Here's why:

* Acids donate protons (H⁺ ions) in solution.

* Bases accept protons (H⁺ ions) in solution.

Sulfurous acid, when dissolved in water, releases H⁺ ions, making the solution more acidic:

H₂SO₃ (aq) ⇌ H⁺ (aq) + HSO₃⁻ (aq)

Therefore, sulfurous acid is a weak acid, not a base.