Acid rain

Acid rain has a lower pH than normal rain. This happen when sulfur dioxide is dissolved in water in clouds, which make it acidic.

Sulfur dioxide
Most hydrocarbon fuels contain impurities such as '''sulfur. When the hydrocarbons burn, the sulfur is oxidised to form sulfur dioxide.'''

Liquid hydrocarbon fuels such as petrol and diesel contain sulfur. Coal also contains sulfur. Natural gas and the gases made from crude oil do not contain sulfur.