# Methanal (HCHO) and ethanal (CH₃CHO) are both aldehydes. Propanone (CH₃COCH₃) is a ketone. Explain how you could use chemical tests to distinguish between methanal, ethanal, and propanone, naming the reagents used and the observations you would expect for each compound.

> WJEC A-Level Chemistry (Wales) — 3.4 Carbonyls and acids · Explain · 5 marks

> Aldehydes contain the functional group –CHO. Ketones contain the C=O group between two carbon atoms. Tollens' reagent (ammoniacal silver nitrate) and Fehling's solution are used to distinguish between aldehydes and ketones. Acidified potassium dichromate(VI) can be used as an oxidising agent.

## Mark scheme (5 marks)

1. Tollens' reagent (or Fehling's solution) can be used to distinguish aldehydes from ketones
2. Both methanal and ethanal (aldehydes) give a positive result with Tollens' reagent — silver mirror forms (or Fehling's solution gives a brick-red precipitate)
3. Propanone (ketone) gives no reaction / no silver mirror / no colour change with Tollens' reagent (or Fehling's solution) because ketones are not oxidised by these reagents
4. Acidified potassium dichromate(VI) can be used to distinguish methanal from ethanal — both decolourise/turn green, but methanal is oxidised further to methaноic acid (a carboxylic acid) while ethanal is oxidised to ethanoic acid; OR Tollens' reagent alone cannot distinguish methanal from ethanal, so a further test is needed (e.g. methanal reduces Tollens' to give a mirror more readily / methanal gives a positive result with Fehling's more vigorously)
5. Acidified potassium dichromate(VI) turns from orange to green with both methanal and ethanal (aldehydes are oxidised) but shows no colour change with propanone, confirming propanone is a ketone

## Key terms

- [aldehyde](https://www.gradenine.co.uk/glossary/aldehyde)
- [ketone](https://www.gradenine.co.uk/glossary/ketone)

## Related

- [Revision notes for WJEC A-Level Chemistry (Wales)](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/methanal-hcho-and-ethanal-ch-cho-are-4d624afe) · Published by Druglandscape Ltd.