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.

NCERT exercises

Exercises (9) Intext Questions (Page 212) (2) Intext Questions (Page 214) (3) Intext Questions (Page 216) (2)

NCERT Exemplar exercises

Multiple Choice Questions (21) Short Answer Questions (9) Long Answer Questions (8)

CBSE Class 10 Science previous year papers, with answers →

Questions · page 12 of 15

Q221 CBSE 2022
In 1987, the United Nations Environment Programme (UNEP) forged an agreement to freeze the production of a certain chemical to year 1986 level. Name the chemical and the manufacturing companies in which this chemical was being mostly used. In what way this chemical damages
1. (i)the upper layers of the atmosphere and ultimately
2. (ii)the organisms on the earth.
Show answer

CFCs, used in refrigerators and fire extinguishers, release chlorine that destroys ozone, so more UV reaches the Earth and harms organisms.
The chemical is chlorofluorocarbons (CFCs). They were used mostly by the companies manufacturing refrigerators (and air-conditioners), where CFCs served as the refrigerant, and fire extinguishers, the two uses NCERT names; CFCs were also used as propellants in aerosol sprays. (i) Upper layers of the atmosphere: CFCs drift up to the upper atmosphere, where ultraviolet radiation breaks them and sets chlorine atoms free. These break ozone ( $O_{3}$ ) down to oxygen ( $O_{2}$ ), so the ozone layer becomes thinner (ozone depletion). (ii) Organisms on the earth: the thinner ozone layer lets more of the Sun's ultraviolet radiation reach the earth. This radiation is highly damaging to organisms: it can cause skin cancer in human beings, damage the eyes and weaken immunity, and it harms plants and other organisms.

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Q222 CBSE 2022
1. (a)Name the group of organisms which form in the first trophic level of all food chains. Why are they called so ?
2. (b)Why are the human beings most adversely affected by bio-magnification ?
3. (c)State one ill-effect of the absence of decomposers from a natural ecosystem.
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Producers make their own food; pesticides accumulate most in humans at the top; without decomposers nutrients would not return to the soil.
(a) Producers — green plants (and other organisms, such as blue-green algae, that make food by photosynthesis). They are called producers because they make their own food: using the energy of sunlight, in the presence of chlorophyll, they produce organic substances such as sugar and starch from carbon dioxide and water, and every other organism of the food chain depends on this food, directly or indirectly. (b) Harmful chemicals such as pesticides are not degradable. They enter the food chain and accumulate progressively at each trophic level (biological magnification). Human beings occupy the top level of the food chains they belong to, so the maximum concentration of these chemicals accumulates in their bodies. (c) The nutrients in dead plants, dead animals and wastes would not be returned to the soil, because nothing would break them down into simple substances; the soil would not be replenished. (Also accepted: dead remains and wastes would pile up.)

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Q223 CBSE 2022
1. (a)We do not clean ponds or lakes, but an aquarium needs to be cleaned regularly. Why ?
2. (b)Why is ozone layer getting depleted at the higher levels of the atmosphere ? Mention one harmful effect caused by its depletion.
Show answer

A pond is a self-sustaining ecosystem with decomposers, an aquarium is not; CFCs deplete ozone, letting in UV that can cause skin cancer.
(a) Ponds and lakes are natural, self-sustaining ecosystems: their decomposers (bacteria and fungi) break down dead plants and animals and other wastes into simple substances, which are recycled into the water and soil. An aquarium is an artificial ecosystem that does not have enough decomposers (or a complete food web), so uneaten food, excreta and dead matter pile up in a small volume of water and pollute it; it must therefore be cleaned regularly. (b) The ozone layer at the higher levels of the atmosphere is being depleted chiefly by synthetic chemicals such as chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. Released CFCs reach the upper atmosphere, where UV radiation breaks them down and the chlorine they release destroys ozone. Harmful effect: more ultraviolet radiation reaches the Earth's surface, which can cause skin cancer in human beings.

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Q224 CBSE 2022
Explain the reason why in a food chain
1. (a)the number of individuals decreases as we move from lower trophic level to higher trophic level, and
2. (b)the flow of energy is unidirectional.
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Only about 10% of energy passes up each level, so numbers fall; lost heat energy cannot return, so energy flow is one-way.
(a) At each trophic level, most of the energy in the food eaten is used by the organisms for their own life processes and is lost to the surroundings, mostly as heat. On average only about 10% of the energy is passed on to the next level (the ten per cent law): of 10 000 J held by the producers, about 1 000 J reaches the herbivores, 100 J the next consumers and 10 J the fourth level. With less and less energy available at each higher level to support the organisms there, the number of individuals generally decreases from the lower to the higher trophic levels. (b) The energy captured by the producers does not go back to the Sun, and the energy passed to the herbivores does not come back to the producers; as energy moves up through the trophic levels it is no longer available to the previous level. The energy lost at each level, mostly as heat, cannot be used by plants to make food. So energy flows in one direction only, from the Sun to the producers, herbivores and carnivores, and is not recycled.

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Q225 CBSE 2022

Draw a diagram showing flow of energy from solar input to a food chain having four trophic levels in an eco system. List two inferences that can be drawn from the energy flow diagram.

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Energy flows one way from the Sun through four trophic levels, and only about $10%$ passes to the next level.
Diagram: Sun (solar input) → Producers (green plants, 1st trophic level) → Herbivores (2nd) → Carnivores (3rd) → Top carnivores (4th), with energy lost as heat at every level and only about one-tenth passed on at each step, for example 10,000 J → 1,000 J → 100 J → 10 J (see the diagram). Inferences (any two): (i) The flow of energy is unidirectional: energy captured by the producers does not return to the solar input, and energy passed to the herbivores does not come back to the producers. (ii) The energy available decreases at each successive trophic level: only about 10% of the energy of one level reaches the next, the rest being used in life processes and lost as heat. (iii) Because of this loss, very little usable energy remains after four trophic levels, so food chains generally have only three or four steps.

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Q226 CBSE 2022
Give reasons for the following :
1. (a)The existence of decomposers is essential in a biosphere.
2. (b)Flow of energy is unidirectional in a food chain.
3. (c)A food chain generally consists of only three or four trophic levels.
Show answer

Decomposers recycle nutrients; energy never returns and is lost as heat; only $10%$ passes on, so chains have three or four levels.
(a) Decomposers (bacteria and fungi) break down the complex organic substances of dead remains and wastes into simple inorganic substances that go into the soil and are used again by plants. Without them dead matter would pile up with its nutrients locked away, and the cycling of materials in the biosphere would break down. (b) The energy captured by producers passes to herbivores and then to carnivores but never comes back: it does not revert to the solar input, the energy passed to herbivores does not return to the producers, and at every level much of it is lost as heat that cannot be used again. So energy flows in one direction only. (c) Only about $10%$ of the energy at one trophic level is passed on to the next (the 10 per cent law). The loss at each step is so great that very little usable energy remains after four trophic levels, so a food chain generally consists of only three or four steps.

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Q227 CBSE 2022

Define Trophic Level. Prepare a food chain comprising the following organisms : Snakes, Plants, Hawks, Rats If 1000 joules of energy is available at the first trophic level for transfer to the organisms of the second trophic level, then how much energy will be made available to the organisms at the fourth trophic level ?

Show answer

Food chain $Plants \to Rats \to Snakes \to Hawks$ ; by the ten per cent law the fourth trophic level gets 1 J.
Trophic level: each step or level of a food chain at which the transfer of food (energy) takes place is called a trophic level. Food chain: Plants $\to$ Rats $\to$ Snakes $\to$ Hawks (plants at the first trophic level, rats at the second, snakes at the third and hawks at the fourth). Energy: by the ten per cent law, about $10%$ of the energy at one level is made available to the next. From $1000$ J at the first level, the second level gets $100$ J, the third $10$ J and the fourth $1$ J. So $1$ J of energy will be made available to the hawks at the fourth trophic level.

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Q228 CBSE 2020
Answer question numbers 3(a) to 3(d) and 4(a) to 4(d) on the basis of your understanding of the following paragraphs and the related studied concepts. Human body is made up of five important components, of which water is the main component. Food as well as potable water are essential for every human being. The food is obtained from plants through agriculture. Pesticides are being used extensively for a high yield in the fields. These pesticides are absorbed by the plants from the soil along with water and minerals and from the water bodies these pesticides are taken up by the aquatic animals and plants. As these chemicals are not biodegradable, they get accumulated progressively at each trophic level. The maximum concentration of these chemicals gets accumulated in our bodies and greatly affects the health of our mind and body.
1. (a)Why is the maximum concentration of pesticides found in human beings ?[1]
2. (b)Give one method which could be applied to reduce our intake of pesticides through food to some extent.[1]
3. (c)Various steps in a food chain represent : (a) Food web (b) Trophic level (c) Ecosystem (d) Biomagnification[1]
4. (d)With regard to various food chains operating in an ecosystem, man is a : (a) Consumer (b) Producer (c) Producer and consumer (d) Producer and decomposer[1]
Show answer

Pesticides accumulate most in humans at the top of the food chain; wash food; steps are trophic levels; man is a consumer.
(a) Human beings are at the top level of the food chains they belong to. Pesticides are not biodegradable, so they accumulate progressively at each trophic level (biological magnification), and the highest concentration is reached at the top, in human beings. (b) Wash fruits, vegetables and grains thoroughly before use; organic farming, or using bio-pesticides in place of chemical ones, is equally accepted. (c) Option (b): Trophic level. (d) Option (a): Consumer.

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Q229 CBSE 2020
Answer question numbers 4(a) – 4(d) on the basis of your understanding of the following paragraph and the related studied concepts : India today is facing the problem of overuse of resources, contamination of water and soil and lack of methods of processing the waste. The time has come for the world to say goodbye to “single-use plastics”. Steps must be undertaken to develop environment-friendly substitutes, effective plastic waste collection and methods of its disposal. Indore treated 15 lakh metric tonnes of waste in just 3 years, through biomining and bioremediation techniques. Bioremediation involves introducing microbes into a landfill to naturally ‘break’ it down and biomining involves using trommel machines to sift through the waste to separate the ‘soil’ and the waste component. The city managed to chip away 15 lakh metric tonnes of waste at a cost of around ₹ 10 crore. A similar experiment was successfully carried out in Ahmedabad also.
1. (a)State two methods of effective plastic waste collection in your school.[1]
2. (b)Name any two uses of “single-use plastic” in daily life.[1]
3. (c)If we discontinue the use of plastic, how can an environment-friendly substitute be provided ?[1]
4. (d)Do you think microbes will work similarly in landfill sites as they work in the laboratory ? Justify your answer.[1]
Show answer

Separate bins collect plastic; bottles and wrappers are single-use; cloth or jute bags replace plastic; microbes act as in the laboratory.
(a) Keep separate bins for plastic and for paper, and separate biodegradable from non-biodegradable waste at the source. Other correct methods, such as a plastic-collection drive, also earn credit. (b) Packaging of water, milk, food and biscuits (bottles, pouches, wrappers and carry bags), and disposable utility items such as bowls, tumblers, plates and straws. (c) By providing cloth or jute bags, paper bags and packaging, earthen pots and cups, and reusable utensils or leaf plates in place of plastic articles, or any other material that serves the same purpose. (d) Yes. The action of the microbes is tested in the laboratory under the same conditions as in the landfill, so they break down the waste in the landfill in the same way, as the work at Indore and Ahmedabad shows.

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Q230 CBSE 2020
Which one of the following is an artificial ecosystem ?
1. (a)River
2. (b)Crop field
3. (c)Forest
4. (d)Ocean
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(b) Crop field

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Q231 CBSE 2020
Of the 50,000 kJ of energy that passes from the second trophic level to the third trophic level in a food chain, about 5,000 kJ of energy is
1. (a)stored in body tissues.
2. (b)burnt in respiration.
3. (c)trapped in the environment.
4. (d)lost as heat.
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(a) stored in body tissues.

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Q232 CBSE 2020
Consider the following ecosystems : I. Ponds II. Forests III. Aquariums IV. Crop fields Out of these, the natural ecosystems are
1. (a)I, II and III
2. (b)II, III and IV
3. (c)I and II only
4. (d)I, II and IV
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(c) I and II only

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Q233 CBSE 2020
A system of inter-dependent food chains represents
1. (a)Food web
2. (b)Trophic levels
3. (c)Ecosystem
4. (d)Community
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(a) Food web

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Q234 CBSE 2020
What will happen if the deer are missing in the following food chain ? Grass $⟶$ Deer $⟶$ Tiger
1. (a)The population of tigers will increase.
2. (b)The amount of grass will decrease.
3. (c)The tigers will die.
4. (d)The tigers will start eating grass.
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(c) The tigers will die.

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Q235 CBSE 2020
Disposable plastic plates should not be used because
1. (a)they are made of non-biodegradable materials.
2. (b)they are made of biodegradable materials.
3. (c)they are made of toxic materials.
4. (d)they are made of materials with light weight.
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(a) they are made of non-biodegradable materials.

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Q236 CBSE 2020
In an ecosystem, 10% of energy available for transfer from one trophic level to the next is in the form of :
1. (a)heat energy
2. (b)chemical energy
3. (c)mechanical energy
4. (d)light energy
Show answer

(b) chemical energy

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Q237 CBSE 2020
Food web is constituted by
1. (a)relationship between the organisms and the environment.
2. (b)relationship between plants and animals.
3. (c)various interlinked food chains in an ecosystem.
4. (d)relationship between animals and environment.
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(c) various interlinked food chains in an ecosystem.

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Q238 CBSE 2020
How much of the net primary productivity of a terrestrial ecosystem is eaten and digested by herbivores ?
1. (a) $100%$
2. (b) $10%$
3. (c) $1%$
4. (d) $0.1%$
Show answer

(b) $10%$

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Q239 CBSE 2020
Soil fertility is determined by its ability to :
1. (a)Decay organic matter
2. (b)Hold organic matter
3. (c)Hold water
4. (d)Support life
Show answer

(d) Support life

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Q240 CBSE 2020
Choose the incorrect statement from the following :
1. (a)Ozone is a molecule formed by three atoms of oxygen.
2. (b)Ozone shields the surface of the Earth from ultraviolet radiations.
3. (c)Ozone is deadly poisonous.
4. (d)Ozone gets decomposed by UV radiations.
Show answer

(a) Ozone is a molecule formed by three atoms of oxygen.
(b) Ozone shields the surface of the Earth from ultraviolet radiations.
(c) Ozone is deadly poisonous.
(d) Ozone gets decomposed by UV radiations.

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