# A student has two unlabelled bottles, each containing a different carbonyl compound. One bottle contains pentan-2-one and the other contains pentanal. The student carries out two chemical tests to identify which bottle contains which compound. Describe how the student could use Fehling's solution and acidified potassium dichromate(VI) solution to distinguish between pentan-2-one and pentanal, and explain the observations expected for each test with each compound.

> Edexcel A-Level Chemistry (9CH0) — 17.2 Aldehydes and ketones · Describe · 5 marks

## Mark scheme (5 marks)

1. Fehling's solution turns from blue to brick-red/orange precipitate with pentanal only
2. Aldehydes (pentanal) are oxidised / act as reducing agents, so reduce Cu²⁺ to Cu⁺ (copper(I) oxide), which is the brick-red precipitate
3. Ketones (pentan-2-one) do not react with Fehling's solution because ketones cannot be oxidised under mild conditions / are not reducing agents
4. Acidified potassium dichromate(VI) changes colour from orange to green with pentanal
5. Acidified potassium dichromate(VI) remains orange with pentan-2-one because ketones are not oxidised by this reagent under these conditions

## Key terms

- [carbonyl compound](https://www.gradenine.co.uk/glossary/carbonyl-compound)
- [Fehling's solution](https://www.gradenine.co.uk/glossary/fehling-s-solution)
- [acidified potassium dichromate(VI)](https://www.gradenine.co.uk/glossary/acidified-potassium-dichromate-vi)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-has-two-unlabelled-bottles-d3ee166f) · Published by Druglandscape Ltd.