Acid rain

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Acid rain is where chemicals in the air combine with rain and make the water more acidic.

Normally, the pH of rain is about 6.5, which means it is a weak acid. Other chemicals can make this a stronger acid if they react with it. When this rain falls to the ground, it can dissolve building materials and kill fish in rivers and lakes.

The most common chemicals which do this are oxides of sulphur and nitrogen, which are put into the air by power stations and vehicles (such as cars). There are also some natural causes, like volcanoes and biological processes that happen on the land, in wetlands, and in the oceans.

Many international countries are trying to reduce the amount of acid rain by agreeing to treaties, and by taking part in emissions trading. In America, many power stations use a process called Flue gas desulfurization (FGD) to reduce the sulphur in the gases that are made.

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