A student has a sample of propanal and a sample of propanone. She wants to confirm which sample is which using a chemical test. Describe how the student could use Fehling's solution to identify which sample is propanal, and explain the chemistry that allows this identification to be made.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Add Fehling's solution to each sample and warm the mixture.
The sample that produces a brick‑red/orange‑red precipitate is propanal.
Propanone gives no colour change and remains blue with Fehling's solution.
Propanal is an aldehyde and is oxidised by Fehling's solution, acting as a reducing agent.
Propanone is a ketone and cannot be oxidised under these conditions; ketones do not react with Fehling's solution.
The sample that produces a brick‑red/orange‑red precipitate is propanal.
Propanone gives no colour change and remains blue with Fehling's solution.
Propanal is an aldehyde and is oxidised by Fehling's solution, acting as a reducing agent.
Propanone is a ketone and cannot be oxidised under these conditions; ketones do not react with Fehling's solution.
Examiner tips
- Use the command words ‘describe’ and ‘explain’ – list the steps first, then give the chemical reason. Mention the colour change and the red precipitate. Explain that aldehydes are oxidised to carboxylic acids, while ketones are not. Keep the answer concise and use the exact terminology (Fehling's solution, reducing agent).
Common mistakes
- Confusing the colour of the precipitate (red vs blue). Forgetting to state that the reaction is a redox process. Mixing up which compound is the aldehyde and which is the ketone.
Mark scheme (5 marks)
- Add Fehling's solution to each sample and warm/heat the mixture
- The sample that produces a brick-red/orange-red precipitate is propanal
- Propanone gives no colour change / remains blue with Fehling's solution
- Propanal is an aldehyde and is oxidised by Fehling's solution / acts as a reducing agent
- Propanone is a ketone and cannot be oxidised under these conditions / ketones do not react with Fehling's solution
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Describe and Explain" question →
- Decode the mark scheme abbreviations →
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