What is Acid Rain?

As the name suggests, acid rain is rain which is acidic. The rain becomes acidic because of gases which dissolve in the rain water to form various acids. Rain is naturally slightly acidic beacuse of the carbon dioxide dissolved in it (which comes from animals breathing), and to a lesser extent from chlorine (which is derived from the salt in the sea). This gives rain a pH of around 5.0, and in some parts of the world it can be as low as 4.0 (this is typical around volcanoes, where the sulphur dioxide and hydrogen sulphide form sulphuric acid in the rain).

Before the Industrial Revolution, the pH of rain was generally between 5 and 6, so the term acid rain is now used to describe rain with a pH below 5.

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