non-biodegradable substances are not broken down by biological processes, so they persist in the environment for a long time and may harm the ecosystem (for example plastics, glass, DDT). Two methods of safe disposal of biodegradable domestic waste: (i) Composting: the kitchen and garden waste is put in a pit and covered with soil, and micro-organisms decompose it into manure (compost). (ii) Vermicomposting: the waste is fed to earthworms in a pit or bin, which turn it into vermicompost, a rich manure. (Producing biogas from the waste in a biogas plant is also a safe method.)

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2. Q182 CBSE 2023
   How do ​harmful chemicals get ​accumulated progressively ​at each trophic level in ​a food chain ​?
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   Non-biodegradable chemicals are never excreted, so each level stores what it eats; the concentration rises up the food chain (biological magnification).

   This is called biological magnification. Chemicals such as pesticides that are sprayed on crops are washed into the soil and into water, and are absorbed by plants along with water and minerals, so they enter the food chain at the producer level. These chemicals are non-biodegradable - the body can neither break them down nor excrete them - so all of what is eaten stays stored in the tissues. Since each organism at a higher trophic level eats a very large number of organisms of the level below, and only about ten per cent of that food becomes its own body, the whole of the chemical from all that food is packed into a much smaller mass of flesh. The concentration therefore increases progressively at each successive trophic level and is highest in the organisms at the top of the food chain, which often includes human beings.

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3. Q183 CBSE 2023
   We ​do ​not clean ​ponds or lakes but ​an ​aquarium ​needs to be cleaned regularly. Why ?
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   A pond is complete and self-sustaining, its decomposers clearing waste; an aquarium is artificial and incomplete, so waste collects and it needs cleaning.

   A pond or lake is a complete, self sustaining **natural** ecosystem. It has its own producers (algae and water plants), consumers of every level, and above all plenty of decomposers such as bacteria and fungi. These decomposers break down the dead remains and the waste on the spot and return the simple substances to the water for the producers to use again, so waste does not pile up and the pond cleans itself. An aquarium is an **artificial** and incomplete ecosystem: it has very few producers and far too few decomposers for the waste made in it, while the food for the fish is added by us from outside. Uneaten food, fish excreta and dead matter therefore collect in the tank and would foul the water, so we must clean it regularly ourselves.

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4. Q184 CBSE 2023
   Use of ​several ​pesticides which results ​in excessive accumulation ​of pesticides in rivers or ponds, is a matter of deep concern. ​Justify ​this statement.
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   Pesticides are non-biodegradable, so they build up along the food chain by biological magnification and reach the highest concentration in humans.

   Pesticides are non-biodegradable, so micro-organisms cannot break them down and they remain in a river or pond for a very long time. From the water they are absorbed by aquatic plants, which are eaten by small fish, which are eaten by larger fish, which are eaten by human beings. At every one of these steps the consumer eats many individuals of the level below but is unable to break the pesticide down or excrete it, so the whole of it stays stored in its body. The concentration of the pesticide therefore increases at each successive trophic level, a process called biological magnification. Because human beings are at the top of most food chains, the maximum concentration finally collects in our own bodies, where it causes serious health damage. The polluted water also becomes unfit for drinking and kills aquatic organisms. This is why the accumulation of pesticides in water bodies is a matter of deep concern.

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5. Q185 CBSE 2023
   In the following food chain, if 50 J of energy was available to the hawk, how much ​energy would ​have been ​present at the first and third ​tropic levels ? Justify your answer. Grass → Grasshopper → ​Frog ​→ ​Snake → Hawk
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   First trophic level (grass): $5\times 10^5$ J; third trophic level (frog): $5\times 10^3$ J.

   First trophic level (grass): $5\times 10^5$ J, that is $500000$ J. Third trophic level (frog): $5\times 10^3$ J, that is $5000$ J. Justification: by the ten per cent law only about $10\%$ of the energy at one trophic level reaches the next, the rest being lost as heat and in respiration and undigested matter. The hawk is the fifth level, so going down to the third multiplies the energy by $10$ twice and going down to the first multiplies it by $10$ four times.

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6. Q186 CBSE 2023
   Construct ​a food chain comprising the following ​organisms ​: Hawk, Frogs, Snakes, Plants Which ​of these will have the ​highest ​concentration of ​the harmful chemicals such as pesticides? Name ​the phenomenon.
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   The chain is $\text{Plants}\to \text{Frogs}\to \text{Snakes}\to \text{Hawk}$; the hawk has the most pesticide, by biological magnification.

   The food chain is Plants $\to$ Frogs $\to$ Snakes $\to$ Hawk. The hawk, being at the last trophic level, will have the highest concentration of the harmful chemicals such as pesticides. The phenomenon is called biological magnification (biomagnification).

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7. Q187 CBSE 2023
   Name two non-biodegradable ​substances. In the ​following food chain, maximum concentration of harmful non-biodegradable ​chemicals ​was ​found ​in the bodies ​of hawks. Why ? Grass $⟶$ Rabbits $⟶$ Snakes $⟶$ Hawks
   Show answer

   Plastics and DDT are non-biodegradable; their concentration rises at each trophic level, so hawks, at the top, hold the maximum.

   Two non-biodegradable substances: plastics (polythene) and DDT (a pesticide). Glass and aluminium cans are also acceptable. Why the hawks have the maximum concentration: these chemicals are not broken down by decomposers, and once inside a body they are neither broken down nor excreted, so they are stored in the tissues. At each step of the food chain a consumer eats a large mass of the level below it and retains all of the chemical contained in it, so the chemical of a large mass of prey is concentrated into a much smaller mass of predator. The concentration therefore increases at every trophic level - a process called biological magnification. The hawks are the top consumers, at the fourth and highest trophic level of this chain, so the concentration in their bodies is the maximum.

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8. Q188 CBSE 2023
   1. (a)State ​any one advantage ​of ​using cloth bags ​over plastic bags.
   2. (b)​List any ​two methods by ​which the solid wastes ​generated in urban areas can be safely disposed off.
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   Cloth bags are biodegradable and reusable; urban solid waste can be safely handled by composting and sanitary landfill.

   (a) A cloth bag is biodegradable, so after it is thrown away micro-organisms break it down and it does not accumulate as waste, whereas a plastic bag is non-biodegradable and stays in the soil for years, chokes drains and can kill animals that swallow it. A cloth bag can also be used again and again, so far fewer bags are discarded. (b) Two safe methods are: (i) Composting or vermicomposting, in which the biodegradable part of the waste, such as kitchen and garden waste, is allowed to decompose in a pit and is converted into manure; (ii) Sanitary landfill, in which the non-biodegradable waste is dumped in a low-lying lined area and covered with a layer of soil. (Recycling of plastic, glass and metal, and incineration of combustible or hospital waste in a proper high-temperature incinerator, are also acceptable.)

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9. Q189 CBSE 2023
   1. (a)What is meant by trophic level ?
   2. (b)On ​an average, ​what percentage of energy is ​transferred to the second trophic ​level from the first trophic level ? Which ​categories of consumers do the ​organisms ​of the ​second trophic level belong to ?
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   Each step of a food chain is a trophic level; about 10% of energy passes to the second, whose organisms are herbivores (primary consumers).

   (a) A trophic level is each step or level of a food chain at which the transfer of energy (food) takes place; the producers form the first trophic level, the herbivores the second and the carnivores the third and higher levels. (b) On an average about 10% of the energy of the first trophic level is transferred to the second. The organisms of the second trophic level are herbivores, that is, primary consumers.

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10. Q190 CBSE 2023
    “Ozone, although ​a ​deadly ​poison, ​performs an essential ​function for ​the earth and ​its inhabitants.” Explain.
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    Ozone is poisonous if breathed, but the ozone layer absorbs most of the Sun's UV radiation, so it protects the Earth's inhabitants.

    Ozone (${\text{O}}_3$) is a molecule of three oxygen atoms. Unlike oxygen (${\text{O}}_2$), which aerobic life needs, ozone is a deadly poison if it is breathed in, and near the ground it is a harmful pollutant. But at the higher levels of the atmosphere ozone forms a layer that absorbs most of the ultraviolet (UV) radiation from the Sun and so shields the surface of the earth from it. UV radiation is highly damaging to organisms — in human beings it is known to cause skin cancer — so this ozone protects the earth and its inhabitants. That is its essential function. It is formed there by the action of UV on oxygen: ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$, then $\text{O}+{\text{O}}_2\to {\text{O}}_3$.

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11. Q191 CBSE 2023
    Explain how some harmful chemicals enter our ​bodies through ​the ​food chain. Why ​is ​the concentration of these harmful chemicals found to be maximum ​in human beings ​?
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    Harmful chemicals enter through plants and build up at each trophic level, so human beings at the top have the maximum concentration.

    Pesticides and insecticides sprayed on crops, and industrial wastes let into rivers and lakes, are washed off into the soil and the water. Plants and algae absorb them along with water and minerals, so the chemicals enter the food chain at the producer level. Being non-biodegradable, they are neither broken down nor excreted, and are stored in the tissues of the organism that takes them in. So when a herbivore eats many such plants, all of their stored chemical collects in its body, and when a carnivore eats many such herbivores it collects all of theirs, and so on. The concentration therefore increases at every successive trophic level, a process called biological magnification, and the chemicals reach our bodies in the food we eat. Human beings are at the **top** of most food chains, so the load accumulated at every level below is concentrated in us, and the concentration of these harmful chemicals is found to be maximum in human beings.

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12. Q192 CBSE 2023
    Write ​one ​difference between biodegradable and non-biodegradable wastes. List two impacts of ​each type ​of the ​accumulated waste ​on environment if not disposed ​off properly.
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    Biodegradable waste is broken down by micro-organisms but non-biodegradable waste persists; each pollutes the environment when accumulated.

    Difference: biodegradable wastes are broken down into simpler, harmless substances by the action of micro-organisms such as bacteria and fungi, whereas non-biodegradable wastes cannot be broken down by micro-organisms and remain in the environment unchanged for a very long time. Impacts of accumulated **biodegradable** waste: (i) it rots, giving off a foul smell and creating unhygienic conditions in which flies, mosquitoes and germs breed and spread diseases; (ii) its decay releases gases such as methane, and the liquid seeping from it pollutes the soil and the ground water, while in a pond or river the decomposition uses up the dissolved oxygen and kills aquatic life. Impacts of accumulated **non-biodegradable** waste: (i) it piles up for years, so that plastics choke drains, cover the soil and lower its fertility, and are swallowed by cattle and marine animals, often killing them; (ii) substances such as pesticides and heavy metals enter food chains and, being neither broken down nor excreted, undergo biological magnification, so their concentration rises at each trophic level and is highest in human beings.

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13. Q193 CBSE 2023
    How is ozone ​formed in the higher levels of the atmosphere ​? "Damage to ​the ​ozone layer is a cause of ​concern." ​Justify this statement.
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    Ultraviolet rays split ${\text{O}}_2$ into atoms that join ${\text{O}}_2$ to form ozone; its damage lets harmful ultraviolet radiation reach Earth.

    Formation: at the higher levels of the atmosphere the ultraviolet radiation of the Sun splits molecular oxygen into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$, and these highly reactive atoms then combine with oxygen molecules to form ozone, $\text{O}+{\text{O}}_2⟶{\text{O}}_3$. Justification: this ozone forms a layer that absorbs the Sun's harmful ultraviolet radiation and shields life on Earth from it. Ultraviolet radiation causes skin cancer and cataract in human beings, damages the immune system, and harms crops and the plankton at the base of food chains. Since about 1980 the layer has been thinning, chiefly because of synthetic chemicals such as CFCs used as refrigerants and in fire extinguishers, so more ultraviolet radiation reaches the surface. That loss of protection is why damage to the ozone layer is a cause of concern, and why an agreement was made in 1987 to freeze CFC production.

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14. Q194 CBSE 2023
    (a) Write the percentage of

    1. (i)​solar energy captured by the autotrophs ​and
    2. (ii)​energy transferred from ​autotrophs to ​the next level in ​a ​food chain. (b) What are trophic levels? Why do different food chains in ​an ecosystem not have more than four to five trophic levels? Give reason.
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    Autotrophs capture about 1% of sunlight and pass on about 10%; the energy lost at each level limits chains to four or five levels.

    (a) (i) About 1% of the energy of the sunlight falling on the leaves is captured by the autotrophs. (ii) About 10% of the food energy of the autotrophs is transferred to the next trophic level. (b) Trophic levels are the successive steps of a food chain at which the transfer of food energy takes place: the producers form the first level, the herbivores the second, the small carnivores the third and the large carnivores the fourth. Because only about 10% of the energy at any level is passed on, and about 90% is lost as heat and used up in the life processes of that level, the energy left after four or five steps is too small to support a further level. That is why food chains generally have only four to five trophic levels.

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15. Q195 CBSE 2023
    Define the ​term ecosystem. List ​its two main components. We ​do ​not clean natural ponds ​or ​lakes, but an aquarium needs to ​be cleaned regularly. Why?
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    An ecosystem is an area's interacting organisms plus its non-living components; a pond recycles its wastes through decomposers, an aquarium does not.

    An ecosystem is a self-contained unit consisting of all the interacting organisms of an area together with the non-living physical factors of their environment. Its two main components are (i) the biotic components, the living organisms, namely producers, consumers and decomposers, and (ii) the abiotic components, the non-living physical factors such as air, water, soil, sunlight, temperature and minerals. A natural pond or lake is a complete, self-sustaining natural ecosystem: it contains decomposers that break down the dead remains and wastes of its organisms and return the nutrients to the water, the food chains are complete, and the system maintains its own balance, so it need not be cleaned. An aquarium is an artificial and incomplete ecosystem; it lacks the full complement of decomposers and complete food chains, and food is supplied to it from outside, so the fish wastes and leftover food are not recycled but accumulate and foul the water. It must therefore be cleaned regularly.

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16. Q196 CBSE 2023
    Define ​ecosystem. List two artificial ​(man-made) ecosystems. Natural ecosystems like forests and ​lakes ​do not ​require regular cleaning, whereas the artificial ecosystems start giving foul smell if ​not cleaned regularly. ​Why ​?
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    An ecosystem is an area's interacting organisms plus its non-living components; artificial ones smell unless cleaned because their wastes pile up.

    Ecosystem: all the interacting organisms of an area, together with the non-living (abiotic) components of their environment, form an ecosystem. Two artificial (man-made) ecosystems: a garden and a crop field (an aquarium, a park or a dam are equally acceptable). Why artificial ecosystems have to be cleaned: a natural ecosystem such as a forest or a lake is complete and self-sustaining. It has producers, consumers of every level and a full population of decomposers in a balanced food web, so the dead remains and wastes are broken down continuously and the nutrients are returned to the soil or water for the producers to use again; nothing accumulates. An artificial ecosystem is incomplete and unbalanced - it holds only a few kinds of organism and too few decomposers to deal with the waste produced, so uneaten food, dead plants and animals and excreta pile up. This waste is then decomposed slowly and anaerobically by bacteria, releasing foul-smelling gases such as hydrogen sulphide and ammonia. Hence artificial ecosystems start giving a foul smell if they are not cleaned regularly.

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17. Q197 CBSE 2023
    Define trophic levels. ​At which trophic level do we find ​

    1. (i)secondary and
    2. (ii)tertiary consumers in a ​food chain ? What will happen ​if all the organisms of ​a trophic level ​die ? ​Give reasons to justify your answer.
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    Secondary consumers are at the third level, tertiary at the fourth; if a level dies, the level below multiplies and the one above starves.

    The various steps in a food chain at which the transfer of food energy takes place are called trophic levels. (i) Secondary consumers are found at the **third** trophic level and (ii) tertiary consumers at the **fourth** trophic level, because the producers occupy the first trophic level and the primary consumers the second. If all the organisms of a trophic level die, the food chain is broken at that point. The organisms of the level just **below** it multiply unchecked, because the predator that kept their numbers down has gone, and they may overconsume the level below them in turn. The organisms of the level just **above** it starve, because energy flows in one direction only and their only source of food has disappeared, so their numbers fall or they migrate. The flow of energy beyond that level stops and the balance of the ecosystem is upset.

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18. Q198 CBSE 2023
    If a harmful chemical enters in ​a food chain ​comprising peacock, plants, ​rats ​and snakes, which of ​these organisms is ​likely to have the highest concentration of the chemical in its body. Justify ​your ​answer. Name the process involved and define it.
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    The peacock, at the top of the chain, has the highest concentration of the chemical; the process is biological magnification.

    The food chain is plants $\to$ rats $\to$ snakes $\to$ peacock, and the peacock is likely to have the highest concentration of the chemical because it is at the highest (fourth) trophic level. Harmful chemicals such as pesticides are not biodegradable: they enter the plants from the soil and water, are not broken down in the body and so are stored, and each consumer takes in the chemical from all the food it eats. The concentration therefore rises at every step and is highest in the peacock. The process is biological magnification: the progressive increase in the concentration of harmful non-biodegradable chemicals at successive trophic levels of a food chain.

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19. Q199 CBSE 2023
    1. (a)​Construct a food chain of four trophic levels operating ​in a grassland. If the energy available to the organisms of $4^{\text{th}}$ trophic level is $75$ joules, how much energy was available ​with the organisms of the $2^{\text{nd}}$ trophic level for transfer to the next ​trophic level ? Justify your answer.
    2. (b)​Why is the flow ​of energy unidirectional ​in a food chain ?
    Show answer

    7500 J was available at the second level by the ten per cent law; energy cannot return to earlier levels, so flow is one-way.

    (a) Food chain: Grass (1st level) $\to$ Grasshopper (2nd level) $\to$ Frog (3rd level) $\to$ Snake (4th level). By the ten per cent law, only about $10\%$ of the energy at one trophic level is passed on to the next, so the energy at each level is ten times that at the level above it: 3rd level $=10\times 75=750$ J, and 2nd level $=10\times 750=7500$ J. So $7500$ J was available with the organisms of the second trophic level, of which about $750$ J passes on to the third level. (b) The energy captured by the producers does not revert to the sun, and the energy that passes to the herbivores does not come back to the producers; at each level much of it is lost as heat, which cannot be used again. As it moves progressively through the trophic levels it is no longer available to the previous level, so the flow of energy is unidirectional.

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20. Q200 CBSE 2023
    (a) Give ​one ​example each of

    1. (i)Natural and
    2. (ii)Artificial ​ecosystems. (b) Write an ​aquatic food chain with four steps. Which group ​of organisms are ​of primary importance and why?
    Show answer

    Pond (natural), crop field (artificial); in the aquatic chain, producers (phytoplankton) matter most because all other organisms depend on them.

    (a) (i) Natural ecosystem: a pond (or a lake, forest or grassland). (ii) Artificial ecosystem: a crop field (or a garden or an aquarium). (b) Phytoplankton $\to$ Zooplankton $\to$ Small fish $\to$ Large fish. The producers (phytoplankton) are of primary importance: they make food by photosynthesis using sunlight, trapping solar energy in the food, and all the other organisms of the chain depend on them for food and energy, the zooplankton directly and the fish indirectly.

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