Webcarbonic acid, (H 2 CO 3), a compound of the elements hydrogen, carbon, and oxygen.It is formed in small amounts when its anhydride, carbon dioxide (CO 2), dissolves in water. CO 2 + H 2 O ⇌ H 2 CO 3 The predominant species are simply loosely hydrated CO 2 molecules.Carbonic acid can be considered to be a diprotic acid from which two series of salts can be formed—namely, hydrogen ...