A coastal chemical plant extracts bromine from sea water. The plant operator notices that when the extracted bromine is shaken with a solution of potassium iodide, the colourless solution turns brown. Explain what is observed, including why the colour change occurs, and describe how a student could confirm the identity of the brown substance using a named reagent.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Bromine is a halogen extracted from sea water at industrial scale. It is a more reactive halogen than iodine. When bromine water is mixed with potassium iodide solution, a displacement reaction occurs.
Model answer (5 marks)
Bromine displaces iodine from potassium iodide because bromine is a more reactive halogen. The reaction:
Br₂(aq) + 2 KI(aq) → 2 KBr(aq) + I₂(aq)
The brown colour comes from the iodine (I₂) that is released into the solution. A student can confirm the iodine by adding a starch solution; the starch forms a blue‑black complex with iodine, giving a dark blue colour. The reaction also produces potassium bromide as the displaced product.
Br₂(aq) + 2 KI(aq) → 2 KBr(aq) + I₂(aq)
The brown colour comes from the iodine (I₂) that is released into the solution. A student can confirm the iodine by adding a starch solution; the starch forms a blue‑black complex with iodine, giving a dark blue colour. The reaction also produces potassium bromide as the displaced product.
Examiner tips
- Show the displacement equation and explain reactivity trend; mention iodine’s brown colour; state the starch test and colour change; note the formation of KBr
Common mistakes
- Writing the reaction with the wrong stoichiometry; confusing the colour change as due to bromine instead of iodine; forgetting to mention the starch test or the blue‑black complex
Mark scheme (5 marks)
- Bromine displaces iodine from potassium iodide solution because bromine is more reactive than iodine
- The brown colour is due to iodine being produced / released in the solution
- To confirm the identity of iodine, add starch solution
- The starch turns blue-black / dark blue in the presence of iodine
- The product potassium bromide is also formed in the displacement reaction
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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