When carbon dioxide (CO₂) is dissolved in water, it reacts to form carbonic acid (H₂CO₃). Carbonic acid is considered a weak acid. The reaction can be represented by the following equation:
\[ \text{CO}_2 (g) + \text{H}_2\text{O} (l) \leftrightarrow \text{H}_2\text{CO}_3 (aq) \]