Our Environment CBQs — CBSE Class 10 (2027) with step-wise solutions

Sample Our Environment CBQs

1. Question 1MCQ1 mark
   bone | metal can | paper sheet | plastic bottle Which option arranges these objects by the time they take to biodegrade, from LEAST time to MOST time?
   • paper sheet, bone, metal can, plastic bottle
   • bone, paper sheet, plastic bottle, metal can
   • plastic bottle, metal can, bone, paper sheet
   • paper sheet, metal can, bone, plastic bottle
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   1. [1 mark] paper sheet, bone, metal can, plastic bottle — paper decomposes fastest, then bone, then a metal can; plastic takes the longest.

   Answer: paper sheet, bone, metal can, plastic bottle

2. Question 2Assertion-Reason1 mark
   Assertion (A): Polythene bags thrown into a compost pit along with vegetable peels remain almost unchanged even after several months. Reason (R): Polythene cannot be broken down by bacteria and other saprophytes.
   • (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.
   • (B) Both Assertion and Reason are true but Reason is NOT the correct explanation of Assertion.
   • (C) Assertion is true but Reason is false.
   • (D) Assertion is false but Reason is true.
   Show step-wise solution

   1. [1 mark] Option (A). Vegetable peels are biodegradable and are decomposed by microbes, but polythene is non-biodegradable because bacteria and other saprophytes cannot break it down; so it stays unchanged in the pit. R correctly explains A.

   Answer: (A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion.

3. Question 3Short answer2 marks
   Suggest two negative impacts of fertiliser entering a lake ecosystem.
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   1. [1 mark] The fertiliser causes increased growth of plants / algae in the lake (an algal bloom).
   2. [1 mark] This leads to eutrophication - the water becomes deoxygenated as decomposers respire while feeding on the decaying algae, killing aquatic life.

   Answer: Excessive algal/plant growth and eutrophication (deoxygenation of the water), harming aquatic organisms.

4. Question 4Case study4 marks
   Students of a school in Bhubaneswar studied a village pond as part of an ecology project. With help from their teacher, they estimated that the phytoplankton in the pond trap 20,000 kJ of energy per square metre in a year from sunlight. They identified the following food chain in the pond: phytoplankton → zooplankton → small fish → kingfisher Their teacher told them that, on average, only about 10% of the energy at one trophic level is available to the next level. (i) Which organism occupies the third trophic level in this food chain? (ii) How much energy per square metre in a year is available to the zooplankton? (iii) Calculate the energy available to the kingfisher, and explain why food chains generally have only three or four trophic levels.
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   1. [1 mark] (i) The small fish occupies the third trophic level (phytoplankton first, zooplankton second).
   2. [1 mark] (ii) Energy available to zooplankton = 10% of 20,000 kJ = 2,000 kJ per square metre per year.
   3. [1 mark] (iii) Small fish get 10% of 2,000 = 200 kJ, so the kingfisher gets 10% of 200 = 20 kJ per square metre per year.
   4. [1 mark] (iii) Since about 90% of the energy is lost (used for life processes/lost as heat) at each level, the energy left after three or four trophic levels is too small to support another level of organisms.

   Answer: (i) Small fish. (ii) 2,000 kJ m⁻² per year. (iii) 20 kJ m⁻² per year; very little energy is left for further levels.

5. Question 5Long answer5 marks
   A team of students surveyed a grassland in the Banni region of Gujarat and recorded these feeding relationships: grasses are eaten by grasshoppers and by rabbits; grasshoppers are eaten by frogs; frogs are eaten only by snakes, and snakes feed only on frogs; snakes are eaten only by eagles; rabbits are eaten by foxes. (a) Write the longest food chain in this grassland and state the trophic level of the snake in it. (b) Explain why the flow of energy in this grassland is unidirectional. (c) If the eagle obtains 30 kJ of energy through this longest food chain, calculate the energy that must have been trapped by the grasses, assuming the ten per cent law. (d) Predict two effects on this grassland if farmers killed most of the frogs. (e) State the role of decomposers in this grassland.
   Show step-wise solution

   1. [1 mark] (a) Longest food chain: grass → grasshopper → frog → snake → eagle; the snake is at the fourth trophic level (tertiary consumer).
   2. [1 mark] (b) Energy captured by the grasses (autotrophs) does not revert to the Sun, and energy passed on to herbivores does not come back to the autotrophs; it moves progressively through the trophic levels and is no longer available to the previous level, so the flow is unidirectional.
   3. [1 mark] (c) Working back with 10% transfer: eagle 30 kJ ← snake 300 kJ ← frog 3000 kJ ← grasshopper 30,000 kJ ← grasses 3,00,000 kJ (300 000 kJ).
   4. [0.5 mark] (d) With few frogs left to eat them, the grasshopper population increases and more grass is eaten.
   5. [0.5 mark] (d) Snakes lose their food and decrease in number, and so the eagles that feed on snakes also decrease.
   6. [1 mark] (e) Decomposers (bacteria and fungi) break down the dead remains and wastes of organisms into simple inorganic substances that go into the soil and are used again by plants.

   Answer: (a) Grass → grasshopper → frog → snake → eagle; fourth level. (b) Energy never returns to the previous level. (c) 3,00,000 kJ. (d) Grasshoppers rise, snakes and eagles fall. (e) Recycle nutrients by decomposing dead matter.

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