If you have eaten a sweet called an 'acid drop' just think back to how it tasted. The sharp or sour taste was due to the acid in the sweet.

Vinegar also contains an acid, and this, too, gives the liquid its sour taste. Lemons also contain acid.

There are many kinds of acid. Some are weak and can be used in cooking and making food. Vinegar and lemon are like this. Others are much stronger and must be treated with great care. They are used in factories to make many different products. They are also used in batteries such as car batteries because acids conduct electricity.

But acids can be produced when gases dissolve in water, too. Two examples of this are important for the environment. When carbon dioxides gas (mainly produced by living things breathing out and by burning fuels) dissolves in rainwater or water droplets in a cloud, it produces carbonic acid. This is a very weak acid, but over time it can do a lot of damage. It is the main agent responsible for dissolving limestone to make caves, but it is also 'poisonous' to plants, and can cause some plants to die back.

When sulphur and nitrogen oxides released from vehicles or otherwise burning oil and coal, gets into the air it, too, can produce acids, in these cases sulphuric acid and nitric acid) these can be very important locally, making soils acid and also killing off plants more effectively than carbonic acid.

Acid rain is one of the most important reasons for cleaning up sources of pollution such as power stations and vehicles.