CBSE Class 11-science Answered

Acid rain:

a) Acid rain:

Normally rain water has a pH of 5.6 due to the presence of H⁺ ions formed by the reaction of rain water with carbon dioxide present in the atmosphere.

Source: Burning of fossil fuels (which contain sulphur and nitrogenous matter) such as coal and oil in power stations and furnaces or petrol and diesel in motor engines produce sulphur dioxide and nitrogen oxides.  SO₂ and NO₂ after oxidation and reaction with water are major contributors to acid rain, because polluted air usually contains particulate matter that catalyses the oxidation.

Harmful effects:

• Harmful for agriculture, trees and plants as it dissolves and washes away nutrients needed for their growth.
• Causes respiratory ailments in human beings and animals.
• Affects plant and animal life in aquatic ecosystem when acid rain falls and flows as ground water to reach rivers, lakes etc.
• Corrodes water pipes resulting in the leaching of heavy metals such as iron, lead and copper into the drinking water.
• Damages buildings and other structures made of stone or metal. The Taj Mahal in India has been affected by acid rain.

b) Ozone layer depletion:

Ozone layer acts as a blanket in the atmosphere above 16 km height from the earth’s surface.

It absorbs the harmful ultraviolet rays coming from the sun and thus protects life on the earth.

It is now established that a more potentially more serious threat to the stratosphere ozone comes from the emissions of the synthetic chlorofluorocarbon (CFC) which are used on a wide scale for various purposes.

They deplete ozone and also contribute to global warming.

The chlorofluoro carbons are decomposed by the ultraviolet rays to highly reactive chlorine which is produced in the atomic form.

CF₂Cl_2(g) → CF₂Cl_(g)  + Cl_(g)

                                    _{(free radical)}

The free radical [Cl] reacts with ozone and chlorine monoxide is formed.

Cl_(g) + O_3(g)  →  ClO_(g) + O_2(g)

This causes depletion of ozone.Chlorine monoxide further reacts with atomic oxygen and produces more free chlorine radicals.

ClO_(g)  + O_(g) →Cl_(g)  + O_2(g)

Again this free radical [Cl] destroys ozone and the process continues thereby giving rise to ozone depletion.