The gas from the air that dissolves in rain to make it slightly acidic is carbon dioxide (CO2).

Here's how it works:

* Carbon dioxide in the atmosphere: CO2 is a natural component of the atmosphere.

* Dissolving in water: When CO2 dissolves in water, it forms carbonic acid (H2CO3).

* Slightly acidic: Carbonic acid is a weak acid, so rainwater is slightly acidic, typically with a pH of around 5.6.

This natural acidity is important for some ecosystems, but human activities, such as burning fossil fuels, have increased the amount of CO2 in the atmosphere, leading to acid rain with a lower pH.