When sulphur dioxide is dissolved in water, then it forms sulphurous acid (H₂SO₃).
Explanation: Metals react with oxygen to form metallic oxides. These metallic oxides are basic in nature because they react with water to form bases. On the other hand, non-metals react with oxygen to form non-metallic oxides and these oxides differ from metallic oxides because they are acidic in nature. Also, non-metallic oxides react with water to form acids.
Sulphur is non-metal, and sulphur dioxide (SO₂) is acidic in nature. Thus, it reacts with water to form sulphurous acid (H₂SO₃  or H₂O.SO₂)

The chemical reaction is: SO₂ + H₂O → H₂SO₃