Acid rain is rain that is highly acidic because of sulphur oxides, nitrogen oxides, and other air pollutants dissolved in it. Normal rain is slightly acidic, with a pH of 6. Acid rain may have a pH value as low as 2.8.

    Acid rain can severely damage both plant and animal life. Certain lakes, for example, have lost all fish and plant life because of acid rain.

    Acid rain comes from sulphur in coal and oil. When they burn, they make sulphur dioxide (SO2 ). Most sulphur leaves factory chimneys as the gaseous sulphur dioxide (SO2 ) and most nitrogen is also emitted as one of the nitrogen oxides (NO or NO2 ), both of which are gases. The gases may be dry deposited–absorbed directly by the land, by lakes or by the surface vegetation. If they are in the atmosphere for any time, the gases will oxidise (gain an oxygen atom) and go into solution as acids.

    Sulphuric acid (H2 SO4 ) and the nitrogen oxides will become nitric acid (HNO3 ). The acids usually dissolve in cloud droplets and may travel great distances before being precipitated as acid rain.

   Catalysts such as hydrogen peroxide, ozone and ammonium help promote the formation of acids in clouds. More ammonium (NH4 ) can be formed when some of the acids are partially neutralised by airborne ammonia (NH3 ). Acidication increases with the number of active hydrogen (H+) ions dissolved in acid.        

    Hydrocarbons emitted by for example, car exhausts will react in sunlight with nitrogen oxides to produce ozone. Although it is invaluable in the atmosphere, low level ozone causes respiratory problems and also hastens the formation of acid rain. When acid rain falls on the ground it dissolves and liberates heavy metals and aluminium (Al).

    When it is washed into lakes, aluminium irritates the outer surfaces of many fish. As acid rain falls or drains into the lake the pH of the lake falls. Forests suffer the effect of acid rain through damage to leaves, through the loss of vital nutrients, and through the increased amounts of toxic metals liberated by acid, which damage roots and soil micro organisms.

Taken from Science and Technology Encycopedia, 1998

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