1
Which of these is a human-made ecosystem?
💡 Hint:
A garden is in the same class.
Explanation:
A crop-field and a garden are human-made. A forest, a pond and a lake are natural.
Class 10 · Science · Chapter 13 · बिहार बोर्ड (BSEB)CBSE · NCERT 2026-27
Our Environment
Our Environment
How to use this page:
1. Read — 13.1 aquarium · 13.2 chain · 13.3 pesticides · 13.4 ozone · 13.5–13.6 waste · 13.7 home · 13.8 sewage · 13.9 e-waste, diagram, worked example, board tip
2. Check — each lesson has its own questions; the number follows the lesson
3. Mastery ★ — all of that lesson correct. Redo the wrong ones
4. Use the food-chain energy figure — 1000 J to 100 J, then 10 J, then 1 J. About ten percent remains at each arrow.
In NCERT 2026-27 this is chapter 13. In the older 16-chapter Bihar book Our Environment is chapter 15. That book’s chapter 14 is Sources of Energy. Progress stays in this browser.
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- 1 The ecosystem and an aquarium — Activity 13.1
- 2 The food chain and ten percent — Activity 13.2
- 3 Biological magnification — Activity 13.3
- 4 The ozone layer — Activity 13.4
- 5 Biodegradable and not — Activities 13.5, 13.6
- 6 Household waste and sewage — Activities 13.7, 13.8
- 7 E-waste — Activity 13.9
- Chapter winner — every lesson at mastery ★
🎬 Shorts
This chapter has no Short on the channel yet, so ten empty cards are not shown. The notes, diagrams and checks are complete.
📄 PDF download — notes + answer key
The ecosystem and an aquarium — Activity 13.1
पारितंत्र और एक्वेरियम — क्रियाकलाप 13.1 · NCERT 13.1 · Activity 13.1
Living things and their surroundings togetherNotes
The interacting organisms of an area, together with the non-living parts of the environment, form an ecosystem. Plants, animals, microorganisms and human beings are the biotic components. Temperature, rainfall, wind, soil and minerals are the abiotic components.
In a garden, grasses, trees, rose, jasmine, sunflower, frogs, insects and birds affect one another. Forests, ponds and lakes are natural ecosystems. Gardens and crop-fields are human-made.
Activity 13.1 — set up an aquariumActivity
A fish needs room to swim, water, oxygen and food. Oxygen can come from a pump (an aerator) and fish food is sold in the market. Add a few aquatic plants and animals and the jar can become a system that sustains itself. An aquarium is an example of a human-made ecosystem.
It cannot be left as it is. It has to be cleaned after some time. A pond or a lake is not scrubbed the same way every week, because there the cycle of producers, consumers and decomposers is already joined on a large scale.
Green plants and some bacteria are producers. With chlorophyll and sunlight they make sugar and starch. Organisms that take this food directly, or by eating other consumers, are consumers — herbivores, carnivores, omnivores and parasites. Bacteria and fungi break dead bodies into simple inorganic substances. These are the decomposers. Without them garbage and dead organisms would stay piled up and the soil would not be replenished.
Worked exampleExample
Question: An aquarium has to be cleaned. Why is a lake not treated the same way?
Answer: A small jar goes out of balance quickly. Waste collects and oxygen falls. In a lake, plants keep making food and oxygen and decomposers keep returning dead matter, so it does not have to be emptied again and again like an aquarium.
10-second revision
- Ecosystem = biotic components + abiotic components
- An aquarium is human-made; a pond is natural
- Without decomposers the soil is not replenished
Board tip · BSEBBoard tip
For producers write both green plants and some bacteria. Only “plants” stays incomplete.
Board tip · CBSEBoard tip
Remember the pair: forest, pond and lake are natural; garden and crop-field are human-made.
Check your understandingall correct = mastery ★
2
Decomposers break complex organic substances into simple inorganic substances.
Explanation:
True — bacteria and fungi do this and the matter returns to the soil.
3
The pump that supplies oxygen in an aquarium is called an ______.
💡 Hint:
The device in Activity 13.1.
Explanation:
An aerator, or oxygen pump.
4
Who breaks dead organisms into simple substances?
💡 Hint:
Bacteria and fungi.
Explanation:
Decomposers break them down. Without them the garbage stays piled up.
5
Define an ecosystem and give one example each of a biotic and an abiotic component.
Model answer (board format)
All the interacting organisms of an area together with the non-living parts form an ecosystem. A biotic example is a frog or grass. An abiotic example is temperature, rainfall or soil.
Check your answer:
The food chain and ten percent — Activity 13.2
खाद्य शृंखला और दस प्रतिशत — क्रियाकलाप 13.2 · NCERT 13.1.1 · Activity 13.2 · ten percent
Activity 13.2 — who eats whomActivity
While making the aquarium, do not put in an aquatic animal that would eat the others. If you do, one organism may be left and the chain breaks. In a group, discuss how producers, consumers and decomposers depend on one another.
Write the aquatic organisms in the order of who eats whom. Make a chain of at least three steps. No one group is the only important one. Producers bring energy in and decomposers return matter. Remove one and the others fall too.
Trophic levels and ten percentNotes
This link of organisms feeding on one another is a food chain. Each step is a trophic level. Producers are at the first level. Herbivores, the primary consumers, are at the second, small carnivores at the third, and larger carnivores at the fourth.
Green plants turn about 1 percent of the sunlight on their leaves into food. The next level, on average, turns 10 percent of the food eaten into its own body. The rest leaves as heat, and some is used in digestion and in work. So a chain is usually three or four steps. There are more individuals at the lower levels, and the greatest number is of the producers.
The flow of energy is one way. Energy that reaches the herbivores does not come back to the plants, and the energy of the plants does not return to the sunlight. Each organism is usually eaten by two or more kinds, so instead of one straight chain there is a branching food web.
Worked exampleExample
Question: 100000 J of sunlight falls on the leaves. Producers turn 1 percent of it into food. How much energy is left at the third and the fourth trophic level?
Calculation: food = 1/100 × 100000 J = 1000 J (first level, producers).
Second level = 10/100 × 1000 J = 100 J.
Third level = 10/100 × 100 J = 10 J.
Fourth level = 10/100 × 10 J = 1 J.
That is the order in the figure: 1000 J → 100 J → 10 J → 1 J.
10-second revision
- 1 percent of sunlight becomes food; the next level gets about 10 percent
- The flow is one way and the chain stays short
- A food web is the branches of many chains
Board tip · BSEBBoard tip
1 percent and 10 percent are different numbers. The sunlight figure is 1, the food figure is 10.
Board tip · CBSEBoard tip
Energy falls as you go up. The amount of pesticide rises as you go up. Do not join the two in one sentence.
Check your understandingall correct = mastery ★
1
Which of the following is a food chain?
💡 Hint:
A producer, then a herbivore, then the next consumer.
Explanation:
Grass → goat → human is a chain. Grass, wheat and mango are all producers.
2
The flow of energy in a food chain is one way.
Explanation:
True — energy that reaches the herbivores does not come back to the plants.
3
The next trophic level receives on average ______ percent of the food energy.
💡 Hint:
The one percent of sunlight is a different number.
Explanation:
About 10 percent.
4
About what part of the sunlight on the leaves do plants turn into food?
💡 Hint:
Ten percent belongs to the next level.
Explanation:
About 1 percent. Ten percent is the share that reaches the next consumer.
5
Producers have 5000 J. How much energy reaches the secondary consumer? Show the steps.
Model answer (board format)
The primary consumer gets 10/100 × 5000 J = 500 J. The secondary consumer gets 10/100 × 500 J = 50 J. Ten percent goes on at each step.
Check your answer:
6
A food web forms because —
💡 Hint:
Branches instead of one straight line.
Explanation:
Each organism is usually eaten by two or more kinds, so a web forms.
Biological magnification — Activity 13.3
जैव आवर्धन — क्रियाकलाप 13.3 · After NCERT 13.1.1 · Activity 13.3
A chemical that does not break down collects as it goes upNotes
Chemicals used to protect a crop from pests are washed into the soil or the water. Plants take them up with water and minerals. Aquatic plants and animals take them from the water. These substances are not degraded, so the amount grows at each trophic level.
Humans occupy the top of any food chain, so the amount in the human body is the greatest. This is biological magnification. Wheat, rice, vegetables, fruits and meat can carry pesticide residues. Washing or ordinary methods do not always remove them.
Energy falls as you go up. The concentration of these chemicals rises as you go up. Remember both together.
Activity 13.3 — a newspaper report and a banActivity
Reports of pesticide levels in ready-made food keep appearing, and some states have banned such products. In a group, debate whether a ban is needed.
Where did the pesticide in these foods come from? Can the same path reach the body through other foods too? Also write ways to cut down what we take in — less spraying at the source, and a preference for food from lower in the chain, because the amount has already grown at the top.
Worked exampleExample
Question: In a pond a pesticide is low in the grass, higher in the fish and highest in the bird that eats the fish. What is the name, and why?
Answer: This is biological magnification. The chemical does not break down, so each eater also takes in the amount already stored in the previous one. The bird is at the top, so the concentration there is the highest.
10-second revision
- A chemical that does not break down collects at each level
- Humans are at the top, so the amount is the greatest
- Washing does not always remove the residue
Board tip · BSEBBoard tip
Biological magnification and the ten percent law point opposite ways. Write both directions in the answer.
Board tip · CBSEBoard tip
Writing wheat, rice, vegetables and meat makes the answer feel complete.
Check your understandingall correct = mastery ★
1
In biological magnification, where is the amount of a harmful chemical the greatest?
💡 Hint:
Humans are usually that place.
Explanation:
The amount is greatest at the top.
2
Washing food always removes a pesticide residue.
Explanation:
False — the book says washing or other ordinary means do not always remove it.
3
Define biological magnification. How is its direction different from the ten percent energy law?
Model answer (board format)
Chemicals that do not break down become more concentrated at each next trophic level. Humans are at the top, so the amount in them is the greatest. Energy falls to about ten percent at each level. The amount of the chemical rises.
Check your answer:
4
A pesticide enters the chain when —
💡 Hint:
The source in Activity 13.3.
Explanation:
The chemical is washed into soil or water and plants absorb it.
The ozone layer — Activity 13.4
ओज़ोन परत — क्रियाकलाप 13.4 · NCERT 13.2.1 · Activity 13.4 · O2 and O3
Ozone at the ground and the layer aboveCaution
A molecule of ozone is three oxygen atoms. Ordinary oxygen, O2, is necessary for aerobic life. Ozone itself is a poison. Near the ground it should not be breathed.
High in the atmosphere this same ozone shields the Earth from the ultraviolet (UV) rays of the Sun. These rays harm organisms and can cause skin cancer in humans. The place is different and the job is different. Do not treat the two as one thing because the molecule has one name.
UV splits oxygenNotes
Ozone high up is formed by the action of UV on oxygen. Higher-energy UV splits some O2 molecules into free oxygen atoms.
O2 UV→ 2O
These atoms join molecular oxygen and make ozone.
O2 + O → O3
The net of the two steps is that three oxygen molecules give two ozone molecules.
3O2 → 2O3
Writing O2 → O3 with coefficients 1 and 1 is not balanced. In the balanced net equation the coefficient of O2 is 3 and the coefficient of O3 is 2.
Activity 13.4 — CFCs and the holeActivity
In the 1980s the ozone in the atmosphere fell sharply. The cause is synthetic chemicals such as chlorofluorocarbons (CFCs). They were used in refrigerators and in 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 companies to make CFC-free refrigerators.
From the library or a newspaper, find which chemicals thin the layer, whether the rules have reduced the damage, and whether the size of the hole has changed in recent years.
Worked exampleExample
Question: Which of O2 + O → O3 and 3O2 → 2O3 is balanced? What does UV do?
Answer: Both are balanced. In the first, oxygen atoms are 2 + 1 = 3 and O3 has 3. In the second, the left has 3 × 2 = 6 and the right has 2 × 3 = 6. The job of UV is to split O2 into free O atoms. Do not write O2 → O3 as balanced without the coefficients 3 and 2.
10-second revision
- High ozone blocks UV; ozone near the ground is a poison
- UV splits O2; then O2 + O → O3; net 3O2 → 2O3
- CFCs are refrigerants; UNEP in 1987 froze them at 1986 levels
Board tip · BSEBBoard tip
Forgetting the coefficients is a common slip. Write 3O2 and 2O3 together.
Board tip · CBSEBoard tip
The skin-cancer reason is UV, not the name ozone itself. When the layer thins, UV increases.
Check your understandingall correct = mastery ★
1
High in the atmosphere the main job of ozone is —
💡 Hint:
Near the ground it is a poison.
Explanation:
High ozone shields us from UV. Near the ground the same molecule is poisonous.
2
O2 → O3 with coefficients 1 and 1 is a balanced equation.
Explanation:
False — two oxygen atoms on the left and three on the right. The balanced net form is 3O2 → 2O3.
3
In 1987 CFC production was frozen at the ______ level.
💡 Hint:
The UNEP agreement.
Explanation:
At the 1986 level.
4
Write the two steps of ozone formation and the net equation. Where does UV act?
Model answer (board format)
UV splits O2 into free oxygen atoms: O2 → 2O. Then O2 + O → O3. The net equation is 3O2 → 2O3.
Check your answer:
5
CFCs were mainly used —
💡 Hint:
The layer thinned because of these.
Explanation:
CFCs were used as refrigerants and in fire extinguishers.
Biodegradable and not — Activities 13.5, 13.6
जैव और अजैव निम्नीकरण — क्रियाकलाप 13.5, 13.6 · NCERT 13.2.2 · Activities 13.5, 13.6
Activity 13.5 — bury the waste and watchActivity
Collect one day of waste from home: spoilt food, vegetable peels, used tea leaves, milk packets, empty cartons, paper, empty medicine bottles and strips, torn clothes and broken footwear.
Bury it in a pit in the garden. If there is no space, keep it in an old bucket or a flower pot and cover it with at least 15 cm of soil. Keep it moist. Look every 15 days.
Peels and leftover food change form, and they change soon. Plastic, medicine strips and footwear stay almost the same for a long time.
Enzymes are specificNotes
Each enzyme in the body breaks one particular substance. That is why eating coal gives no energy. For the same reason, human-made materials such as plastics are not broken by bacteria or other saprophytes in ordinary conditions. Heat and pressure can change them, but in our surroundings they stay for a very long time.
Substances broken by biological processes are biodegradable. Those that are not broken this way are non-biodegradable. They may lie inert for a long time or they may harm members of the ecosystem.
Activity 13.6 — how many years it will lastActivity
From the library or the internet, find how long various non-biodegradable substances can last in the environment. Some new plastics are now called biodegradable. Read about them too, and see whether they harm the environment or not. This check is needed before the name is trusted.
Worked exampleExample
Question: Why is plastic not biodegradable?
Answer: Enzymes are specific. The enzymes of bacteria cannot break plastic at ordinary temperature and pressure. So it stays for a long time. A peel is broken by these same organisms.
10-second revision
- 15 cm of soil and an observation every 15 days
- Enzymes are specific, so coal and plastic are not broken
- Biodegradable material breaks down; the other kind stays for a long time
Board tip · BSEBBoard tip
Leather breaks slowly; plastic does not in ordinary conditions. When the question asks which group has a non-biodegradable item, pick the group with plastic.
Board tip · CBSEBoard tip
Both 15 cm and 15 days belong to the activity. Do not write one in place of the other.
Check your understandingall correct = mastery ★
1
Which group contains a non-biodegradable item?
💡 Hint:
Plastic does not break down in ordinary conditions.
Explanation:
Plastic is non-biodegradable. Peels, cake and grass are biodegradable.
2
Eating coal gives the body energy, because enzymes break everything.
Explanation:
False — enzymes are specific. Coal is not broken and no energy is gained.
3
Cover the waste in the pit or the pot with at least ______ cm of soil.
💡 Hint:
The watching interval is also 15 days.
Explanation:
15 cm of soil. The observation is every 15 days.
4
Substances broken down by biological processes are called —
💡 Hint:
A peel is in this class.
Explanation:
They are called biodegradable.
5
Why are some substances biodegradable and some not? Give one example of each.
Model answer (board format)
Enzymes are specific, so organisms can break only some substances. A vegetable peel is biodegradable. Plastic does not break down in ordinary conditions, so it is non-biodegradable.
Check your answer:
6
If a new plastic is labelled biodegradable, it is enough to accept it without a check.
Explanation:
False — Activity 13.6 says to find out whether they cause harm or not.
Household waste and sewage — Activities 13.7, 13.8
घर का कचरा और सीवेज — क्रियाकलाप 13.7, 13.8 · NCERT 13.2.2 · Activities 13.7, 13.8
The way we live increases wasteNotes
Convenience and new packaging have increased what we throw away. Many things are used once and then go into the bin. So the disposal problem is not solved only by “someone will collect it”. First see how much waste is made and how much of it organisms can break down.
Activity 13.7 — the account of home and classroomActivity
Find out where the waste of the home goes and whether anyone collects it. Find whether the panchayat, the municipal corporation or the resident welfare association treats biodegradable and non-biodegradable waste separately.
Work out how much waste the home makes in a day and how much of it is biodegradable. Do the same for the classroom. Suggest ways to deal with it.
Two bins
Wet waste — peels, leftover food — is biodegradable. Keep dry waste — plastic, glass, metal foil — in a separate bin. Packaging and things thrown after one use increase non-biodegradable waste.
Tea on trains used to come in plastic glasses that were returned. Disposable cups arrived for hygiene, but millions began to be thrown away every day. Making clay kulhads on a large scale would use up fertile top-soil. Paper cups are better than plastic cups because paper is biodegradable.
Activity 13.8 — sewage and factoriesActivity
Find out how the sewage of your locality is treated. Is there a mechanism so that untreated sewage does not pollute local water bodies.
Find how the local industries treat their waste. There should also be a mechanism so that this waste does not pollute the soil and the water.
Worked exampleExample
Question: Write two ways to reduce the problem of waste disposal.
Answer: Put wet and dry waste in separate bins, so the biodegradable part can become compost. Carry a cloth bag, so plastic bags do not increase. Do not let untreated sewage into a pond.
10-second revision
- Wet and dry waste in separate bins
- Sewage should reach a water body only after treatment
- A kulhad uses fertile soil; a paper cup is better than a plastic cup
Board tip · BSEBBoard tip
In a disposal answer write both separation and producing less. Only “burn it” is not enough.
Board tip · CBSEBoard tip
Environment-friendly habits: a cloth bag, switching off an idle fan, walking to school. All three are right.
Check your understandingall correct = mastery ★
1
Which is an environment-friendly habit?
💡 Hint:
All three cut waste or fuel.
Explanation:
All three are environment-friendly.
2
Letting untreated sewage into the local pond is safe.
Explanation:
False — Activity 13.8 asks for a mechanism that stops this pollution.
3
Keep wet waste such as peels in a different bin from ______ waste.
💡 Hint:
Two bins.
Explanation:
Separate from dry waste or plastic.
4
Write one comment each on kulhads and on paper cups as replacements for disposable plastic cups.
Model answer (board format)
If clay kulhads are made on a large scale, fertile top-soil is lost. A paper cup is better than a plastic cup because paper is biodegradable and does not build a daily heap of plastic.
Check your answer:
5
An example of a local body is —
💡 Hint:
Activity 13.7 asks these about the waste system.
Explanation:
A panchayat, a municipal corporation or a resident welfare association is a local body.
E-waste — Activity 13.9
ई-कचरा — क्रियाकलाप 13.9 · NCERT Activity 13.9 · hazardous materials
Activity 13.9 — throwing a phone and a circuitActivity
From the internet or the library, find which hazardous materials have to be dealt with while disposing of electronic items. How would these materials affect the environment.
The search usually turns up poisonous metals in batteries and circuit boards, such as lead and mercury. Thrown in the open, they can reach the soil and the water. Also ask about recycling plastics: whether the process itself leaves smoke or leftover waste.
Even if all waste were biodegradable, an effect would remainNotes
Even if all household waste were biodegradable, a heap can still overload the decomposers. Smell, disease and a shortage of space remain. So producing less and separating the waste are both needed.
If every organism of one trophic level is killed, the next level gets no food and the organisms of the previous level may increase. If the producers are removed, the whole chain falls. No level can be removed without harm.
Worked exampleExample
Question: Why should a broken mobile phone not go into the wet-waste bin?
Answer: It can contain plastic and the hazardous materials of a battery. With the wet waste these would join the compost or the soil. E-waste should be collected separately, so that poisonous metals do not reach water and soil and the plastic is recycled in a controlled way.
10-second revision
- E-waste can carry poisonous metals, such as lead and mercury
- Do not leave it in the wet waste or in a pond
- A heap can still cause harm even if all waste is biodegradable
Board tip · BSEBBoard tip
In an e-waste answer write both the material and the path: the metal, then the soil or the water.
Board tip · CBSEBoard tip
In the “remove one level” question, write the effect of removing producers and of removing the top consumer separately.
Check your understandingall correct = mastery ★
1
E-waste thrown in the open is harmful because —
💡 Hint:
Activity 13.9 asks about hazardous materials.
Explanation:
Metals such as lead and mercury can reach the soil and the water.
2
If all the waste we make is biodegradable, there will be no effect on the environment.
Explanation:
False — a heap can still bring smell, disease and a shortage of space.
3
What happens if all the organisms of one trophic level are removed? Write the producer level separately.
Model answer (board format)
The next level gets no food and the organisms of the previous level may increase. If the producers are removed, the entry of energy stops and the whole chain is damaged. No level can be removed without harm.
Check your answer:
4
Batteries often contain poisonous metals such as lead and ______.
💡 Hint:
The search in Activity 13.9.
Explanation:
Lead and mercury. They should not be thrown in the open.
❓ Full question bank — with answers and explanations — 67 questions
Pick a type. The 35 lesson checks are separate — each lesson has as many as its topic needs. All correct earns mastery ★.
BSEB modelpractice · not annual60 tried · 0 correctMultiple choiceMCQ140 tried · 0 correctTrue / falseTrue / false80 tried · 0 correctFill in the blanksFill80 tried · 0 correctMatchMatch20 tried · 0 correctAssertion–reasonAssertion–reason50 tried · 0 correctBalancingBalancing40 tried · 0 correctIdentifyIdentify20 tried · 0 correctShort and longVSA, SA, LA120 tried · 0 correctCBSE-stylenot a PYQ60 tried · 0 correct
No question is marked as a verified past paper. The BSEB set is a model for practice. CBSE items are CBSE-style, not a copy of any year’s paper.
Multiple choice
0/14Pick one option. A wrong try brings a hint.
1
An abiotic component of an ecosystem is —
💡 Hint:
Organisms are biotic.
Explanation:
Soil is abiotic. A frog, a fungus and grass are biotic.
2
A correct example of a food chain is —
💡 Hint:
A feeding relation in order.
Explanation:
Grass → goat → human is a chain.
3
The part of sunlight on the leaves that becomes food is about —
💡 Hint:
Ten percent belongs to the next level.
Explanation:
About 1 percent.
4
From 1000 J in the producers, the tertiary consumer receives —
💡 Hint:
Ten percent, three times.
Explanation:
1000 → 100 → 10 → 1 J. The tertiary consumer gets 1 J.
5
In biological magnification the highest concentration is in —
💡 Hint:
Humans are usually at the top.
Explanation:
The amount is greatest in the top consumer.
6
Ozone near the ground —
💡 Hint:
The layer above has a different job.
Explanation:
Near the ground ozone is a poison. Up above it blocks UV.
7
The high ozone layer protects against —
💡 Hint:
Skin cancer is linked to UV.
Explanation:
The layer protects against UV rays.
8
UNEP froze CFC production —
💡 Hint:
The agreement is of 1987.
Explanation:
In 1987 it was frozen at the 1986 level.
9
A non-biodegradable item is —
💡 Hint:
Enzymes do not break it in ordinary conditions.
Explanation:
A plastic bag is non-biodegradable.
10
When the waste is buried, the soil cover should be at least —
💡 Hint:
The watching interval is 15 days.
Explanation:
At least 15 cm of soil.
11
A paper cup is better than a plastic cup because —
💡 Hint:
The soil point belongs to the kulhad.
Explanation:
Paper is biodegradable, so it is better than plastic.
12
Untreated sewage is harmful because —
💡 Hint:
Activity 13.8.
Explanation:
Untreated sewage pollutes local water.
13
E-waste can contain the hazard of —
💡 Hint:
Batteries and circuits.
Explanation:
Metals such as lead and mercury are hazardous.
14
If decomposers are removed —
💡 Hint:
Dead matter stays piled up.
Explanation:
The soil will not receive the simple substances.
True or false
0/81
A garden is a natural ecosystem.
Explanation:
False — a garden and a crop-field are human-made. A forest is natural.
2
In a food chain, energy returns to the previous level.
Explanation:
False — the flow is one way.
3
The amount of pesticide in a human can be higher than in an organism lower in the chain.
Explanation:
True — this is biological magnification.
4
Ozone near the ground is the safe form of the ozone layer.
Explanation:
False — near the ground it is a poison. The protection is the layer high up.
5
UV rays can split O2 into free oxygen atoms.
Explanation:
True — then O2 + O makes O3.
6
Plastic is quickly broken by bacteria in ordinary conditions.
Explanation:
False — enzymes are specific and plastic stays.
7
If kulhads are made on a large scale, fertile top-soil can be lost.
Explanation:
True — the book asks you to think about this.
8
All the organisms of one trophic level can be removed without any harm.
Explanation:
False — the next level goes hungry, and if producers are removed the whole chain falls.
Fill in the blanks
0/81
Green plants and some bacteria are called ______.
💡 Hint:
Producers.
Explanation:
They make food.
2
The next level receives about ______ percent of the energy.
💡 Hint:
10 percent.
Explanation:
The sunlight figure is 1.
3
The branching link of many chains is called a food ______.
💡 Hint:
A food web.
Explanation:
One straight link is a chain.
4
The formula of ozone is ______.
💡 Hint:
O3.
Explanation:
Three oxygen atoms.
5
The 1987 agreement is linked to the body called ______.
💡 Hint:
UNEP.
Explanation:
The environment programme.
6
A substance that is not broken by biological processes is called ______.
💡 Hint:
Non-biodegradable.
Explanation:
Plastic is an example.
7
Observe the buried waste every ______ days.
💡 Hint:
15 days.
Explanation:
The soil is also 15 cm.
8
One example of a hazardous metal in a broken battery is ______.
💡 Hint:
Lead or mercury.
Explanation:
Activity 13.9.
Match
0/21
Match the word with its meaning.
Column B: A. Breaking dead matter · B. One step of a chain · C. Branching links · D. Making food
1. Producer
2. Decomposer
3. Trophic level
4. Food web
Explanation:
A producer makes food, a decomposer breaks matter, a trophic level is one step, and a web is branches.
2
Match the ozone fact with the right place.
Column B: A. A poison · B. Shield against UV · C. A refrigerant · D. Splitting O2
1. O3 near the ground
2. The high layer
3. CFCs
4. The first job of UV
Explanation:
A poison at the ground, a shield above, CFCs as refrigerants, and UV splits oxygen.
Assertion–reason
0/5Check both statements, then see whether the reason explains the assertion.
1
Assertion (A): The flow of energy in a food chain is one way.
Reason (R): Energy that has reached the herbivores does not return to the plants.
Explanation:
Both are true and R is the correct explanation.
2
Assertion (A): The amount of pesticide in the human body can be high.
Reason (R): A chemical that does not break down collects at each next trophic level.
Explanation:
Both are true and R explains A as biological magnification.
3
Assertion (A): The ozone layer protects against ultraviolet rays.
Reason (R): Ozone near the ground is safe to breathe.
Explanation:
A is true. R is false — ozone near the ground is a poison.
4
Assertion (A): Plastic is biodegradable.
Reason (R): Enzymes are specific and do not break every substance.
Explanation:
A is false. R is true — this is why plastic does not break down in ordinary conditions.
5
Assertion (A): An aquarium has to be cleaned from time to time.
Reason (R): An aquarium is a natural lake.
Explanation:
A is true. R is false — an aquarium is a human-made ecosystem.
Balance the equation
0/4These are the ozone steps. UV splits O2. O2 → O3 is balanced only when the coefficients are 3 and 2.
1
UV splits an oxygen molecule into free atoms. Balance by filling coefficients (blank = 1).
O2 → O
Explanation: O2 → 2O
2
A free oxygen atom joins molecular oxygen to make ozone. Balance by filling coefficients (blank = 1).
O2 + O → O3
Explanation: O2 + O → O3
3
The net change is 3O2 → 2O3. Do not treat O2 → O3 as balanced with 1 and 1. Balance by filling coefficients (blank = 1).
O2 → O3
Explanation: 3O2 → 2O3
4
Write the joining step the other way: ozone gives oxygen and one free atom. Balance by filling coefficients (blank = 1).
O3 → O2 + O
Explanation: O3 → O2 + O
Classify
0/21
Place each item as biodegradable or not.
Vegetable peel
Plastic bag
Paper
Medicine strip
Explanation:
Peels and paper break down. Plastic and a medicine strip stay in ordinary conditions.
2
Place each organism in its trophic level. Chain: grass → grasshopper → frog → snake.
Grass
Grasshopper
Frog
Snake
Explanation:
Grass is the producer, the grasshopper primary, the frog secondary and the snake tertiary.
Very short answer
0/51
Define an ecosystem in one line.
Model answer (board format)
All the interacting organisms of an area together with the non-living parts form an ecosystem.
Check your answer:
2
Write one job of decomposers.
Model answer (board format)
They break the complex substances of dead organisms and garbage into simple inorganic substances and return them to the soil.
Check your answer:
3
What is biological magnification?
Model answer (board format)
Biological magnification is the increasing build-up of non-degradable chemicals at each next trophic level. The amount is greatest at the top.
Check your answer:
4
Why is damage to the ozone layer a cause for concern?
Model answer (board format)
The layer blocks UV rays. If it is damaged these rays increase and harm such as skin cancer can follow.
Check your answer:
5
What is a biodegradable substance?
Model answer (board format)
A substance that is broken down by biological processes, such as bacteria, is biodegradable.
Check your answer:
Short answer
0/41
Producers have 2000 J. How much energy will the primary and the secondary consumer get?
Model answer (board format)
The primary consumer gets 10/100 × 2000 = 200 J. The secondary consumer gets 10/100 × 200 = 20 J.
Check your answer:
2
Write two problems caused by non-biodegradable waste.
Model answer (board format)
They stay for a long time and take up space. Some enter the food chain and cause biological magnification, or they harm organisms.
Check your answer:
3
Write two ways to reduce waste, one of them segregation.
Model answer (board format)
Put wet biodegradable waste and dry plastic in separate bins. Carry a cloth bag so that new bags are not made.
Check your answer:
4
Write two differences between ozone at the ground and the ozone layer.
Model answer (board format)
The molecule is O3 in both places. Near the ground it is a poison and should not be breathed. The layer above shields the Earth from UV rays.
Check your answer:
Long answer
0/31
Write a four-level chain of a grassland. From 1000 J in the producers, find the energy at each level and say why the chain does not grow long.
Model answer (board format)
Grass → grasshopper → frog → snake. The energy is 1000 J, 100 J, 10 J and 1 J. At each level most of the energy goes into heat and work, and only about ten percent goes on. After four levels almost no usable energy is left.
Check your answer:
2
How is ozone formed? Write the cause of damage to the layer, one harm, and the UNEP step. Also write why ozone near the ground is not safe.
Model answer (board format)
UV splits O2 into free O, and O2 + O makes O3. The net form is 3O2 → 2O3. The cause of the damage is CFCs, which were used in refrigerants and fire extinguishers. More UV can cause skin cancer. In 1987 UNEP froze CFC production at the 1986 level. Ozone near the ground is a poison; only the high layer protects.
Check your answer:
3
Write the method of Activity 13.5. Give the difference between biodegradable and non-biodegradable waste with examples. How would you segregate waste at home?
Model answer (board format)
Bury one day of waste in a pit or a pot, cover it with at least 15 cm of soil, keep it moist and look every 15 days. A peel is biodegradable because organisms break it. Plastic is non-biodegradable because enzymes do not break it in ordinary conditions. At home, keep wet and dry waste in separate bins.
Check your answer:
BSEB model paper · practice
0/6This model set is for practice. It is not a question from any year’s annual examination. Annual questions will be added only when a source page is available.
1
Which of the following is a natural ecosystem?
💡 Hint:
The others are human-made.
Explanation:
A lake is natural.
2
A secondary consumer is —
💡 Hint:
The third trophic level.
Explanation:
A small carnivore is a secondary consumer.
3
In the net ozone equation the coefficient of O3 is ______.
💡 Hint:
2.
Explanation:
3O2 → 2O3.
4
Why is an animal that would eat the others not put in an aquarium?
Model answer (board format)
It can eat the other organisms and the chain breaks. A link of at least three steps does not survive.
Check your answer:
5
Balance the joining step of ozone formation. UV has already split O2 before this. A blank coefficient means 1.
O2 + O → O3
Explanation: O2 + O → O3
6
Explain the ten percent law with an example. How is biological magnification different? Give one numerical example.
Model answer (board format)
Each next level receives about ten percent of the energy of the previous level. From 1000 J in the producers, the primary gets 100 J, the secondary 10 J and the tertiary 1 J. The flow is one way. In biological magnification the amount of a chemical that does not break down rises as you go up; it does not fall the way energy does.
Check your answer:
CBSE-style questions
0/6These are competency-based practice questions. They are not copies of a CBSE paper.
1
A student puts into an aquarium a fish that eats the smaller fish. The most likely result is —
💡 Hint:
The caution of Activity 13.2.
Explanation:
The predator fish can eat the others and break the chain.
2
200000 J of sunlight falls on the leaves. Producers make food from 1 percent. The primary consumer receives —
💡 Hint:
First 1 percent, then 10 percent.
Explanation:
Food = 2000 J. The primary consumer gets 10 percent of that = 200 J.
3
Assertion (A): A pesticide residue can remain in rice even after washing.
Reason (R): These chemicals are not degraded and they collect in the chain.
Explanation:
Both are true and R is the correct reason.
4
Assertion (A): O2 → O3 with coefficients 1 and 1 is equal in the count of atoms.
Reason (R): Higher-energy UV splits some O2 molecules into free oxygen atoms.
Explanation:
A is false — with 1 and 1 the oxygen atoms stay 2 and 3. The balanced net form is 3O2 → 2O3. R is true.
5
In a town, wet and dry waste go into one bin and a drain falls into a pond without treatment. Write two harms and one improvement.
Model answer (board format)
Plastic mixed with biodegradable waste spoils compost and stays for a long time. Untreated sewage pollutes the pond water. Improvement: keep two bins, and treat the sewage before it is released into the pond.
Check your answer:
6
An old phone is dropped into the household wet waste. What is the risk? How is this different from the ozone issue?
Model answer (board format)
A battery and a circuit can contain a poisonous metal such as lead or mercury. With the wet waste it can reach the soil and the water. That is local pollution. The ozone layer blocks UV high up, and ozone near the ground is a poison in a different way. The phone should be given separately as e-waste.
Check your answer:
🏛️ Board exam corner — Bihar Board (BSEB)— CBSE
Switch board with BSEB | CBSE above. The lessons follow the same NCERT chapter.
BSEB model · practiceBoard tip
This page has no verified annual-exam question, because no source page has been added. The model set below is practice in the board pattern.
🏛️ Model questions on one page →
The verified label will be used only when a source page for the question is available.
CBSE-style · competency-based
These are case and assertion-reason practice items. Do not treat them as past CBSE questions.
🔁 Spaced review — today’s questions
Wrong questions return soon; correct ones return after a few days.
🧠 What you learned + equation sheet
What you learned
- In an ecosystem the biotic and abiotic components depend on each other.
- Producers are the first trophic level; then herbivores, then small and large carnivores.
- The next level receives about ten percent of the energy, and the flow is one way.
- Chemicals that do not break down collect more at higher levels. That is biological magnification.
- The high ozone layer blocks ultraviolet rays; ozone near the ground is a poison. CFCs thinned the layer.
- Keep biodegradable waste separate. Do not leave sewage and e-waste in the open.
| What | Keep this |
|---|---|
| Sunlight to food | about 1 percent on the leaves |
| Next level | about 10 percent |
| Ozone joining | O2 + O → O3 |
| Net change | 3O2 → 2O3 |
| CFCs | UNEP 1987, freeze at 1986 levels |
| Waste bins | wet biodegradable, dry not |
The notes are original writing. The textbook was used only for activity order and numbers. “Verified” will be used only on a question that has a source page.