# 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.

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

> 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.

## Mark scheme (5 marks)

1. Bromine displaces iodine from potassium iodide solution because bromine is more reactive than iodine
2. The brown colour is due to iodine being produced / released in the solution
3. To confirm the identity of iodine, add starch solution
4. The starch turns blue-black / dark blue in the presence of iodine
5. The product potassium bromide is also formed in the displacement reaction

## Key terms

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

## Related

- [Revision notes for OCR A-Level Chemistry B: Salters (H433)](https://www.gradenine.co.uk/learn)
- [How to answer "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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Source: [GradeNine](https://www.gradenine.co.uk/q/a-coastal-chemical-plant-extracts-bromine-79ae1a2c) · Published by Druglandscape Ltd.