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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
1. Treat each compound with Tollens' reagent (ammoniacal AgNO₃).
2. Methanal and ethanal both give a positive result – a silver mirror forms (or Fehling's gives a brick‑red precipitate).
3. Propanone gives no reaction – no silver mirror, no colour change.
4. To distinguish methanal from ethanal, add acidified potassium dichromate(VI).
5. Both aldehydes decolourise the orange dichromate to green, but methanal is oxidised further to methanoic acid (stronger, more rapid change) whereas ethanal gives a slower, less intense change to ethanoic acid; propanone shows no colour change.
2. Methanal and ethanal both give a positive result – a silver mirror forms (or Fehling's gives a brick‑red precipitate).
3. Propanone gives no reaction – no silver mirror, no colour change.
4. To distinguish methanal from ethanal, add acidified potassium dichromate(VI).
5. Both aldehydes decolourise the orange dichromate to green, but methanal is oxidised further to methanoic acid (stronger, more rapid change) whereas ethanal gives a slower, less intense change to ethanoic acid; propanone shows no colour change.
Examiner tips
- Use the command word ‘explain’ – state the reagent, the reaction and the observation for each compound.
- Show the sequence of tests: first Tollens/Fehling to separate aldehydes from ketones, then dichromate to separate the two aldehydes.
- Mention the colour change (orange to green) and the formation of a silver mirror or brick‑red precipitate.
Mark scheme (5 marks)
- Tollens' reagent (or Fehling's solution) can be used to distinguish aldehydes from ketones
- Both methanal and ethanal (aldehydes) give a positive result with Tollens' reagent — silver mirror forms (or Fehling's solution gives a brick-red precipitate)
- 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
- 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)
- 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 in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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