Test for Chlorine

Testing a gas for chlorine

To test whether a gas is chlorine, dampen a piece of litmus paper with a few drops of water and place the damp end in the gas. A positive result is that the litmus paper is bleached and turns white. Either red or blue litmus paper can be used.

This is a qualitative test: it identifies chlorine from a distinctive observation rather than measuring how much chlorine is present.

Why the paper must be damp

The water is essential because chlorine first dissolves and reacts in the moisture on the paper:

$$\mathrm{Cl_2(g) + H_2O(l) \rightleftharpoons HCl(aq) + HOCl(aq)}$$

The solution formed is acidic and contains chloric(I) acid, $\mathrm{HOCl}$, which is a strong oxidising agent. It oxidises the coloured litmus indicator, disrupting the chemical structures responsible for its colour. The indicator therefore becomes colourless, so the white paper underneath is seen.

With damp blue litmus paper, the acidic solution may make the paper turn red before the bleaching action turns it white. The identifying final observation is the bleaching to white.

Carrying out the test safely

Chlorine is toxic, so it must not be inhaled or tested by smelling it. In a controlled school test, only a small quantity is used, the damp paper is held in the gas rather than touched to a reaction mixture, and tweezers may be used to hold it. Record the observation as “the damp litmus paper is bleached white”.

Test and result

• Test: place damp litmus paper in the gas.
• Positive result for chlorine: the paper is bleached and turns white.
• Red or blue litmus paper may be used.

Essential distinction

The paper must be damp because chlorine reacts in the water on it. Damp blue litmus may turn red first, but the final identifying result is bleaching to white.

Safety

Chlorine is toxic: do not inhale or smell the gas.