When carbon dioxide (CO2) dissolves in water (H2O), it undergoes a chemical reaction to form carbonic acid (H2CO3). Carbonic acid is a weak acid that partially dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). This process can be represented by the following chemical equation:

CO2 + H2O ⇌ H2CO3 ⇌ H+ + HCO3-

The extent to which CO2 dissolves in water and forms carbonic acid depends on several factors, including temperature, pressure, and the presence of other dissolved substances. In general, the solubility of CO2 in water increases as temperature decreases and pressure increases. Conversely, the presence of other dissolved substances, such as salts, can reduce the solubility of CO2.

Carbonic acid is an important component of the Earth's carbon cycle and plays a crucial role in regulating the pH of natural waters. It also contributes to the formation of carbonate rocks, such as limestone and dolomite, which are essential components of the Earth's crust.