# A student tests an unknown organic liquid using a variety of chemical tests. The liquid does not decolourise bromine water, gives a silver mirror with Tollens' reagent, and produces a cream precipitate when tested with acidified silver nitrate solution. Explain what each of these results tells the student about the functional groups present in the unknown liquid.

> Eduqas A-Level Chemistry — 4.8 Organic synthesis and analysis · Explain · 5 marks

> An unknown organic liquid is subjected to three chemical tests: bromine water, Tollens' reagent, and acidified silver nitrate solution.

## Mark scheme (5 marks)

1. No decolourisation of bromine water shows there is no C=C double bond (no alkene / unsaturation) present
2. Silver mirror with Tollens' reagent shows an aldehyde group (–CHO) is present
3. Tollens' reagent works because the aldehyde reduces the silver ions to silver metal / the aldehyde is oxidised
4. Cream precipitate with acidified silver nitrate shows a bromide ion (Br⁻) is present / the liquid contains a C–Br bond (bromoalkane)
5. The cream precipitate is silver bromide (AgBr)

## Key terms

- [bromine water](https://www.gradenine.co.uk/glossary/bromine-water)
- [Tollens' reagent](https://www.gradenine.co.uk/glossary/tollens-reagent)
- [acidified silver nitrate](https://www.gradenine.co.uk/glossary/acidified-silver-nitrate)

## Related

- [Revision notes for Eduqas A-Level Chemistry](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-student-tests-an-unknown-organic-fdd20b02) · Published by Druglandscape Ltd.