Test for chlorine — AQA GCSE Chemistry
Test yourself on Test for chlorine with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Test for chlorine explained
Chlorine is a pale yellow-green toxic gas, so it must never be smelled directly or prepared in open laboratory conditions without suitable ventilation.
Read the full explanation
To test for it, damp a piece of litmus paper with distilled water, hold it in the gas with forceps, and observe. Chlorine dissolves in the water film to form an acidic solution, so the paper first turns red; the chlorine then bleaches the dye, so the colour is removed and the paper turns white. The dampness is essential because dry litmus does not respond. The bleaching effect distinguishes chlorine from acidic gases such as hydrogen chloride, which redden damp litmus but do not bleach it.
Your focus
- Carry out the damp litmus test for chlorine safely and record the colour changes in order.
- Explain why the litmus must be damp and why the paper is finally white.
- Use the bleaching observation to identify an unknown gas as chlorine.
Test for chlorine exam tips
Marking Points
- States that the litmus paper must be damp, for example moistened with distilled water, before exposure to the gas.
- Describes holding the damp litmus paper in the chlorine gas, for example using forceps, and observing the colour change.
- Explains that the litmus first turns red because chlorine dissolves to form an acidic solution.
- Explains that the dye is then bleached, so the paper loses its colour and turns white.
- Distinguishes chlorine from an acidic gas that only reddens damp litmus, because only chlorine bleaches it.
- Links the observation to the conclusion that the gas is chlorine, rather than to a named product.
Examiner Tips
- 💡Write the observation as a sequence: damp litmus turns red, then is bleached to white.
- 💡Name the safety precaution of not inhaling chlorine and testing in a fume cupboard or well-ventilated area.
- 💡If asked to identify an unknown gas, quote the bleaching of damp litmus as the specific evidence for chlorine.
Common Mistakes
- Using dry litmus paper: the error is that no colour change is seen, and the correction is to damp the paper with distilled water first.
- Reporting only that the paper turns white: the error is that the initial reddening is omitted, and the correction is to record both the red colour and the subsequent bleaching.
- Confusing bleaching with a colour change to blue or another colour: the error is a wrong final colour, and the correction is to state that the paper turns white because the dye is destroyed.