The acid formed by mixing water with carbon dioxide is carbonic acid (H₂CO₃).

Here's the reaction:

CO₂ (g) + H₂O (l) ⇌ H₂CO₃ (aq)

* CO₂ is carbon dioxide gas

* H₂O is liquid water

* H₂CO₃ is carbonic acid, which exists in aqueous solution (aq)

Carbonic acid is a weak acid, meaning it only partially ionizes in water. This reaction is reversible, and the equilibrium lies towards the left side, meaning that most of the carbon dioxide remains dissolved in water.

This process is important in the formation of acid rain and the regulation of pH in natural waters.