Carbonic acid

Carbonic acid is an acid with the chemical formula of H₂CO₃. It is the reaction product of water and carbon dioxide and exists in an equilibrium with water and carbon dioxide whenever the latter is dissolved in the former, for instance in soda water or blood. It is not possible to obtain pure carbonic acid.

In solution, carbonic acid can lose one or two protons. Removing the first proton from carbonic acid forms the bicarbonate ion; removing the second proton leads to the carbonate ion.

• H₂CO₃ → HCO₃^- + H⁺ (pK_d = 6.35)
• HCO₃^- → CO₃^2- + H⁺ (pK_d = 10.33)

When carbonic acid is combined with positive or basic atoms or radicals, salts called carbonates or bicarbonates can form. For instance, combined with lime (calcium oxide) it constitutes marble and chalk (calcium carbonate).

Carbonic acid is present in carbonated soft drinks. In such beverages it occurs in approximately 150 times the concentration than in the natural occurrence which aided in the carving of Mammoth Cave.