Rain is naturally acidic, but acid gasses make it even more acidic. Acid gasses are mainly caused by humans burning fossil fuels like coal and oil; but nature also creates these gasses with volcanoes.
The opposites of acid are alkalis; for example, toothpaste and baking powder are both alkalis. Strong alkalis can also be dangerous, such as ammonia and bleach.
The ph scale is used to measures the strength of acids and alkalis. A low ph number lets us know that a substance is acid; a high number lets us know that a substance is alkali.
Rain is normally a bit acidic, with a ph of around 5.5, if the ph of rain is below 5.5, then the rain is most likely contaminated by acid gases.
Gasses that cause acid rain are sulphur and nitrogen. When these gasses mix with the oxygen and water vapour in the air it causes sulphur dioxide and nitrogen oxide to be formed. Most of the sulphur released into the atmosphere comes from power stations; volcanoes also produce lots of sulphur when they erupt. Most of the nitrogen oxides come from the vehicles people around the world travel in daily, from planes, cars and trucks.
Acid rain is a problem all over the world, when acid gases are released, they go up in the sky, and then they are carried by strong winds. Acid rain in Scandinavian countries is caused by air pollution in Britain and other countries of Europe. In the USA, winds blow the air pollution to certain areas in Canada.
When rain is acidic, it affects trees, lakes, buildings and agricultural land. Sometimes rain is not very acidic and does not cause a lot of problems, but when it is acidic, it can be very harmful to the environment.
The acid in acid rain drains important minerals from the leaves and the soil, and is very bad for plants, trees and agricultural land. If the soil is alkaline; when acid rain falls on it the acid becomes neutral and so the plants are not hugely affected, but it the soil is slightly acidic, it can be disastrous. When sufficient acid rain falls in to lakes and rivers, life can all but die out in a relatively short period of time depending on the mass of water.
Humans are affected when we breathe in air pollution, this can cause breathing problems, and even cancer. Drinking water which has been contaminated with acid rain can cause brain damage over time.
Acid rain also eats into stone and metal, so buildings can be affected by erosion over time, especially sandstone and limestone which are examples of soft stones.