ACID RAIN

When sulphuric acid, nitric acid or carbonic acid in wet or dry forms (such as rain, cloud water, fog, hail, sleet, snow, dew, dust) fall from atmosphere to the ground is termed as acid rain or acid deposition or acid precipitation.

Causes of acid rain----

Sulphur di oxide (SO₂), carbon di oxide (CO₂), nitrogen di oxide (NO₂) [released by industry sector, automobile sector (cars, buses, truck etc), or due to combustion of fuels, eruption of volcanoes, burning of fossil fuels] are emitted into the atmosphere and transported by wind and air currents.

1.Formation of sulphuric acid rain and its harmful effect----

In presence of sunlight SO₂ dissociated photochemically as;

SO₂------> SO (sulphur monoxide) + O (very reactive)

SO₂+O ------> SO3

SO₃ + H₂O ------> H₂SO₄

H₂SO₄ forms in the above process mixed with rain water and falls on ground in the form of acid rain.

When this sulphuric acid rain falls over a historical monument, it gradually eats up the marble stone (CaCO₃) of the monument and corrodes it slowly.

CaCO₃ + H₂SO₄ ------> CaSO₄ + CO₂ + H₂O

Due to corrosive nature of SO₂ and H₂SO₄ the acid rain decolourises building materials such as lime stone, marble, roof slate and mortar. The corrosion of metals (Fe,steel,Zn,Al,bronze) is accelerared by acid rain.

2.Formation of nitric acid rain and its harmful effect---

NO₂ present in the air mixed with moisture or O₃ present in the atmosphere and forms HNO₃.

4NO₂ + O₂ + 2H₂O ------> 4HNO₃

2NO₂ +H₂O +O₃ -------> 2HNO₃ + O₂

Another process for the formation of HNO₃ in the atmosphere-

In presence of sunlight NO₂ dissociated photochemically as;

NO₂ ---------> NO + O (very reactive)

NO₂ + O ----> NO₃

NO₂ + NO₃ -----> N₂O₅

N₂O₅ + H₂O -----> 2HNO₃

HNO₃ forms in the above process mixed with rain water and falls on ground in the form of acid rain.

When this nitric acid rain falls over a historical monument, it gradually eats up the marble stone (CaCO₃) of the monument and corrodes it slowly.

CaCO₃ + 2HNO₃ -------> Ca (NO₃)₂ + CO₂ + H₂O

The acid rain fades the colour of the fabrics (cotton, nylon, rayon), lather and paper. The acid rain makes the water of lakes acidic and hence fish life comes in danger in this water. The yield of agricultural crops is also reduced.

3.Formation of carbonic acid and its harmful effect--- 

CO₂ present in the air mixed with moisture present in the atmosphere and forms H₂CO₃.

CO₂ + H₂O ----> H₂CO₃

H₂CO₃ forms in the above process mixed with rain water and falls on ground in the forms of acid rain.

Acid rain cause harmful effect on soil, the character of soil changed due to acid rain, the P^H balance of soil affected. High altitude forests are especially vulnerable as they are often surrounded by clouds and fogs which are more acidic than rain. Plants, foods crops can also be damaged by acid rain. Acid rain caused harmful effect on fish and other living species that lives in water.