# 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.

> OCR A-Level Chemistry B: Salters (H433) — ES Elements from the sea · Describe · 5 marks

> Iodide ions (I⁻) are present in the aqueous solution produced from seaweed ash. Iodine (I₂) can be displaced from this solution and then purified.

## Mark scheme (5 marks)

1. Chlorine water (or chlorine gas) is added to the solution containing iodide ions
2. Chlorine displaces iodine because chlorine is more reactive than iodine (higher in the halogen group / further up Group 7)
3. The solution turns brown / orange-brown, showing iodine has been produced
4. An organic solvent (such as cyclohexane) is added and the mixture is shaken; iodine dissolves preferentially into the organic layer, which turns purple / violet
5. The organic layer is separated and evaporated / the solvent is evaporated to leave pure, dry iodine

## Key terms

- [iodide ions](https://www.gradenine.co.uk/glossary/iodide-ions)

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-company-extracts-iodine-from-seaweed-10f606ca) · Published by Druglandscape Ltd.