Chlorine water is added separately to two test tubes, one containing potassium bromide solution and another containing potassium iodide solution. State what is observed in each test tube and explain why these reactions occur.

Pearson Edexcel International GCSE Chemistry (4CH1) — 2.2 Group 7 (halogens) · State and Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Chlorine is a halogen in Group 7 of the periodic table. It can react with solutions of other halide salts.

Model answer (4 marks)

Chlorine displaces bromine from the potassium bromide solution, giving an orange‑brown colour.
Chlorine displaces iodine from the potassium iodide solution, giving a dark brown colour.
The reason is that chlorine is more reactive than bromine and iodine; it can therefore remove them from their salts.
This is because, down Group 7, the halogens become less reactive as the atoms have more electron shells, making it harder to gain an electron.

Examiner tips

  • Use the command words ‘State’ and ‘Explain’ – give the colour change first, then the reason. Show the displacement reaction and the relative reactivity trend. Mention the electron‑shell argument for decreasing reactivity.
  • Use clear, exam‑style wording: ‘orange‑brown’ and ‘dark brown’ are the colours expected.

Mark scheme (4 marks)

  1. Chlorine displaces bromine from potassium bromide solution, so the solution turns orange/brown
  2. Chlorine displaces iodine from potassium iodide solution, so the solution turns brown/dark brown
  3. Chlorine is more reactive than both bromine and iodine, so it can displace them from their compounds
  4. Down Group 7, halogens become less reactive because the atoms have more electron shells, making it harder to gain an electron

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