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All rain is weakly acidic due to dissolved carbon dioxide. However, certain pollutants released into the atmosphere can cause the rain to become strongly acidic.

Many fossil fuels contain sulfur impurities. When these fuels are burned, the sulfur is oxidised to form sulfur dioxide.

S(s) + O₂(g) → SO₂(g)

This sulfur dioxide then dissolves in droplets of rainwater to form sulfuric acid:

H₂SO₄

When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures, nitrogen and oxygen from the air combine to produce oxides of nitrogen, which refers to a number of different compounds formed from the chemical reaction of nitrogen and oxygen.

Oxides of nitrogen (referred to collectively as NO_x) also cause acid rain.

Nitrogen monoxide and nitrogen dioxide are jointly referred to as NO_x.

Effects of acid rain

Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result, particularly those made of limestone (calcium carbonate). Acid rain also increases the rate of corrosion of metal structures such as bridges and statues.

Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result.

Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.