(a) Construct a terrestrial food chain comprising four trophic levels. (b) What will happen if we kill all the organisms in one trophic level? (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.

Q: (a) Construct a terrestrial food chain comprising four trophic levels. (b) What will happen if we kill all the organisms in one trophic level? (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.

(a) The completed table is as follows: | Feature | Oxygen | Ozone | | :--- | :--- | :--- | | **Formula** | (i) $O_2$ | (ii) $O_3$ | | **Benefits to biotic component** | (iii) Essential for all aerobic forms of life (for respiration). | (iv) Shields the surface of the Earth from harmful ultraviolet (UV) radiation from the Sun. | (b) Ozone is formed at the higher levels of the atmosphere (stratosphere) through the following steps: 1. High-energy ultraviolet (UV) radiation from the Sun splits molecular oxygen ($O_2$) into free, highly reactive oxygen ($O$) atoms: $$O_2 \xrightarrow{\text{UV}} O + O$$ 2. These free oxygen atoms then combine with intact molecular oxygen to form ozone ($O_3$): $$O + O_2 \rightarrow O_3$$

Our Environment

Ozone Layer and its Depletion

Official Answer

(a) The completed table is as follows:

Feature	Oxygen	Ozone
Formula	(i) $O_2$	(ii) $O_3$
Benefits to biotic component	(iii) Essential for all aerobic forms of life (for respiration).	(iv) Shields the surface of the Earth from harmful ultraviolet (UV) radiation from the Sun.

(b) Ozone is formed at the higher levels of the atmosphere (stratosphere) through the following steps:

High-energy ultraviolet (UV) radiation from the Sun splits molecular oxygen ( $O_2$ ) into free, highly reactive oxygen ( $O$ ) atoms:

$O_2 \xrightarrow{\text{UV}} O + O$

These free oxygen atoms then combine with intact molecular oxygen to form ozone ( $O_3$ ):

$O + O_2 \rightarrow O_3$

O2O3aerobic lifeultraviolet radiationUVO2 -> O + OO + O2 -> O3

Marking Scheme

10.5 marks for correctly writing formulas (i) and (ii).
21.0 mark for correctly stating the benefits (iii) and (iv).
31.5 marks for explaining the formation of ozone along with both balanced chemical equations.

Hint

Remember that oxygen is diatomic and ozone is triatomic. For the formation of ozone, write down the two-step chemical reaction initiated by UV light.

Quick Oral Answer

Ozone is formed in the stratosphere when high-energy UV radiation splits oxygen molecules ( $O_2$ ) into free oxygen atoms ( $O$ ). These free atoms then combine with molecular oxygen ( $O_2$ ) to form ozone ( $O_3$ ).

Analysis & Explanation

This question assesses the student's understanding of the chemical nature and atmospheric role of oxygen and ozone.

Oxygen ( $O_2$ ) is a diatomic molecule essential for sustaining aerobic life on Earth through cellular respiration.
Ozone ( $O_3$ ) is a triatomic allotrope of oxygen. While it is a deadly poison at ground level, it plays a vital protective role in the stratosphere by absorbing highly damaging ultraviolet (UV) radiation.
The formation of ozone is a photochemical process. High-energy UV radiation breaks the strong double bond in $O_2$ to create free oxygen radicals, which are extremely reactive and quickly bind with surrounding $O_2$ molecules to form stable $O_3$ .

Common Mistakes

1Confusing the formula of ozone as $O$ or $O_4$ instead of $O_3$ .
2Omitting the chemical equations when explaining the formation of ozone.
3Describing ozone as a beneficial gas to breathe directly; it is actually a deadly poison at ground level.

Interesting Facts

Although ozone is a deadly poison at ground level, it performs a crucial shielding function in the upper atmosphere. Without this layer, life could not have safely emerged onto land due to intense solar UV radiation.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Our Environment" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Ozone Layer and its Depletion" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.