Chapter No.14Chemistry 10

How ozone layer forms in stratosphere?


  1. No. 10.How ozone layer forms in stratosphere?

Answer. As majority of the ultraviolet radiations coming from the sun have been absorbed in the upper stratosphere Therefore, the mid stratosphere has less UV light passing through it. Here O and O2 recombine to form ozone which is an exothermic reaction. Ozone formation in this region results in formation of ozone layer. Thus, ozone layer exists in mid stratosphere as shown in figure.



Ultraviolet radiations absorbed by ozone layer

  1. No. 11.Why the 75% of the atmospheric mass lies within the troposphere?

Answer. Atmosphere consists of gases. That means atmospheric mass is because of presence of gases; nitrogen, oxygen, carbon dioxide etc. The two major gases are nitrogen and oxygen that comprise 99% by volume of the atmosphere. About 99% of atmospheric mass lies within 30 kilometer of the surface and 75% lies within the lowest 11 kilometers that is the troposphere.

  1. No. 12.How ozone layer is being depleted by Chlorofluocarbons?

Answer. Chlorofluorocarbons (CFCs) used as refrigerants in air conditioners and refrigerators are major cause of depletion of ozone layer. These compounds leak in one way or other; escape and diffuse to stratosphere. There ultraviolet radiations break the C-C1 bond in CFCl3 and generate chlorine free radicals.



CFCl2               CFCl2

These free radicals are very reactive. They react with ozone to form oxygen as

O3+ Cl           O2 + OCl·

OCl·          O· + Cl·

O· + O·         O2

A single chlorine free radical released by the decomposition of CFCs is capable of destroying up to 1 lac ozone molecules as shown in figure.

Depletion of ozone layer by chlorofluoro carbons


  1. No.1. Write down the significance of atmospheric gases.

Answer. The study of composition of atmosphere provides us the knowledge about significance of gases present in the atmosphere. Atmosphere is divided into four regions. Each region has its natural characteristics. Atmosphere is the envelope of different gases around the Earth. Major concentration of Na, O2 and Ar are present in atmosphere. The presence of CO2 and water vapours play a significant role in maintaining temperature of the atmosphere. Both of these gases are transparent to visible light but absorb infrared radiations emitted by the Earth’s surface. Therefore, these gases absorb much of the outgoing radiations and warm the atmosphere. The presence of O2 in stratosphere saves us from dangerous UV radiations.