Carbonic acid is an inorganic compound that occurs when carbon dioxide dissolves in water. Its chemical formula is H2CO3, and it is a weak acid.
Carbonic acid is a natural byproduct of respiration and is the only acid produced as a gas by the lungs. Two types of salts are formed by carbonic acid: carbonates and bicarbonates. Carbonic acid creates calcium bicarbonate when it dissolves limestone; this process forms geological structures such as stalactites and stalagmites. The post-industrial release of carbon dioxide into the air is responsible for ocean acidification, as approximately 50 percent of the additional carbon dioxide is dissolved into the oceans.