SO₂ (g) + H₂O (l) ⇌ H₂SO₃ (aq)
This reaction is an equilibrium reaction, meaning it can go in both directions. However, the formation of sulfurous acid is favored under normal conditions.
Sulfurous acid is a weak acid, meaning it doesn't fully ionize in water. It can further react with water to produce bisulfite ions (HSO₃⁻) and sulfite ions (SO₃²⁻):
H₂SO₃ (aq) ⇌ H⁺ (aq) + HSO₃⁻ (aq)
HSO₃⁻ (aq) ⇌ H⁺ (aq) + SO₃²⁻ (aq)
The presence of sulfurous acid in water contributes to the acidity of acid rain.
