Test for chlorine: The test for chlorine uses litmus paper. When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.

The idea

Chlorine gas is detected using damp litmus paper. The paper must be damp because chlorine reacts with water to form acidic species that first turn the litmus red, and chlorine also bleaches the dye so the colour is then removed and the paper turns white. If the litmus paper is dry, the bleaching effect is not seen clearly, so the damp condition is essential. The method is to hold or place the damp litmus paper into the gas, for example at the mouth of a test tube or gas jar containing chlorine, and observe the colour change. The positive result is bleaching to white, not simply a colour change to red. Chlorine is toxic, so the test is performed in a fume cupboard or well-ventilated area, and the paper is not touched with bare hands.

What earns the marks

• The reagent is damp litmus paper, and the dampness is essential because chlorine reacts with water on the paper.
• The litmus paper is put into chlorine gas, for example by holding it at the mouth of a gas jar or test tube.
• Chlorine bleaches the litmus dye, so the paper loses its colour and turns white.
• A red colour may appear first because the solution formed on the damp paper is acidic, but the definitive observation for chlorine is bleaching to white.
• The test must be carried out safely because chlorine is toxic, so a fume cupboard or good ventilation is used.

Common mistakes and how to put them right

• Using dry litmus paper.

  Put it right: Without water the bleaching effect is not observed clearly, so the paper must be dampened before exposure to the gas.

• Reporting only that litmus turns red.

  Put it right: Reddening shows an acidic gas, but the distinctive result for chlorine is bleaching to white, so the final observation must be stated.

• Confusing bleaching with burning or dissolving.

  Put it right: The litmus dye is chemically destroyed by chlorine, so the paper turns white rather than being set on fire or simply wetted.

Exam tips

• State that the litmus paper must be damp, and explain that the dampness allows chlorine to react with water on the paper.
• Give the full observation: the litmus may turn red first, then it is bleached and turns white.
• Mention a safety precaution such as using a fume cupboard because chlorine is toxic.