Weathered limestone
Chemical weathering occurs when rocks are broken down by a chemical change. Rainwater can become slightly acidic by absorbing carbon dioxide in the atmosphere and this reacts with the mineral grains in the rock giving rise to new minerals and salts.
The degree of chemical weathering depends on the type of rock for example limestone is more readily chemically weathered than granite. Other factors such as temperature also play a role as the chemical reactions occur more quickly in areas of high temperatures.
Acid rain is also a contributor to chemical weathering. When fossil fuels such as coal, gas and gasoline are burnt they release oxides sulphur, carbon and nitrogen into the atmosphere. These gases combine with moisture in the air to form sulphuric acid, carbonic acid and nitric acid, making the resulting rainwater more acidic than normal. Original public domain image from Flickr