To carry out a test for the presence of chlorine gas, follow these steps:
1. First, you will need a test tube that contains the sample gas.
2. Remove the bung and insert a piece of damp blue litmus paper into the opening of the test tube.
3. Observe the litmus paper carefully. If chlorine is present, the paper will turn from blue to white.
However, the paper may turn red briefly before turning white. This is because the chlorine reacts with the water on the damp paper, to form hydrochloric acid, which is acidic. As a result, the blue litmus paper turns red before eventually turning white as it gets bleached by the chlorine.