NCERT Solutions for Class 10 Science Chapter 13: Our Environment
These Class 10 Science Chapter 13 solutions cover Our Environment from the NCERT textbook (session 2026–27). The chapter explains how the living (biotic) and non-living (abiotic) parts of an ecosystem interact, how energy flows through food chains and food webs, the build-up of harmful chemicals by biological magnification, and two major environmental problems — depletion of the ozone layer and disposal of waste. Below you will find every in-text question and all end-of-chapter Exercises reproduced verbatim and answered in exam-ready style.
Chapter 13, Our Environment, studies how the components of the environment interact and how human activity disturbs that balance. An ecosystem is made of biotic components (producers, consumers and decomposers) and abiotic components (temperature, rainfall, wind, soil, minerals). Producers trap solar energy; consumers depend on producers directly or indirectly; decomposers break down dead matter and recycle nutrients to the soil. Energy moves through food chains and branching food webs, with each level called a trophic level. Energy flow is unidirectional and only about 10% passes to the next level, which limits food chains to three or four steps. Non-degradable chemicals like pesticides build up along the chain — biological magnification — harming top consumers most. The chapter ends with two human-made problems: thinning of the protective ozone layer by CFCs, and the growing burden of biodegradable and non-biodegradable waste.
Key Concepts & Definitions
Ecosystem: all the interacting organisms in an area together with the non-living constituents of the environment; it has biotic and abiotic components. Natural ecosystems include forests, ponds and lakes; gardens and crop-fields are human-made (artificial) ecosystems.
Producers (autotrophs): green plants and certain bacteria that make organic compounds (sugar, starch) from inorganic substances using sunlight in the presence of chlorophyll.
Consumers (heterotrophs): organisms that eat food made by producers, directly or indirectly — classed as herbivores, carnivores, omnivores and parasites.
Decomposers: microorganisms (bacteria and fungi) that break down dead remains and waste into simple inorganic substances returned to the soil.
Trophic level: each step or feeding level of a food chain — producers (1st), primary consumers (2nd), secondary consumers (3rd), tertiary consumers (4th).
Biological magnification: progressive accumulation of non-degradable chemicals (e.g. pesticides) at successive trophic levels, with the maximum concentration in top consumers such as humans.
Biodegradable / non-biodegradable: substances that are (or are not) broken down by biological processes such as the action of bacteria and other saprophytes.
Ozone (O3): a molecule of three oxygen atoms; in the upper atmosphere it shields Earth from harmful ultraviolet (UV) radiation.
Formation of ozone in the upper atmosphere:
O2 ⟶UV O + O
O + O2 → O3 (Ozone)
Ten per cent law (energy flow): only about 10% of the energy at one trophic level is passed on to the next; the rest is lost mostly as heat.
In-text Questions — Answers
Questions (Page 212)
1. What are trophic levels? Give an example of a food chain and state the different trophic levels in it.
ANSWERTrophic levels are the different feeding steps or levels in a food chain through which energy passes from one group of organisms to the next.Example of a food chain: Grass → Grasshopper → Frog → Snake → Hawk.The trophic levels are: First (T1) – Grass (producer); Second (T2) – Grasshopper (primary consumer / herbivore); Third (T3) – Frog (secondary consumer); Fourth (T4) – Snake (tertiary consumer); Fifth (T5) – Hawk (top consumer).
2. What is the role of decomposers in the ecosystem?
ANSWERDecomposers (bacteria and fungi) break down the dead bodies and waste products of plants and animals into simple inorganic substances.These simple substances mix back into the soil and are absorbed again by plants, so decomposers recycle nutrients and keep the natural replenishment of the soil going. They also clean the environment by removing dead organic matter; without them, dead remains and garbage would pile up and nutrients would not return to the soil.
Questions (Page 214)
1. Why are some substances biodegradable and some non-biodegradable?
ANSWERBiodegradable substances can be broken down by the action of microorganisms (bacteria, fungi) and other biological processes, because suitable enzymes exist to digest them — for example vegetable peels, paper and cloth.Non-biodegradable substances such as plastics, DDT and many synthetic materials are not broken down by these microorganisms because no enzymes can act on them. Enzymes are specific in their action, so man-made materials persist in the environment for a very long time.
2. Give any two ways in which biodegradable substances would affect the environment.
ANSWER(i) When biodegradable wastes decompose they produce a foul smell and release gases that can pollute the air.(ii) Their decomposition supports the growth of disease-causing germs and flies, and large amounts can cause unhygienic conditions; at the same time, when managed well they enrich the soil with nutrients.
3. Give any two ways in which non-biodegradable substances would affect the environment.
ANSWER(i) They persist in the environment for a very long time and cause land, water and air pollution; plastics can block drains and choke animals.(ii) Non-degradable chemicals like pesticides enter food chains and accumulate by biological magnification, harming organisms at higher trophic levels, including humans.
Questions (Page 216)
1. What is ozone and how does it affect any ecosystem?
ANSWEROzone (O3) is a molecule made of three atoms of oxygen. At ground level it is a deadly poison, but at the higher levels of the atmosphere it forms a layer that shields Earth’s surface from the Sun’s harmful ultraviolet (UV) radiation.By absorbing UV radiation, the ozone layer protects all living organisms in the ecosystem. UV rays are highly damaging — they can cause skin cancer in humans, damage the eyes, and harm crops and other organisms. So depletion of the ozone layer threatens the whole ecosystem.
2. How can you help in reducing the problem of waste disposal? Give any two methods.
ANSWER(i) Reduce, reuse and recycle: use fewer disposable items, reuse bags and containers, and recycle paper, glass, metal and plastic.(ii) Segregate waste into biodegradable and non-biodegradable; compost kitchen and garden waste and send recyclables for recycling, avoiding open burning of plastics.
NCERT Exercises — Solutions
1. Which of the following groups contain only biodegradable items?
(a) Grass, flowers and leather(b) Grass, wood and plastic(c) Fruit-peels, cake and lime-juice(d) Cake, wood and grass
ANSWERThe correct answer is (c) Fruit-peels, cake and lime-juice and (d) Cake, wood and grass. Both groups contain only biodegradable items, so either option is acceptable.Option (a) contains leather (which decomposes very slowly and is often treated as non-biodegradable) and option (b) contains plastic, which is non-biodegradable. In groups (c) and (d) every item — fruit-peels, cake, lime-juice, wood and grass — is broken down by microorganisms, so they are fully biodegradable.
2. Which of the following constitute a food-chain?
(a) Grass, wheat and mango(b) Grass, goat and human(c) Goat, cow and elephant(d) Grass, fish and goat
ANSWERThe correct answer is (b) Grass, goat and human.A food chain needs a producer followed by consumers feeding one upon the other. Here grass (producer) → goat (herbivore, primary consumer) → human (consumer). Option (a) lists only producers; option (c) lists only herbivores at the same level; option (d) is not a valid chain because a goat does not eat fish.
3. Which of the following are environment-friendly practices?
(a) Carrying cloth-bags to put purchases in while shopping(b) Switching off unnecessary lights and fans(c) Walking to school instead of getting your mother to drop you on her scooter(d) All of the above
ANSWERThe correct answer is (d) All of the above.Carrying cloth bags reduces plastic waste, switching off unused lights and fans saves energy, and walking instead of using a vehicle saves fuel and cuts pollution. All three reduce our impact on the environment.
4. What will happen if we kill all the organisms in one trophic level?
ANSWERKilling all the organisms in one trophic level disturbs the entire ecosystem because every level is interdependent.The level above would starve from lack of food, and the level below would over-multiply because its predators are gone. For example, if all the herbivores (deer, goats) in a food chain are killed, the carnivores that eat them die of starvation, while the producers (plants) grow uncontrollably. This breaks the balance of nature, and once a trophic level is destroyed its organisms cannot be brought back.
5. 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?
ANSWERYes, the impact of removing a trophic level is different for different levels.Removing the producers is most damaging, as all other levels depend on them for energy and the whole ecosystem would collapse. Removing herbivores starves the carnivores and lets plants overgrow. Removing top carnivores causes the herbivore population to rise sharply, which then destroys the vegetation.No, the organisms of no trophic level can be removed without causing damage, because all trophic levels are interlinked and removing any one upsets the natural balance of the ecosystem.
6. What is biological magnification? Will the levels of this magnification be different at different levels of the ecosystem?
ANSWERBiological magnification is the progressive increase in the concentration of harmful, non-degradable chemicals (such as pesticides) in the bodies of organisms at each successive trophic level of a food chain.Yes, the level of magnification is different at different trophic levels. The concentration of these chemicals goes on increasing from lower trophic levels to higher ones, so it is lowest in producers and highest in the top consumers (such as humans). This is because these chemicals are not degradable and keep accumulating as we move up the chain.
7. What are the problems caused by the non-biodegradable wastes that we generate?
ANSWERNon-biodegradable wastes such as plastics, glass, metals and synthetic chemicals create many problems:(i) They are not broken down by microorganisms, so they persist in the environment for a very long time and cause land, water and air pollution.(ii) Chemicals like pesticides enter food chains and accumulate by biological magnification, harming organisms including humans.(iii) Plastics choke drains and water bodies, and animals may die after swallowing them.(iv) They reduce soil fertility and may release toxic gases when burnt.
8. If all the waste we generate is biodegradable, will this have no impact on the environment?
ANSWERNo, even if all our waste were biodegradable it would still affect the environment.Large amounts of biodegradable waste take time to decompose and, while decomposing, produce foul smells and gases that pollute the air. The decaying matter promotes the breeding of flies, mosquitoes and disease-causing germs, creating unhygienic conditions. So a huge load of biodegradable waste, if not managed properly, still causes pollution and health hazards.
9. Why is damage to the ozone layer a cause for concern? What steps are being taken to limit this damage?
ANSWERWhy it is a concern: The ozone layer shields Earth from the Sun’s harmful ultraviolet (UV) radiation. Damage to it allows more UV rays to reach the surface, which can cause skin cancer, cataracts (eye damage) and a weakened immune system in humans, and can also harm crops, plants and aquatic life. So the entire ecosystem is at risk.Steps being taken: The decrease in ozone was linked to synthetic chemicals like chlorofluorocarbons (CFCs) used in refrigerants and fire extinguishers. In 1987 the United Nations Environment Programme (UNEP) forged an agreement to freeze CFC production at 1986 levels. It is now mandatory for manufacturing companies worldwide to make CFC-free refrigerators and products.
Extra Practice Questions
Short Answer Type Questions
Q1. Define an ecosystem and name its two main components.
ANSWERAn ecosystem is all the interacting organisms in an area together with the non-living parts of their environment. Its two components are biotic (living organisms — producers, consumers, decomposers) and abiotic (physical factors like temperature, rainfall, soil and minerals).
Q2. Why are food chains generally limited to three or four trophic levels?
ANSWERBecause only about 10% of the energy at one trophic level passes to the next; the rest is lost mostly as heat. After three or four steps so little usable energy remains that it cannot support another level, so the food chain ends.
Q3. State two differences between a food chain and a food web.
ANSWERA food chain is a single straight-line sequence of organisms feeding one upon another, where each organism usually eats only one type below it. A food web is a network of many interconnected food chains, where each organism is eaten by two or more kinds of organisms, giving more feeding choices and greater stability.
Q4. Why is the flow of energy in an ecosystem said to be unidirectional?
ANSWEREnergy captured by producers moves to herbivores, then to carnivores, and is finally lost as heat. It never returns to the previous level or back to the Sun, so it flows only in one direction — from producers to consumers to decomposers.
Q5. Name one natural and one artificial ecosystem.
ANSWERNatural ecosystem — a forest, pond or lake. Artificial (human-made) ecosystem — a garden, crop-field or aquarium.
Long Answer Type Questions
Q1. Explain energy flow through a food chain using the ten per cent law, and state two conclusions drawn from the energy-flow diagram.
ANSWERGreen plants capture about 1% of the sunlight falling on their leaves and convert it into food energy. When a herbivore eats a plant, only about 10% of that food energy is built into its body and made available to the next level; the rest is used in respiration, digestion and movement, and lost as heat. The same 10% rule applies at every step, so energy decreases sharply up the chain. Two conclusions from the energy-flow diagram are: (i) energy flow is unidirectional — energy passes from the Sun to producers to consumers and never returns; and (ii) the energy available diminishes progressively at each higher trophic level because of losses at every step. This is why food chains rarely have more than four levels.
Q2. What is biological magnification? Explain how pesticides enter and accumulate in a food chain, and why humans are most affected.
ANSWERBiological magnification is the progressive increase in concentration of non-degradable chemicals at successive trophic levels. Pesticides sprayed on crops are washed into the soil and water bodies. From the soil, plants absorb them along with water and minerals; in water, aquatic plants and animals take them up. Since these chemicals are not broken down (non-degradable), they are passed on to herbivores that eat the plants, then to the carnivores that eat the herbivores, accumulating more at each step. As humans occupy the highest trophic level in most food chains, the maximum concentration of these chemicals collects in our bodies. That is why our food grains, vegetables, fruits and even meat contain pesticide residues, which often cannot be removed by washing.
Q3. Describe how human activities have damaged the ozone layer and the management of waste, and suggest measures to reduce both problems.
ANSWERThe ozone layer in the upper atmosphere protects Earth from harmful UV radiation. From the 1980s its amount dropped sharply because of synthetic chemicals, mainly chlorofluorocarbons (CFCs) used as refrigerants and in fire extinguishers; the resulting thinning lets in more UV rays that cause skin cancer and harm crops and aquatic life. The remedy is to stop using ozone-depleting chemicals — the UNEP agreement of 1987 froze CFC production, and CFC-free products are now mandatory. Waste is the second problem: improvements in lifestyle and packaging have increased both biodegradable and non-biodegradable waste, causing pollution and health hazards. It can be managed by segregating waste at source, composting biodegradable waste, recycling and reusing materials, reducing the use of plastics and disposable items, and treating sewage and industrial effluents before release. Following the 3Rs — reduce, reuse, recycle — helps cut both problems.
MCQs & Assertion–Reason
1. The non-living physical factors of an ecosystem such as soil, temperature and rainfall are called:
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Decomposers are essential for an ecosystem.
Reason: They break down dead organic matter into simple substances that are returned to the soil and reused by plants.
A-R 2. Assertion: Food chains usually do not have more than four trophic levels.
Reason: Only about 10% of energy is transferred to the next level, so little usable energy remains after a few steps.
A-R 3. Assertion: The maximum concentration of pesticides is found in producers.
Reason: Non-degradable chemicals accumulate progressively at each higher trophic level.
A-R 4. Assertion: Ozone is harmful at ground level but beneficial in the upper atmosphere.
Reason: Upper-atmosphere ozone absorbs the Sun’s ultraviolet radiation and protects living organisms.
A-R 5. Assertion: The flow of energy in an ecosystem is unidirectional.
Reason: Energy that passes from one trophic level to the next can flow back to the previous level.
Answer key: 1-(A), 2-(A), 3-(D), 4-(A), 5-(C).
Common Mistakes & Exam Tips
Common mistakes to avoid
Confusing biotic (living) with abiotic (non-living) components — soil, water and temperature are abiotic.
Writing an invalid food chain — it must start with a producer and each organism must actually eat the one before it.
Saying energy cycles in an ecosystem — energy flow is unidirectional; only nutrients are recycled.
Thinking biological magnification is highest at the bottom — it is highest in top consumers like humans.
Calling all slow-decomposing items biodegradable — plastics and DDT are non-biodegradable.
Mixing up ground-level ozone (a poison) with upper-atmosphere ozone (a protective shield).
How to score full marks in this chapter
Always support trophic-level answers with a clear example food chain and label each level. State the 10% law precisely when explaining energy flow, and give the two conclusions (unidirectional + progressive decrease). For biological magnification, name pesticides, trace their path soil/water → plants → consumers → humans, and conclude with humans being most affected. For ozone, mention O3, UV protection, CFCs and the 1987 UNEP agreement. Use exact terms — producers, consumers, decomposers, biodegradable, non-biodegradable — to gain full marks.
Frequently Asked Questions
What is Class 10 Science Chapter 13 Our Environment about?
It explains how biotic and abiotic components of an ecosystem interact, how energy flows through food chains and food webs, how harmful chemicals build up by biological magnification, and two major environmental problems — ozone-layer depletion and waste disposal.
What is the 10% law of energy flow?
The 10% law states that on average only about 10% of the energy present at one trophic level is passed on to the next; the rest is lost mostly as heat. This limits food chains to three or four trophic levels.
Why does the ozone layer matter for the ecosystem?
Ozone in the upper atmosphere absorbs the Sun’s harmful ultraviolet (UV) radiation. Its depletion lets more UV reach Earth, causing skin cancer, eye damage and harm to crops and aquatic life, so it threatens the whole ecosystem.
Are these Class 10 Science Chapter 13 solutions free?
Yes. All solutions are free and follow the official NCERT Science textbook for session 2026–27, with every in-text and exercise question answered.
