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 13 of 15

1. Q241 CBSE 2020
   1. (a)Construct a terrestrial food chain ​comprising four trophic ​levels.
   2. (b)What will ​happen if we kill all the organisms in one trophic ​level ?
   3. (c)Calculate the amount ​of energy available to the ​organisms at the fourth trophic level if the ​energy available to the organisms at the second trophic level is 2000 J.
   Show answer

   A four-level chain such as grass, grasshopper, frog, snake; killing one level unbalances the ecosystem; the fourth level gets 20 J.

   (a) $\text{Grass}\to \text{Grasshopper}\to \text{Frog}\to \text{Snake}$ (producer, herbivore, carnivore, top carnivore); any correct four-level terrestrial chain is accepted. (b) Food energy can no longer pass to the higher levels: the organisms of the next higher level lose their food and decline, while the organisms of the level below are no longer eaten and increase in number. The food chain and the ecosystem become unbalanced. (c) By the 10% law, energy at level III $=10\%$ of $2000$ J $=200$ J, and energy at level IV $=10\%$ of $200$ J $=20$ J.

   Step-by-step solution →
2. Q242 CBSE 2020
   1. (a)State one important ​function of ozone layer at the higher ​level in the atmosphere.
   2. (b)How ​is ​ozone ​formed ?
   3. (c)It has been observed that ozone layer is ​getting depleted. Name the compound ​responsible ​for ozone depletion.
   Show answer

   Ozone shields the Earth from UV; UV splits ${\text{O}}_2$ into atoms that form ${\text{O}}_3$; CFCs deplete it.

   (a) The ozone layer shields the surface of the earth from the harmful ultraviolet (UV) radiation of the sun, which can cause skin cancer in human beings. (b) At the higher levels of the atmosphere, UV radiation from the sun splits oxygen molecules into free oxygen atoms, and each free oxygen atom then combines with an oxygen molecule to form ozone: ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$ and $\text{O}+{\text{O}}_2⟶{\text{O}}_3$. (c) Chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers, are responsible for the depletion of the ozone layer.

   Step-by-step solution →
3. Q243 CBSE 2020
   1. (a)From the following group ​of ​organisms create a ​food chain which is the most advantageous for ​Human beings in terms of energy. Hawk, Rat, Cereal plant, Goat, Snake, Human Being
   2. (b)State the possible disadvantage if ​the cereal plant ​is growing in soil rich in ​pesticides.
   3. (c)Construct a food web using the organisms mentioned above.
   Show answer

   The shortest chain, cereal plant to human, gives humans the most energy; soil pesticides accumulate up the chain and harm humans most.

   (a) Cereal plant $\to$ Human being. Feeding directly on the producer, human beings stand at the end of the shortest chain; since only about $10\%$ of the energy passes from one trophic level to the next, the most energy reaches them this way. (b) Pesticides in the soil are absorbed by the cereal plant and enter the food chain. Being non-biodegradable, they accumulate progressively at each trophic level (biological magnification), so human beings and other top consumers receive the highest concentration, which harms their health. (c) Food web: Cereal plant $\to$ Rat $\to$ Snake $\to$ Hawk; Rat $\to$ Hawk; Cereal plant $\to$ Goat $\to$ Human being; Cereal plant $\to$ Human being (as drawn in the figure).

   Step-by-step solution →
4. Q244 CBSE 2020
   1. (a)State with reason the consequence of decrease in number of ​carnivores ​in an ​ecosystem.
   2. (b)​In a food chain, ​state the trophic level at which the ​concentration of harmful ​chemicals is maximum. Why is ​it so ?
   Show answer

   Fewer carnivores let herbivores increase and over-graze plants; harmful chemicals are highest in top carnivores because they accumulate at each level (biological magnification).

   (a) The population of herbivores will increase, because the carnivores that prey on them are fewer, so fewer herbivores are eaten. The extra herbivores then over-graze the plants, so the vegetation decreases and the balance of the ecosystem is disturbed. (b) At the highest trophic level, the top carnivores (tertiary consumers). Harmful chemicals such as pesticides are non-biodegradable, so they are not broken down; as they pass from one trophic level to the next they accumulate progressively at each level and reach their maximum concentration at the top. This is biological magnification.

   Step-by-step solution →
5. Q245 CBSE 2020
   What is meant ​by trophic level in a food chain ? Construct a terrestrial ​food ​chain with four trophic levels. ​The energy ​flow in a food chain is always unidirectional. Why ​?
   Show answer

   A trophic level is a step of the food chain where food (energy) is transferred; energy flow is one-way as it never returns.

   Trophic level: each step or level of a food chain, at which the transfer of food (energy) takes place, forms a trophic level; producers form the first level, herbivores the second, and carnivores the third and fourth. Terrestrial food chain with four trophic levels: Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake (any other correct chain, such as Grass $\to$ Insect $\to$ Frog $\to$ Hawk, is acceptable). The energy flow is unidirectional because the energy captured by the autotrophs does not revert to the solar input, and the energy that passes to the herbivores does not come back to the autotrophs; as it moves progressively through the various trophic levels, it is no longer available to the previous level. A large part of it is also lost as heat at every level.

   Step-by-step solution →
6. Q246 CBSE 2020
   1. (a)Create a food ​chain ​of the following ​organisms : Insect, Hawk, Grass, Snake, Frog
   2. (b)Name the organism at ​the third trophic level of the created food chain.
   3. (c)Which organism ​of ​this food chain will have the highest concentration of non-biodegradable chemicals ?
   4. (d)Name the phenomenon associated with it.
   5. (e)If ​10,000 joules of energy is available to frogs, how much energy will be available to ​snakes in this food chain ?
   Show answer

   Frog is at the third level; the hawk has the highest chemical concentration (biological magnification); snakes get 1000 J.

   (a) $\text{Grass}⟶\text{Insect}⟶\text{Frog}⟶\text{Snake}⟶\text{Hawk}$ (b) The frog is at the third trophic level (grass is the first and the insect the second). (c) The hawk, at the top of the food chain, has the highest concentration of non-biodegradable chemicals. (d) Biological magnification (biomagnification). (e) By the ten per cent law, the energy available to snakes $=\frac{10}{100}\times 10,000$ J $=1000$ J.

   Step-by-step solution →
7. Q247 CBSE 2020
   1. (a)Complete ​the following table :
   2. (b)​How ​is ​ozone ​formed at ​the higher ​levels of ​atmosphere ?

   Table for part (a)

   	Oxygen	Ozone
   Formula	(i) ________	(ii) ________
   Benefits to biotic component	(iii) ________	(iv) ________

   Show answer

   (i) ${\text{O}}_2$, (ii) ${\text{O}}_3$; oxygen is needed for respiration, ozone shields from UV; UV splits ${\text{O}}_2$ and the atoms join ${\text{O}}_2$ to form ${\text{O}}_3$.

   (a) (i) ${\text{O}}_2$; (ii) ${\text{O}}_3$; (iii) Oxygen is needed for respiration (breathing): aerobic organisms use it to release energy from food; (iv) Ozone absorbs the harmful ultraviolet (UV) radiation of the sun and shields living organisms from it. (b) At the higher levels of the atmosphere, high-energy UV radiation splits some oxygen molecules into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$. These atoms combine with oxygen molecules to form ozone, $\text{O}+{\text{O}}_2⟶{\text{O}}_3$.

   Step-by-step solution →
8. Q248 CBSE 2020
   What ​are decomposers ​? List ​two ​consequences of ​their absence ​in ​the ecosystem.
   Show answer

   Decomposers are micro-organisms that break dead matter into simple substances; without them nutrients would not be recycled and dead matter would pile up.

   Decomposers are micro-organisms, mainly bacteria and fungi, that break down the complex organic substances of the dead remains and waste products of organisms into simple inorganic substances, which go into the soil and are used again by plants. Two consequences of their absence in the ecosystem: (i) Nutrients would not be recycled: the minerals locked in dead bodies and wastes would not return to the soil, so the soil would not be replenished and plants would lack nutrients. (ii) The natural cleaning of the environment would stop: dead plants, dead animals and wastes would not be broken down and would pile up.

   Step-by-step solution →
9. Q249 CBSE 2020
   1. (a)Write two harmful effects of using plastic bags on the ​environment. ​Suggest alternatives to the ​usage of plastic bags.
   2. (b)List ​any two practices that ​can ​be followed to dispose off the waste produced in our ​homes.
   Show answer

   Plastic bags pollute land, water and air, so use cloth, jute or paper bags; segregate household waste and compost kitchen waste.

   (a) Harmful effects (any two): plastic bags are non-biodegradable, so when thrown away they pollute land and water; burning them releases toxic gases that pollute the air; they block drains and sewers, causing water-logging; animals that swallow them can choke or die. Alternatives: cloth or jute bags, paper bags, and metal or glass containers. (b) Practices (any two): segregate household waste into biodegradable (wet) and non-biodegradable (dry) waste for proper disposal and recycling; compost the biodegradable kitchen waste; reuse articles such as glass bottles for storage. Other correct practices also earn credit.

   Step-by-step solution →
10. Q250 CBSE 2020
    How ​is ozone layer formed ? State ​its importance ​to all ​life ​forms on ​earth ? Why the ​amount of ozone in ​the atmosphere dropped sharply in the 1980s ?
    Show answer

    UV splits ${\text{O}}_2$ into atoms that form ozone, which absorbs harmful UV; CFCs released in the 1980s broke down ozone.

    Formation: high in the atmosphere, UV radiation from the Sun splits some oxygen molecules into free oxygen atoms, ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$; these atoms then combine with oxygen molecules to form ozone, $\text{O}+{\text{O}}_2⟶{\text{O}}_3$. Importance: the ozone layer absorbs most of the harmful ultraviolet radiation from the Sun, so it protects all life forms on earth from its harmful effects, such as skin cancer in human beings. Fall in the 1980s: synthetic chemicals like chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers, were released into the atmosphere and broke down the ozone.

    Step-by-step solution →
11. Q251 CBSE 2020
    Complete ​the ​following flow chart based ​on ​ecosystem ​and ​its components.

    Show answer

    Ecosystems are terrestrial or aquatic; their components are biotic (living organisms) and abiotic (non-living factors such as air and water).

    (i) Aquatic (ii) Abiotic (iii) Non-living factors: air, water, soil, temperature (also rainfall, wind and minerals) (iv) Living organisms: plants, animals and micro-organisms (producers, consumers and decomposers) (v) All the interacting organisms in an area together with the non-living constituents of the environment form an ecosystem.

    Step-by-step solution →
12. Q252 CBSE 2020
    1. (a)​What is an ecosystem ?
    2. (b)List any two ​natural ecosystems.
    3. (c)​We do not clean ponds ​or lakes, but an aquarium needs to be cleaned ​regularly. Why ​?
    Show answer

    An ecosystem is the interacting living and non-living parts of an area; an aquarium has few or no decomposers, so it needs cleaning.

    (a) All the interacting organisms in an area together with the non-living constituents of their environment form an ecosystem; it has biotic (living) and abiotic (non-living) components. (b) A forest and a pond (a lake, a grassland, a desert or the sea are other natural ecosystems; any two correct examples earn credit). (c) Ponds and lakes are natural ecosystems in which decomposers (bacteria and fungi) break down dead remains and wastes and return the nutrients to the water, so wastes do not collect. An aquarium is an artificial ecosystem in which decomposers are absent (or too few), so wastes and dead matter pile up and pollute the water; hence it must be cleaned regularly.

    Step-by-step solution →
13. Q253 CBSE 2020
    1. (a)Define ecosystem.
    2. (b)​Autotrophs are at ​the first level of food chain. Give reason.
    3. (c)In a ​food chain of frogs, grass, insects ​and snakes assign trophic level ​to ​frogs. To which category of consumers ​do ​they belong to ?
    Show answer

    An ecosystem is interacting living and non-living components; autotrophs make food for all; frogs are at the third trophic level, secondary consumers.

    (a) An ecosystem is made up of all the interacting organisms of an area (biotic components) together with the non-living components of their environment (abiotic components); the living and non-living components interact with each other. (b) Autotrophs trap solar energy and convert it into food (chemical energy) by photosynthesis. All other organisms depend on them for food, directly or indirectly, so they form the first level and pass the food energy on to the next level of the food chain. (c) Grass $\to$ insects $\to$ frogs $\to$ snakes: frogs are at the third trophic level and are secondary consumers (carnivores).

    Step-by-step solution →
14. Q254 CBSE 2020
    1. (a)​Explain the role ​of UV radiation in producing ozone layer.
    2. (b)Mention the reaction involved.
    3. (c)​Why is ​excessive use ​of ​CFCs a cause of ​concern ​?
    Show answer

    UV splits oxygen molecules into atoms that combine with oxygen to form ozone; CFCs deplete ozone, letting more UV reach earth.

    (a) At the higher levels of the atmosphere, high-energy UV radiation splits some molecular oxygen (${\text{O}}_2$) into free oxygen atoms ($\text{O}$); these free atoms then combine with molecular oxygen to form ozone (${\text{O}}_3$). (b) ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$ and $\text{O}+{\text{O}}_2⟶{\text{O}}_3$ (ozone). (c) CFCs, used as refrigerants and in fire extinguishers, deplete the ozone layer. Since the ozone layer shields the earth from the Sun’s UV radiation, more UV then reaches the earth, and it can cause skin cancer in human beings and harm other organisms.

    Step-by-step solution →
15. Q255 CBSE 2019
    List ​two ​main ​components of ​an ​ecosystem.
    Show answer

    The two main components are biotic (living) and abiotic (non-living) components.

    The two main components of an ecosystem are: (i) Biotic components: the living organisms, that is, the producers (green plants), the consumers (animals) and the decomposers (bacteria and fungi). (ii) Abiotic components: the non-living physical factors, such as sunlight, temperature, rainfall, wind, air, water, soil and minerals.

    Step-by-step solution →
16. Q256 CBSE 2019
    Write ​various ​steps ​of a ​terrestrial ​food chain of ​four trophic levels.
    Show answer

    Plants $\to$ goat $\to$ fox $\to$ lion is a terrestrial food chain with four trophic levels.

    Plants (producers, first trophic level) → Goat (herbivore, primary consumer, second trophic level) → Fox (carnivore, secondary consumer, third trophic level) → Lion (top carnivore, tertiary consumer, fourth trophic level). Any other correct terrestrial food chain of four trophic levels, such as grass → grasshopper → frog → snake, is equally correct.

    Step-by-step solution →
17. Q257 CBSE 2019
    Generally, ​we do not clean natural ponds ​or lakes ​but ​an aquarium ​needs ​periodic cleaning. ​Why ?
    Show answer

    A natural pond is self-sustaining because decomposers clean it, whereas an aquarium lacks enough decomposers, so wastes pile up.

    A natural pond or lake is a complete, self-sustaining ecosystem: its decomposers (bacteria and fungi) break down wastes and dead remains and return the nutrients to the water, so it cleans itself. An aquarium is an artificial ecosystem that lacks enough decomposers, so wastes, uneaten food and dead matter pile up in it; hence it needs periodic cleaning.

    Step-by-step solution →
18. Q258 CBSE 2019
    Define a food chain. Design ​a terrestrial ​food ​chain ​of four trophic levels. ​If a pollutant enters at the producer level, the organisms of which ​trophic level will have the maximum concentration of the pollutant in ​their bodies ? What is this phenomenon called ?
    Show answer

    The top-level organisms, such as the snake in a grass-grasshopper-frog-snake chain, hold the maximum pollutant, called biological magnification.

    A food chain is a series of organisms in which each organism feeds on the one before it (organisms eating and being eaten in turn), through which food energy passes from one trophic level to the next. A terrestrial food chain of four trophic levels: Grass (producer) $\to$ Grasshopper (herbivore) $\to$ Frog (carnivore) $\to$ Snake (top carnivore). (Any other correct four-level land chain earns credit.) The organisms of the fourth, highest trophic level, the tertiary consumers (the snake in this chain), will have the maximum concentration of the pollutant. This phenomenon is called biological magnification (biomagnification).

    Step-by-step solution →
19. Q259 CBSE 2019
    1. (a)​Natural ​water bodies are not regularly cleaned whereas an aquarium needs regular cleaning. Why ?
    2. (b)What are decomposers ? ​What will be ​the consequence if the decomposers are ​completely eradicated from an ecosystem ? ​Give justification in support ​of ​your answer.
    Show answer

    Natural water bodies clean themselves through decomposers, unlike aquariums; without decomposers dead matter would pile up and nutrients would not return to soil.

    (a) A natural water body is a self-sustaining ecosystem. Its decomposers (micro-organisms such as bacteria and fungi) break down dead remains and wastes and recycle their nutrients, so it cleans itself. An aquarium is an artificial ecosystem without enough decomposers (and without a complete food chain), so uneaten food, wastes and dead matter pile up and pollute the water; it therefore needs regular cleaning. (b) Decomposers are micro-organisms, such as bacteria and fungi, that break down the complex organic substances of dead remains and wastes into simple inorganic substances, which return to the soil. If they were completely eradicated, dead bodies and wastes would not decompose and would pile up, and the nutrients in them would not be returned to the soil. Justification: the soil would lose its fertility and become unsuitable for plants; producers, and therefore all food chains, would suffer, and the ecosystem would become unbalanced.

    Step-by-step solution →
20. Q260 CBSE 2019
    What is a ​food ​chain ? Why ​is the flow ​of ​energy in an ecosystem ​unidirectional ? Explain ​briefly.
    Show answer

    A food chain is a series of organisms, each feeding on the one before; energy flows one way and never returns to earlier levels.

    A food chain is a series of organisms in which each feeds on the one before it, so that food and energy pass from one trophic level to the next; for example, grass $\to$ deer $\to$ lion. The flow of energy is unidirectional because (any two): the energy captured by the autotrophs does not revert to the solar input; the energy passed to the herbivores does not come back to the autotrophs; only about $10\%$ of the energy at one level is passed on and the rest is lost as heat, so energy that has moved on is no longer available to the previous level.

    Step-by-step solution →