When SO₂ is passed through acidified potassium permanganate(VII) solution it decolourises the solution. This can be used for the detection of the SO₂ gas. The below reaction represents the test,

5SO₂ + 2MnO₄^- + 2H₂O → 5SO₄^2- + 4H⁺ + 2Mn²⁺