A company extracts iodine from seaweed on an industrial scale. After harvesting the seaweed, it is burned to produce ash. The ash is dissolved in water to produce an aqueous solution containing iodide ions. Describe and explain the steps needed to convert the iodide ions in this solution into a pure, dry sample of iodine. Include the observations a chemist would make at each stage.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Iodide ions (I⁻) are present in the aqueous solution produced from seaweed ash. Iodine (I₂) can be displaced from this solution and then purified.
Model answer (5 marks)
1. Add chlorine gas (or chlorine water) to the aqueous iodide solution.
2. Chlorine oxidises the iodide ions, displacing iodine:
2 I⁻ + Cl₂ → I₂ + 2 Cl⁻.
3. The solution changes colour from colourless to a brown/orange‑brown, indicating iodine has formed.
4. Add an immiscible organic solvent (e.g. cyclohexane) and shake; iodine dissolves into the organic phase, turning it purple/violet.
5. Separate the organic layer and evaporate the solvent, leaving a dry, pure iodine sample.
2. Chlorine oxidises the iodide ions, displacing iodine:
2 I⁻ + Cl₂ → I₂ + 2 Cl⁻.
3. The solution changes colour from colourless to a brown/orange‑brown, indicating iodine has formed.
4. Add an immiscible organic solvent (e.g. cyclohexane) and shake; iodine dissolves into the organic phase, turning it purple/violet.
5. Separate the organic layer and evaporate the solvent, leaving a dry, pure iodine sample.
Examiner tips
- Use the correct reaction equation and explain the displacement by reactivity; show colour change; mention immiscibility and separation; state evaporation to dryness.
Common mistakes
- Writing the wrong oxidising agent (e.g. NaCl instead of Cl₂); forgetting the colour change; not recognising the need to evaporate the solvent to obtain dry iodine.
Mark scheme (5 marks)
- Chlorine water (or chlorine gas) is added to the solution containing iodide ions
- Chlorine displaces iodine because chlorine is more reactive than iodine (higher in the halogen group / further up Group 7)
- The solution turns brown / orange-brown, showing iodine has been produced
- An organic solvent (such as cyclohexane) is added and the mixture is shaken; iodine dissolves preferentially into the organic layer, which turns purple / violet
- The organic layer is separated and evaporated / the solvent is evaporated to leave pure, dry iodine
Key terms in this question
Related
- All OCR A-Level Chemistry B: Salters (H433) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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