Tests for Chlorine

Physical Properties of Chlorine

Chlorine is a greenish-yellow gas at room temperature. It has a strong, pungent smell that is easily noticeable. Chlorine gas is denser than air, so it tends to stay close to the ground when released.

Test for Chlorine Gas

The standard test for chlorine gas involves using damp blue litmus paper. When chlorine gas comes into contact with damp blue litmus paper, it bleaches the paper, turning it white. After bleaching, the paper may then turn red due to the formation of acidic compounds from chlorine reacting with water.

This bleaching effect is distinctive and helps to identify chlorine gas. The reaction can be summarised as:

Chlorine reacts with water in the damp paper to form hydrochloric acid and hypochlorous acid, which causes the litmus paper to first bleach and then turn red:

$$\mathrm{Cl_2 + H_2O \rightarrow HCl + HClO}$$

Because chlorine is a strong oxidising agent, it removes the colour from the dye in the litmus paper.

Chlorine gas also irritates the eyes and respiratory system, so it should be handled carefully.

For instance, if you expose damp blue litmus paper to chlorine gas, you will see the paper lose its blue colour and become white, then after a short time, it may turn red due to acidity.

Safety Precautions When Testing for Chlorine

Chlorine gas is toxic and irritating, so safety is very important:

• Always perform tests in a well-ventilated area or under a fume cupboard to avoid inhaling chlorine gas.
• Avoid breathing in the gas as it irritates the respiratory system and eyes.
• Wear safety goggles and gloves to protect your eyes and skin from irritation.

Confirmatory Observations for Chlorine

The key confirmatory test for chlorine is the bleaching of damp blue litmus paper. This is a distinctive and reliable test because:

• Chlorine bleaches the paper white due to its oxidising properties.
• The paper then turns red because of the acidic conditions created by hydrochloric acid formed.
• There is no flame test for chlorine gas, unlike some other gases.
• Comparing with other halogens: chlorine’s greenish-yellow colour and bleaching action on damp litmus paper help distinguish it from bromine (reddish-brown gas) and iodine (purple vapour).