When sodium sulphite (Na₂SO₃) reacts with hydrochloric acid (HCl), it produces sulphur dioxide gas (SO₂), which readily decolourises acidified potassium permanganate (KMnO₄) solution. 

Reaction: Na₂SO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g) 

The reaction produces sodium chloride (NaCl), water (H₂O), and sulphur dioxide gas (SO₂). Sulphur dioxide gas (SO₂) is a reducing agent, and when it reacts with acidified potassium permanganate (KMnO₄), it decolourises the purple solution.