Chemical Weathering is the process by which rocks break down as a result of chemical reactions.
Common agents of chemical weathering are water, weak acids and air.
Water
If you place a sugar cube in a glass filled with water it will dissolve because of chemical weathering. The same thing will happen with granite, just at a slightly slower pace.
Weak Acids
Acid Precipitation is rain, sleet, or snow that contain a high concentration of acids. Precipitation contains acid. Acid precipitation contains more acid than normal precipitation.
Acids in Groundwater
Some places of groundwater contain weak acids. Carbonic and Sulfuric acids are examples of acids in groundwater. When groundwater and limestone join they form a chemical reacton occurs. Over along period of time the dissolving limestone creates a karst landscape. Which is also know as caverns.
Acids in Living Things
Take a look at the lichens on the picture below. Lichens produce acids that can slowly and over time break down rock. Lichens grow on trees and rocks. Lichens grow in deserts, arctic areas, and high areas above timberline.