# A student is investigating the chemistry of halogens extracted from the sea. They have a solution containing iodide ions (I⁻) and want to confirm whether a more reactive halogen can displace iodine from this solution. They then wish to identify the iodine produced using a named reagent. Describe how the student could carry out this displacement reaction and identify the iodine formed, explaining why the displacement occurs in terms of reactivity.

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

> Iodine can be obtained from solutions containing iodide ions by reacting them with a more reactive halogen. The reactivity order of the halogens decreases down Group 7 of the periodic table.

## Mark scheme (5 marks)

1. A more reactive halogen (e.g. chlorine or bromine) is added to the solution containing iodide ions
2. The solution turns brown/orange, showing that iodine has been produced
3. Starch solution is used to identify/confirm the presence of iodine
4. Starch turns blue-black in the presence of iodine
5. Displacement occurs because chlorine/bromine is more reactive than iodine and so displaces it from the solution / a more reactive halogen can displace a less reactive halogen from its salt solution

## Key terms

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

## 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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-investigating-the-chemistry-d55804ba) · Published by Druglandscape Ltd.