ChlorineOxford AQA IGCSE Chemistry: Revision notes
Section 1
How can you recognise chlorine gas by smell?
Chlorine gas has a characteristic sharp, choking smell. This smell alone is a useful clue when identifying an unknown gas, but it should always be confirmed with the litmus paper test, since chlorine is toxic and should never be smelled directly or in large amounts.
In an exam, describe the smell as 'sharp' and 'choking' — vague descriptions like 'strong smell' are less likely to gain the mark.
Section 2
What is the test for chlorine gas?
The test for chlorine gas uses damp blue litmus paper. When chlorine gas is present:
- The damp blue litmus paper first turns red, showing that chlorine forms an acidic solution with the water on the paper.
- The paper is then bleached white, as chlorine has a strong bleaching effect.
Section 3
Why does litmus paper turn red then white?
Chlorine dissolves in the water on the damp litmus paper to form an acidic solution, which turns the blue litmus red. Chlorine is also a powerful bleaching agent, so it quickly destroys the coloured dye in the paper, turning it white.
This two-stage colour change (red, then white) is unique to chlorine among common gases and is a reliable way to confirm its identity.
Do not stop the observation at 'turns red' — the full mark scheme answer for chlorine requires both stages: red, then bleached white.
Must Know
- Chlorine gas has a characteristic sharp, choking smell
- The test for chlorine: use damp blue litmus paper
- A positive result: the litmus paper turns red, then is bleached white
- The red colour occurs because chlorine forms an acidic solution
- The bleaching occurs because chlorine is a strong bleaching agent
That's the notes covered.
Carry on to the next subtopic.