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How do carbon dioxide and water form carbonic acid?

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A labeled diagram of the periodic table entry for carbon, showing its atomic number 6, symbol C, electron configuration, atomic weight range, solid physical state, hexagonal crystal structure, and weakly acidic oxide properties. Color coding and icons indicate it is a nonmetal, solid at room temperature, with hexagonal structure and weak acidity. — carbon Carbon and its properties.

Carbonic acid ( $H_{2} C O_{3}$ ) is formed when its anhydride, carbon dioxide ( $C O_{2}$ ), dissolves in water. This chemical process is represented by the following equilibrium reaction:

$C O_{2} + H_{2} O ⇌ H_{2} C O_{3}$

In this reaction, the predominant species are simply loosely hydrated $C O_{2}$ molecules. While this natural conversion is a relatively slow process, it can be significantly accelerated by carbonic anhydrase, a protein enzyme found in red blood cells that catalyzes the reaction to occur in a fraction of a second.

Once formed, carbonic acid acts as a diprotic acid, meaning it can dissociate into hydrogen ions and bicarbonate ions ( $H C O_{3}^{-}$ ), and subsequently into carbonate ions ( $C O_{3}^{2 -}$ ). This chemistry is essential to various natural and biological processes, including the formation of caves through the dissolution of limestone and the transport of carbon dioxide in the blood.1