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 14 of 15

1. Q261 CBSE 2019
   How is ​ozone formed in the upper atmosphere ? ​State ​its importance. What is responsible ​for its ​depletion ? Write one harmful effect of ​ozone ​depletion.
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   UV splits ${\text{O}}_2$ into atoms that form ozone, which absorbs harmful UV; CFCs deplete it, so more UV causes skin cancer.

   Formation: In the upper atmosphere, high-energy ultraviolet (UV) radiation from the Sun splits some oxygen molecules into free oxygen atoms, which then combine with molecular oxygen to form ozone: ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$; $\text{O}+{\text{O}}_2\to {\text{O}}_3$. Importance: The ozone layer shields the surface of the Earth from the harmful ultraviolet radiation of the Sun by absorbing it. Depletion: Chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers and aerosol sprays, are responsible for the depletion of ozone. Harmful effect: More UV radiation reaches the Earth and causes skin cancer in human beings (it can also cause cataract and eye damage, and weaken the immune system).

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2. Q262 CBSE 2019
   What is ozone ? How is ​it formed at the higher level of ​the atmosphere ? What ​is likely to happen ​if the ozone ​layer is continuously damaged, ​which is ​in fact happening at ​the moment ? List any two consequences of it.
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   Ozone (${\text{O}}_3$) shields the Earth from UV and forms from oxygen atoms and ${\text{O}}_2$; damage lets in more UV, causing skin cancer and cataract.

   Ozone (${\text{O}}_3$) is a molecule made of three oxygen atoms. At the higher levels of the atmosphere it forms a layer that shields the Earth from the Sun's ultraviolet (UV) radiation. Formation: high-energy UV radiation splits some oxygen molecules into free oxygen atoms, which then combine with oxygen molecules to form ozone: ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$; $\text{O}+{\text{O}}_2⟶{\text{O}}_3$. If the ozone layer is continuously damaged (mainly by CFCs), more UV radiation will reach the surface of the Earth. Consequences (any two): (i) skin cancer in human beings; (ii) damage to the eyes, such as cataract; (iii) a weakened immune system; (iv) harm to crops and to aquatic life.

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3. Q263 CBSE 2019
   In a village of South ​India people started cultivating crops all around a lake. They added fertiliser to ​their ​field in order to enhance the yield. Soon after it ​was discovered that the water body was completely covered with green floating ​plants and the aquatic animals started dying in large numbers. Analyse the situation and state reasons for the excessive growth ​of plants and death of aquatic animals in the lake.
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   Fertiliser nutrients drain into the lake and make floating plants multiply (eutrophication); they block light and use up dissolved oxygen, so animals die.

   Reason for the excessive growth of plants: the fertilisers contain plant nutrients such as nitrates and phosphates. Rain and irrigation water drain the excess fertiliser from the surrounding fields into the lake. In this nutrient-rich water the green floating plants (algae and other water plants) grow and multiply very fast and cover the lake. This is called eutrophication. Reason for the death of aquatic animals: the layer of floating plants blocks sunlight, so the plants under the surface cannot photosynthesise and die, and no oxygen is added to the water. The floating plants use up the dissolved oxygen in respiration, and bacteria decomposing the dead plants use up still more. With so little dissolved oxygen left, the fish and other aquatic animals cannot respire and die in large numbers.

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4. Q264 CBSE 2019
   How can we ​help in ​reducing the problem ​of waste ​disposal ​? Suggest any ​three methods.
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   Segregate waste, compost kitchen and garden waste, and recycle paper, plastic, glass and metal.

   Any three of the following (other correct methods also earn credit): (1) Segregation: keep biodegradable and non-biodegradable waste in separate bins, so that each can be dealt with properly. (2) Composting: turn kitchen and garden waste into manure in a compost pit. (3) Recycling: send paper, plastic, glass and metal to be recycled into new articles. (4) Reuse: use bottles, jars, boxes and bags again instead of throwing them away. (5) Reducing the use of non-biodegradable materials: for example, carry a cloth or jute bag instead of polythene bags.

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5. Q265 CBSE 2019
   Define ​an ​ecosystem. Draw ​a block diagram ​to ​show ​the flow of ​energy in an ecosystem.
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   An ecosystem is all organisms of an area interacting with one another and their non-living surroundings; energy flows one way from the Sun.

   An ecosystem is the system formed by all the living organisms in an area interacting with one another and with the non-living constituents of their environment, such as air, water, soil, sunlight and temperature. Block diagram of the flow of energy (see the figure): Sun (solar energy) → producers (green plants, which capture about 1% of the sunlight) → primary consumers (herbivores) → secondary consumers (carnivores) → tertiary consumers (top carnivores). Only about 10% of the energy passes from one level to the next; the rest is lost as heat at every level, the dead remains of every level go to the decomposers, and the flow of energy is in one direction only.

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6. Q266 CBSE 2019
   Write the essential function ​performed ​by ​ozone at the higher levels of the Earth’s ​atmosphere ? How is it produced ? Name the synthetic chemicals mainly responsible for ​the drop ​of amount of ozone in the atmosphere. How can the use of these ​chemicals be reduced ?
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   Ozone shields Earth from UV and forms when UV splits ${\text{O}}_2$; CFCs deplete it, so minimise CFC leakage and use CFC-free substitutes.

   Function: at the higher levels of the atmosphere, ozone (${\text{O}}_3$) shields the surface of the Earth from the harmful ultraviolet (UV) radiation of the Sun, which damages organisms and can cause skin cancer in human beings. Production: high-energy UV radiation splits some oxygen molecules into free oxygen atoms, and these atoms combine with oxygen molecules to form ozone: ${\text{O}}_2\stackrel{\text{UV}}{\to }\text{O}+\text{O}$, then $\text{O}+{\text{O}}_2\to {\text{O}}_3$. Chemicals responsible: chlorofluorocarbons (CFCs), used as refrigerants and in fire extinguishers. Reducing their use: minimise the leakage of CFCs from refrigerators and air conditioners, and replace CFCs with ozone-friendly substitute chemicals (CFC-free refrigerators, air conditioners and fire extinguishers).

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7. Q267 CBSE 2019
   1. (a)A food chain generally ​has three or four ​trophic levels. Explain.
   2. (b)What is biological ​magnification ? ​Explain.
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   Only about $10\%$ of energy passes on, so chains have three or four levels; biological magnification is rising chemical concentration up the levels.

   (a) At each step of a food chain only about $10\%$ of the energy in the food is passed on to the next trophic level; the rest is used in the organisms' life processes and lost as heat. The usable energy therefore falls very steeply (for example $10000\to 1000\to 100\to 10$ J), so after three or four levels too little energy is left to support another level of organisms. Hence a food chain generally has only three or four trophic levels. (b) Biological magnification is the progressive increase in the concentration of harmful non-biodegradable chemicals, such as pesticides, at each successive trophic level of a food chain. Pesticides used on crops are washed into the soil and water, absorbed by plants and passed on to the animals that eat them. Being non-biodegradable, they are not broken down or excreted and accumulate in the body; as each organism eats many organisms of the level below, their concentration rises at every level and is highest in the top consumers, such as human beings.

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8. Q268 CBSE 2018
   What are decomposers ? ​State their ​role ​in ​our ​biotic ​environment.
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   Decomposers are bacteria and fungi that break down dead matter into simple substances; they cleanse the environment and return nutrients to the soil.

   Decomposers are micro-organisms, mainly bacteria and fungi, that break down the complex organic substances of dead plants and animals and their wastes into simple inorganic substances. Role (any two): 1. They act as natural cleansing agents, clearing away dead remains and wastes. 2. They return the nutrients to the soil, where plants can absorb them again. 3. They help to recycle matter between the biotic and abiotic components of the environment. Other correct points also earn credit.

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9. Q269 CBSE 2018
   What ​are trophic levels ​? ​Make a ​food chain ​of ​four trophic ​levels.
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   Trophic levels are the steps of a food chain, e.g. $\text{Grass}\to \text{Grasshopper}\to \text{Frog}\to \text{Snake}$.

   Trophic levels are the various steps (levels) in a food chain at which the transfer of food, and so of energy, takes place; each organism in a food chain occupies one trophic level. A food chain of four trophic levels: Grass $\to$ Grasshopper $\to$ Frog $\to$ Snake (producer $\to$ primary consumer $\to$ secondary consumer $\to$ tertiary consumer)

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10. Q270 CBSE 2018
    Students ​in a school listened to the news read ​in the ​morning assembly that the ​mountain of garbage in Delhi, suddenly exploded and various vehicles got buried under it. ​Several people were ​also injured and there was ​traffic jam all around. In the brain storming session the teacher also ​discussed this issue and asked the students to find out a solution to the problem of garbage. Finally they arrived at two main points – one is self management of the garbage we produce and the second is to generate less garbage at individual level.

    1. (a)Suggest two measures to manage the garbage we produce.
    2. (b)As an individual, what can we do to generate the least garbage ? Give two points.
    3. (c)List two values the teacher instilled in his students in this episode.
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    Segregate and compost the garbage; reuse things and reduce non-biodegradable materials; the teacher instilled environmental awareness and scientific attitude.

    (a) Any two of these: segregate the garbage at home into biodegradable waste (kitchen waste, peels, leaves) and non-biodegradable waste (plastics, glass, metals) and dispose of each safely; turn the biodegradable waste into manure by composting or vermicomposting; use it to make biogas; recycle plastic, glass, paper and metal; incineration; waste compaction. Other correct measures also earn credit. (b) Reuse things instead of throwing them away, such as empty bottles, jars and old books; and reduce the use of non-biodegradable materials such as polythene bags and thermocol, for example by carrying a cloth or jute bag. Other correct points also earn credit. (c) Any two of these: awareness of and concern for the environment; scientific attitude; concern for community health and personal health.

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11. Q271 CBSE 2018
    While teaching the chapter ​“Our ​Environment” the teacher stressed upon the harmful ​effects of burning of fossil fuels, plastic, ​paper etc. The students ​noticed the extensive use of plastic and polythene in ​daily life, which can be avoided and the surroundings can be kept clean. They decided to make their ​school “plastic and polythene” free and motivated each other for its minimum use.

    1. (a)Why should the use of polythene and plastic be reduced in daily life ?
    2. (b)In what ways the students would have avoided the use of plastic and polythene in their school ?
    3. (c)How the students would have motivated each other for the success of their decision ?
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    Plastic is non-biodegradable and harmful, so use cloth bags and steel tiffins, and spread the message through pledges, posters and an eco-club.

    (a) Polythene and plastics are non-biodegradable: decomposers (bacteria and fungi) cannot break them down, so they persist in the environment for a very long time and keep piling up. They choke drains and sewers, pollute soil and water, and animals such as cows that swallow polythene bags may die. Burning them releases poisonous gases that pollute the air and harm health. So their use should be reduced. (b) The students could have carried books in cloth or jute bags, brought lunch and water in steel or glass tiffins and bottles, covered books with paper or cloth, used reusable or paper cups and plates instead of plastic ones in the canteen and at school functions, and decorated the school for functions with paper, cloth and flowers instead of plastic and thermocol. Any other practical substitute also earns credit. (c) They could have motivated one another through a pledge and talks in the morning assembly, posters and slogans, rallies and skits, an eco-club, making cloth bags from old clothes and distributing them, and appreciating or rewarding the classes that remained plastic free; they could also carry the message to their families and neighbours.

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12. Q272 CBSE 2017
    What ​is ​a biodegradable ​substance ​?
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    A biodegradable substance is broken down into simpler substances by micro-organisms such as bacteria; vegetable peels and paper are examples.

    A biodegradable substance is a substance that can be broken down into simpler substances by biological processes, that is, by the action of micro-organisms such as bacteria and other saprophytes (decomposers). Examples: vegetable peels, paper, cotton cloth and cow dung.

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13. Q273 CBSE 2017
    Why should ​biodegradable and ​non-biodegradable wastes ​be ​discarded in ​two different dustbins ?
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    They are disposed of differently: biodegradable waste can be composted, while non-biodegradable waste needs recycling or separate disposal.

    Because the two kinds of waste are disposed of in different ways. Biodegradable waste (peels, leftover food, dry leaves) is broken down by decomposers, so it can be turned into compost (manure) or used in a biogas plant. Non-biodegradable waste (polythene, plastics, glass, metal) is not broken down by decomposers, so it has to be recycled, reused or disposed of separately. Collecting them in separate dustbins lets each kind go straight to its own treatment; mixed together, neither can be composted or recycled easily, and non-biodegradable waste left in the environment can harm it.

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14. Q274 CBSE 2017
    List two biotic ​components ​of ​an ​eco-system.
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    Producers (green plants) and consumers (animals) are two biotic components of an ecosystem.

    Any two of the following: producers, such as green plants, which make their own food; consumers, such as herbivores and carnivores, which feed on other organisms; and decomposers, such as bacteria and fungi, which break down dead remains and wastes. For example: producers (green plants) and consumers (animals).

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15. Q275 CBSE 2017
    Why is depletion ​of ​ozone ​layer a cause ​of ​concern ​?
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    The ozone layer shields the Earth from harmful UV radiation; its depletion lets more UV reach the Earth, harming organisms.

    The ozone layer high up in the atmosphere shields the surface of the Earth from the harmful ultraviolet (UV) radiation of the Sun. When the layer is depleted, more UV radiation reaches the Earth, and this radiation is highly damaging to living organisms — for example, it can cause skin cancer in human beings. So the depletion of the ozone layer is a cause of concern.

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16. Q276 CBSE 2017
    What ​is meant ​by ​‘biological ​magnification’ ​?
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    The concentration of harmful non-biodegradable chemicals increases progressively at each trophic level of a food chain.

    Biological magnification is the progressive increase in the concentration of harmful non-biodegradable chemicals, such as some pesticides, in the bodies of organisms at each successive trophic level of a food chain. These chemicals are neither broken down nor excreted, so they accumulate, and their concentration is highest in the organisms at the top of the food chain.

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17. Q277 CBSE 2017
    What is an ecosystem ?
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    An ecosystem is the living organisms of an area interacting with the non-living parts of their environment.

    An ecosystem is formed by all the interacting living organisms of an area (the biotic components: producers, consumers and decomposers) together with the non-living components of their environment (the abiotic components, such as temperature, rainfall, wind, soil and minerals). Examples: a forest or a pond (natural), a garden or a crop field (human-made).

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18. Q278 CBSE 2017
    Why ​is forest ​considered ​a ​natural ​ecosystem ​?
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    A forest has come into being and maintains itself by natural processes, without being created or looked after by human beings.

    A forest is considered a natural ecosystem because it has come into being and maintains itself by natural processes, without being created or looked after by human beings. Its producers, consumers and decomposers interact with sunlight, air, water and soil on their own, so nutrients are recycled and the forest keeps itself going; nobody sows, waters, manures or weeds it. Gardens and crop-fields, by contrast, are made and maintained by people and are artificial ecosystems.

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19. Q279 CBSE 2017
    Why is a ​lake ​considered to ​be a ​natural ​ecosystem ​?
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    A lake has interacting living and non-living parts and sustains itself by natural processes, without being created or maintained by people.

    A lake is an ecosystem because its living organisms, producers (phytoplankton and water plants), consumers (zooplankton, fish) and decomposers (bacteria, fungi), interact with one another and with the non-living parts of their surroundings (water, sunlight, temperature, dissolved gases and minerals). It is a natural ecosystem because it forms and sustains itself by natural processes, with its nutrients recycled by decomposers; people do not create or maintain it, as they do a garden, a crop field or an aquarium.

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20. Q280 CBSE 2017
    Name ​any two ​man-made ​ecosystems.
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    A crop field and a garden are two man-made ecosystems.

    Two man-made (artificial) ecosystems: a crop field and a garden. (An aquarium is another.)

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