The test to show the presence of gas chlorine is by using damp starch iodide paper. Chlorine gas, being an oxidizing agent, easily liberates iodine from potassium iodide which, in turn, reacts with starch to produce a blue-black complex.

Procedure:

1. Take a piece of white paper and add a drop of starch solution on it.

2. Dip the starch paper in a container containing potassium iodide solution.

3. Now, expose the paper to the suspected gas chlorine.

Observation:

If the paper turns blue-black, it indicates the presence of chlorine gas.

Chemical reaction:

Cl2 + 2KI -> 2KCl + I2

Starch + I2 -> Blue-black complex