Rain occurs when vapours condense in clouds & fall to earth. As it begins to fall, it is neutral  (pH = 7). While it traves through the air, it dissolves floating chemical & washes down the particles that are suspended in air. 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), mist of hydrochloric acids & phosphoric acid etc. These substances dissolve in falling rain making it more acidic than normal with pH ranging between (5.6 – 3.5).

Chemistry of acid rain

Natural processes such as volcanic eruption, forest fires & biological decomposition of organic matter produce oxides of sulphur & nitrogen.

Man made sources such as power plants, smelting plants, industrial plants, burning of coal & gasoline also release SO2, nitrogen oxides & acidic soots. Sulphurdioxide & nitrogen dioxide interact with water vapours in presence of sunlight to form sulphuric acid & nitric acid units.

Harmful effect of acid rain

(i) Damage to animals

Most of aquatic animals can not survive when pH is less than 4. Certain fishes die even when the pH is less than 5.5.

(ii) Damage to plants

Acidic water is dangerous for plants leaf pigments are decolourised, acid affects green pigments of plants. Agricultural productivity is also decreased.

(iii) Material Damage

Metallic surfaces exposed to acid rain are readily corroded. Textile fabrics, paper & leather products lose their material strength or disintegrate by the acid rain.

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