Here's why:

* Carbon dioxide reacts with water: CO2 dissolves in water to form carbonic acid (H2CO3).

* Carbonic acid is a weak acid: While carbonic acid is an acid, it doesn't fully dissociate in water, making it a weak acid. This means it only releases a small amount of hydrogen ions (H+), resulting in a mild acidity.

The reaction is as follows:

CO2 (g) + H2O (l) ⇌ H2CO3 (aq)

This process is responsible for the slight acidity of rainwater, contributing to phenomena like acid rain.