# Iodine is an element that can be extracted from seaweed. A student carries out a series of reactions to obtain pure iodine from seaweed and then tests the iodine they have made. Describe how iodine can be identified in the laboratory, and explain why chlorine is able to displace iodine from a solution of potassium iodide during its extraction.

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

> Seaweed contains iodide ions. To extract iodine, the seaweed is burned to ash, the ash is dissolved in water, and chlorine water is added to the solution of iodide ions. The iodine produced can then be identified using a simple chemical test.

## Mark scheme (5 marks)

1. Iodine turns starch solution blue-black (accept: dark blue/blue-black colour with starch).
2. Chlorine is a more reactive (non-metal) than iodine / chlorine is higher in the reactivity series of halogens than iodine.
3. More reactive halogens displace less reactive halogens from their salt solutions / a more reactive halogen can take the place of a less reactive one.
4. Chlorine gains electrons more easily than iodine / chlorine atoms are better at gaining electrons (accept: chlorine is a better oxidising agent).
5. Iodide ions lose electrons to form iodine molecules / iodide ions are oxidised to iodine (I⁻ → I₂ / 2I⁻ → I₂ + 2e⁻).

## Related

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

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