Q. 27

Assertion (A) : Ozone is destroyed by solar radiations in upper stratosphere.

Reason (R) : Thinning of ozone layer allows excessive UV radiations to reach the surface of earth.

A. A and R both are correct but R is not the explanation of A.

B. A is false but R is correct.

C. A and R both are correct and R is the correct explanation of A.

D. A and R both are incorrect.

Answer :

B. A is false but R is correct.

The upper stratosphere consists of considerable amount of ozone (O₃), which protects us from the harmful ultraviolet (UV) radiations coming from the sun. Ozone is not destroyed by solar radiations rather it is destroyed because of release of chlorofluorocarbon compounds (CFCs), also known as freons.

Due to CFCs ozone layer is depleting and allows excessive UV radiations to reach the surface of earth. CFCs mix with the normal atmospheric gases and reach the stratosphere. In stratosphere, they get broken down by powerful UV radiations, releasing chlorine free radical which then reacts with ozone and results in depletion of ozone.

CF₂Cl₂ ^UV→ Cl^. + F₂Cl

Cl^. + O₃ → ClO^. + O₂

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