1 markVery Short AnswerSection A

What happens when higher energy ultraviolet radiations act on the oxygen at the higher level of the atmosphere ?

Our Environment

Ozone Layer Depletion

Official Answer

When higher energy ultraviolet (UV) radiation acts on molecular oxygen ( $O_2$ ) at higher levels of the atmosphere, it splits the diatomic oxygen molecules into free, highly reactive oxygen ( $O$ ) atoms. These free oxygen atoms then combine with intact molecular oxygen ( $O_2$ ) to form ozone ( $O_3$ ).

The chemical reactions representing this process are:

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

$O + O_2 \rightarrow O_3 \text{ (Ozone)}$

ultraviolet radiationsplits molecular oxygenfree oxygen atomscombine with molecular oxygenozone

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

O + O_2 \rightarrow O_3

Marking Scheme

1Stating that UV radiation splits molecular oxygen ( $O_2$ ) into free oxygen ( $O$ ) atoms. (0.5 marks)
2Stating that these free oxygen atoms combine with $O_2$ to form ozone ( $O_3$ ) along with the chemical equations. (0.5 marks)

Hint

Think about how UV light breaks the double bond of $O_2$ to create free oxygen atoms, which then react with other $O_2$ molecules.

Quick Oral Answer

At higher levels of the atmosphere, high-energy UV radiation splits molecular oxygen ( $O_2$ ) into free oxygen ( $O$ ) atoms. These free atoms then combine with molecular oxygen to form ozone ( $O_3$ ).

Analysis & Explanation

Ozone ( $O_3$ ) is a triatomic molecule of oxygen. While diatomic oxygen ( $O_2$ ) is essential for respiration, ozone is a deadly poison at ground level. However, in the stratosphere, it forms a protective shield that absorbs harmful UV radiation from the Sun, preventing it from reaching the Earth's surface where it can cause skin cancer, cataracts, and damage to crops.

Common Mistakes

1Students often forget to write the chemical equations.
2Confusing the role of UV radiation, thinking UV is only absorbed by ozone rather than realizing UV is also responsible for its synthesis.

Interesting Facts

Although ozone is a protective shield high up in the stratosphere, at ground level it is a major component of photochemical smog and is highly toxic to human lungs.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 1 mark in the CBSE Class 10 Science 2012 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 Depletion" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a Very Short Answer question from Section A in the CBSE Class 10 Science 2012 paper.