To carry out a test for the presence of chlorine gas, follow these steps:
1. Obtain a test tube that contains the sample gas.
2. Remove the bung from the test tube and insert a piece of damp blue litmus paper into its opening.
3. Carefully observe the litmus paper. If chlorine is present, the paper will change colour, turning from blue to white.
However, the paper might initially turn red before turning white. This brief change to red occurs because the chlorine reacts with the water on the damp paper, forming hydrochloric acid. As a result, the blue litmus paper turns red before eventually turning white as it gets bleached by the chlorine.