Carbonic acid is a chemical compound with the formula H₂CO₃. It is formed when carbon dioxide dissolves in water. Carbonic acid is a weak acid, which means that it does not completely dissociate in water. When carbonic acid dissociates, it produces hydrogen ions (H+) and bicarbonate ions (HCO₃-).

When carbon dioxide is dissolved in water, it reacts with water molecules to form carbonic acid. This reaction is reversible, which means that carbonic acid can also decompose to form carbon dioxide and water. The equilibrium constant for this reaction is very small, which means that the concentration of carbonic acid in water is typically very low.

Carbonic acid is important in many natural processes, such as the weathering of rocks and the transport of carbon dioxide in the atmosphere. It is also used in some industrial processes, such as the production of beverages and the manufacture of paper.