Chlorine is a toxic, pale green gas, so any test for it must be done in a fume cupboard. The chemical test uses damp litmus paper. Hold a piece of damp blue litmus paper in the gas.
If the gas is chlorine, the litmus paper first turns red, because chlorine dissolves in the water on the paper to form an acidic solution. The paper is then bleached white, because chlorine is a bleaching agent. The bleaching is what shows the gas is chlorine and not just any acidic gas.
The paper must be damp. Chlorine needs water to bleach the dye. Do not confuse this with the other gas tests: a glowing splint relights in oxygen, a lit splint gives a squeaky pop in hydrogen and limewater turns cloudy with carbon dioxide.
