Carbonic acid is a weak 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 usually not possible to obtain pure hydrogen bicarbonate as the presence of even a single molecule of water causes the carbonic acid to revert to carbon dioxide and water fairly quickly. However, pure carbonic acid has been found to be quite stable in the absence of water, with a calculated half-life of 180,000 years.

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 more diluted form it plays a major part in cave formation.

External links

• Ask a Scientist: Carbonic Acid Decomposition
• Why was the existence of carbonic acid unfairly doubted for so long?