Carbonic acid is also an archaic name for carbon dioxide. Carbonic acid is a chemical compound with the chemical formula H 2 CO 3 (equivalently OC(OH) 2). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H 2 CO 3.In physiology, carbonic acid is described as volatile acid or respiratory acid, because it is the only acid excreted as a gas by the lungs.arbonic acid is also an archaic name for carbon dioxide. Carbonic acid is a chemical compound with the chemical formula H 2 CO 3 (equivalently OC(OH) 2). It is also a name sometimes given to solutions of carbon dioxide in water (carbonated water), because such solutions contain small amounts of H 2 CO 3.