Aqueous carbonic acid (H2CO3).

Carbonic acid (H2CO3) is a molecule right at the interface between organic and inorganic chemistry. In fact, according to some definitions it is an organic molecule, while according to others it is an inorganic molecule. Its exceptionality can also be recognized from the fact that it formally belongs neither to the mononor dicarboxylic acids, even though it is a C1 diprotic acid. The diprotic nature, that is, the ability to react to its conjugate base bicarbonate (HCO3 ) and to carbonate (CO3 2 ), provides a link to geology and mineralogy. Carbonate minerals such as calcite, dolomite or siderite are ubiquitous in sedimentary rock. In addition to the acid–base equilibrium, carbonic acid is in equilibrium with carbon dioxide and water. Both equilibria can be summarized as Equation (1):