Bromine water is added to a solution of potassium iodide. Describe what is observed and explain, in terms of reactivity and electron transfer, why this reaction occurs.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Bromine water is added to a potassium iodide solution and the colour changes from a pale yellow to a deep orange‑brown and then to a dark brown.
This occurs because bromine is more reactive than iodine. Bromine molecules accept electrons from iodide ions:
Br₂ + 2 I⁻ → 2 Br⁻ + I₂
Thus bromine is reduced (gains electrons) and iodide is oxidised (loses electrons) to give iodine, which gives the brown colour.
This occurs because bromine is more reactive than iodine. Bromine molecules accept electrons from iodide ions:
Br₂ + 2 I⁻ → 2 Br⁻ + I₂
Thus bromine is reduced (gains electrons) and iodide is oxidised (loses electrons) to give iodine, which gives the brown colour.
Examiner tips
- Use the exact colour change wording (orange‑brown to dark brown).
- Show the balanced redox equation.
- Explain that Br₂ is more reactive than I₂.
Common mistakes
- Saying the solution becomes clear instead of brown.
- Forgetting to mention that bromine is reduced and iodide is oxidised.
Mark scheme (4 marks)
- The solution turns darker / orange-brown to dark brown (or iodine is seen to form)
- Bromine displaces iodide ions because bromine is more reactive than iodine
- Bromine gains electrons / is reduced (bromine molecules accept electrons from iodide ions)
- Iodide ions lose electrons / are oxidised (iodide ions donate electrons to form iodine molecules)
Related
- All Edexcel GCSE Chemistry (1CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →