Rain occurs when water vapours condense in clouds and fall to earth. As it begins to fall, it is neutral (pH = 7). While it travels through the air, it dissolves floating chemicals and washes down particles that are suspended in air. In clean air, rain picks up materials that occurs naturally such as dust, pollen, some CO2 and other chemicals produced by lightening or volcanic activities. These substances make the rain slightly acidic (pH.= 6). This level of acidity is not dangerous
However, when the rain falls through polluted air, it comes across chemicals such as gaseous oxides of sulphur (SOx) oxides of nitrogen (NOx) mists of hydrochloric acid and phosphoric acid, etc. These substances dissolve in falling rain making it more acidic than normal with pH ranging between (5:6 – 3.5), In some case, the pH lowers to the extent of 2. This leads to acid rain. Similarly, acid fog is formed when chemical pollutants dissolve in moist air. Now-a-days, the term of acid rain is used to describe all types of precipitation,
namely ; rain, snow, fog and dew more acidic than normal.
CHEMISTRY OF ACID RAIN
Natural processes such as volcanic eruptions, forest fires and bacterial decomposition of organic matter produce oxides of sulphur and nitrogen. Man-made sources such as power plants, smelting plants, industrial plants, burning of coal and gasoline also release sulphur dioxide, nitrogen oxides and acidic soots. Sulphur dioxide and nitrogen dioxide interact with water vapours in presence of sunlight to form sulphuric acid and nitric acid mist.
SO2 + H2O → H2 + SO3
Trioxosulphate (IV) acid
SO2 + O3 → SO3 + O2;
SO3 + H2O → H2SO4
Tetraoxosulphate (VI) acid
2NO + O2 → 2NO2;
2NO2 + H2O → HNO3 + HNO2
(V) acid (III) acid
The H2SO4 and HNO3, thus, formed remain as vapour at high temperatures and begin to condense slowly as the temperature falls. These acids mix with rain or snow, on its
to the earth and make it sufficiently acidic.
HARMFUL EFFECTS OF ACID RAIN
Some of the significant ill effects of acid rain are as follows:
- Damage to Animals. Most of the aquatic animals cannot survive when the pH is less than 4. Some species of fish, such, as salmon, die even when the pH is less than 5.5. Certain species of algae and zooplankton are eliminated at pH less than 6. A reduction in the zooplankton and bottom fauna ultimately affects, the food availability for the fish population.
- Damage to Plants. Acidic water is dangerous for plants. Leaf pigments are decolourised, acid affects green pigment (chlorophyll) of plants. Agricultural productivity is also decreased. Several non-woody plants, such as barley, cotton and fruit trees like apple, pear, etc., are severely affected by acid rain.
- Material Damage. Metallic surfaces exposed to acid rain are readily corroded. Textile fabrics, paper and leather products lose their material strength or disintegrate by the acid rain
Building materials such as lime stone, marble, dolomite, mortar and slate are weakened on reaction with acid rains because of the formation of soluble compounds. Thus, acid
rain is dangerous for historical monuments.
CaCO3 + H2SO4 → CaSO4 + H2O + CO2
Effect of acid rain on lakes with lime stone deposits Although the acid rain adversely affect the buildings made of lime stone, marble, etc., yet it does not affect the lakes with lime stone deposits severely. It is because of the fact that the acid rain gets diduted largely by the lake water and is also neutralised by the lime stone present in the lake. Hence, its effect on the aquatic life is almost nullified.