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