* Dissolving in water: When sulfur dioxide dissolves in water, it forms sulfurous acid (H2SO3):

SO2 (g) + H2O (l) ⇌ H2SO3 (aq)

* Sulfurous acid is a weak acid: Sulfurous acid can donate hydrogen ions (H+) to solutions, making them acidic.

Therefore, sulfur dioxide is acidic due to its ability to form an acidic solution when dissolved in water.