How does sulfur dioxide react with water?

When sulfur dioxide (SO₂) dissolves in water, it creates an acidic solution. While this is often referred to as a solution of sulfurous acid (H₂SO₃), pure anhydrous sulfurous acid has never been isolated, and the solution contains very little, if any, H₂SO₃.1

Instead, studies indicate that the solution primarily consists of hydrated SO₂ molecules (SO₂ · nH₂O). Depending on factors such as temperature, concentration, and pH, the solution contains various ions, including H₃O⁺ and HSO₃⁻. This resulting mixture acts as a moderately strong acid.12

In the atmosphere, sulfur dioxide reacts with water and other chemicals to contribute to acid rain. Over time, the sulfurous acid in these solutions can be oxidized by oxygen in the air to form the more stable and stronger sulfuric acid.31