Chlorine is a toxic, pale green gas with a sharp, choking smell. Tests with chlorine must be done in a fume cupboard. The test for chlorine uses damp litmus paper.
Put a piece of damp litmus paper into the gas. If the gas is chlorine, the litmus paper is bleached and turns white. The paper may turn red for a moment first, because chlorine dissolves in the water on the paper to make an acidic solution, but it then loses its colour completely.
The paper must be damp. Chlorine needs water to bleach the dye, so dry litmus paper is not bleached.
Do not confuse bleaching with a simple colour change. Many gases make litmus paper change colour, but only chlorine bleaches it white.
