Chlorine water is added to a solution of potassium bromide and then to a separate solution of potassium iodide. Describe what is observed in each reaction and explain why chlorine is able to displace both bromine and iodide from these solutions.

Cambridge International IGCSE Chemistry (0620) — 8.3 Group VII properties · Describe and Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Chlorine water turns the potassium bromide solution orange‑brown, indicating bromine has been liberated.
Chlorine water turns the potassium iodide solution dark brown, indicating iodine has been liberated.
Chlorine is more reactive than bromine and iodine.
A more reactive halogen displaces a less reactive halogen from a salt solution because chlorine atoms gain electrons more readily than bromine or iodine atoms.

Examiner tips

  • Use the colour change to identify the displaced halogen.
  • State the reactivity series of halogens.
  • Explain the displacement in terms of electron gain (reduction).

Common mistakes

  • Confusing the colour of the displaced halogen with the colour of chlorine water.
  • Failing to mention that chlorine is more reactive than both bromine and iodine.
  • Not linking the displacement to the relative ease of electron gain by chlorine.

Mark scheme (4 marks)

  1. Chlorine water turns the potassium bromide solution orange/brown (bromine is displaced/produced)
  2. Chlorine water turns the potassium iodide solution brown/dark brown (iodine is displaced/produced)
  3. Chlorine is more reactive than bromine and iodine
  4. A more reactive halogen displaces a less reactive halogen from a solution of its salt / chlorine atoms gain electrons more easily than bromine or iodine atoms

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