# A forensic scientist is analysing an unknown organic liquid found at a crime scene. The scientist suspects the liquid is either a haloalkane or an alcohol. Describe the chemical tests the scientist could carry out to distinguish between a haloalkane and an alcohol, including the expected observations for each compound.

> OCR A-Level Chemistry A (H432) — 4.2 Alcohols, haloalkanes and analysis · Describe · 5 marks

> Forensic scientists routinely use chemical tests to identify unknown organic compounds. Two common classes of organic compound are alcohols, which contain the –OH functional group, and haloalkanes, which contain a halogen atom bonded to a carbon chain.

## Mark scheme (5 marks)

1. Add the unknown liquid to water and then add sodium (or state reaction with sodium metal) — alcohol reacts with sodium to produce bubbles of hydrogen gas / effervescence
2. Haloalkane does not react (or reacts very slowly / no bubbles) with sodium compared to an alcohol
3. To identify the halogen in the haloalkane: heat the haloalkane with sodium hydroxide solution (to hydrolyse it) then acidify with dilute nitric acid and add silver nitrate solution
4. A precipitate forms with the haloalkane: white precipitate indicates chloroalkane, cream precipitate indicates bromoalkane, yellow precipitate indicates iodoalkane
5. No precipitate forms when the silver nitrate test is applied to the alcohol (after the same acidification step), confirming it is not a haloalkane

## Key terms

- [haloalkane](https://www.gradenine.co.uk/glossary/haloalkane)
- [alcohol](https://www.gradenine.co.uk/glossary/alcohol)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-forensic-scientist-is-analysing-an-f3c971c0) · Published by Druglandscape Ltd.