Plants are dying day by day comment.


  1. No.iv. Plants are dying day by day comment.

Answer. Acid rain increases the acidity of the soil. Many crops and plants cannot grow properly in such soil. Even old trees are being affected due to acidity of soil. Their growth is retarded. They get dry and die. Acid rain directly damages the leaves of trees and plants, thus limiting their growth. Depending upon the severity of the damage, plants growth can be hampered. Plants capability to resist cold or diseases reduces and ultimately they die as shown in figure.


Effect of acid rain on plants


  1. No. i. Justify, ozone is beneficial for human kind.

Answer. Ozone layer surrounds the globe and protects Earth like a shield from harmful ultraviolet radiations of sunlight. Otherwise, ultraviolet radiations would reach to the Earth and cause skin cancer. Thus ozone layer in stratosphere is beneficial for life on the Earth.

  1. No. ii. Why ozone is depleting in atmosphere?

Answer. The ozone molecule absorbs solar radiations and dissociate readily,   i.e.,   self   dissociation   of   ozone   takes   place.   However 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 as


CFCl3             CFCl2+Cr

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.


  1. No. iii. What do you mean by ozone hole?

Answer. Ozone hole means the region of atmosphere in which ozone layer has been depleted. Signs of ozone depletion were first noticed over Antarctica in 1980s. since 1990s depletion have also been recorded over the Arctic as well.

  1. No.iv. Where ozone layer is found?

Answer. Ozone is an allotropic form of oxygen consisting of three oxygen atoms. It is formed in atmosphere by the, association of an oxygen atom with an oxygen molecule in the mid of stratosphere.

Ozone is present throughout the atmosphere. But its maximum concentration called ozone layer lies in stratosphere region about 25 to 30 km away from the Earth’s surface. This layer surrounds the globe and protects Earth like a shield from harmful ultraviolet radiations of sunlight as shown in figure.

  1. No. 1.Explain the phenomenon of decreasing temperature in troposphere.

Answer. Troposphere lies just above the Earth’s surface and extends upward up to 12 km. Although concentration of carbon dioxide and water vapours in negligible in this region, but both of these gases absorb much of the outgoing infrared radiation that keeps the atmosphere warm. As concentration of these gases gradually decreases upward, correspondingly temperature also decreases at a rate of 6°C per kilometer. In this region temperature decreases upwards in a regular way from 17°C to -58°C. Thus carbon dioxide and water vapours are responsible for decreasing of temperature in troposphere.


  1. No. 2.Differentiate between primary and secondary air pollutants.

Answer. Primary pollutants are the Waste or exhaust products driven out because of combustion of fossil fuels and organic matter. These are oxides of sulphur (SO2 and SO3); oxides of carbon (CO2 and CO); oxides of nitrogen (especially nitric oxide NO); hydrocarbon (CH4); ammonia and compounds of fluorine.

Secondary pollutants are produced by various reactions of primary pollutants. These are sulphuric acid, carbonic acid, nitric acid, hydrofluoric acid, ozone arid peroxy acetyl nitrate (PAN).


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