When a colorless gas is passed through bromine water, only decolourisation takes places. The gas is (AFMC 2012)

Bromine water is an oxidizing agent it oxidises SO₂ to SO₃ and itself gets reduces to colorless HBr. 2 2 Colourless Orange red Br H O 2HBr [O] → SO₂ + [O] → SO₃ H₂O + SO₃ → H₂SO₄ H₂S is also oxidized by bromine water but in this case, decolourisation is also accompanied by the formation of sulphur. H₂ S + [O] → H₂O + S