A chemist is analysing an unknown organic compound. The compound is a colourless liquid that turns orange dichromate solution green when warmed with acidified potassium dichromate. When the compound is reacted with sodium carbonate solution, no bubbles are produced. The compound also gives a silver mirror when warmed with Tollens' reagent. Explain what these observations tell the chemist about the identity of the functional group present in the unknown compound, and name the type of compound it is.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An unknown organic compound is subjected to three chemical tests. Acidified potassium dichromate solution turns from orange to green. Sodium carbonate solution produces no effervescence. Tollens' reagent produces a silver mirror.
Model answer (5 marks)
The orange–green change with acidified potassium dichromate shows the compound is oxidisable, i.e. it contains a functional group that can be oxidised to a carboxylic acid.
No effervescence with sodium carbonate means the compound does not release CO₂, so it is not a carboxylic acid (no –COOH group).
A silver mirror with Tollens’ reagent indicates the compound can reduce Ag⁺ to Ag, which is characteristic of an aldehyde (–CHO) group.
Thus the unknown is an aldehyde.
Because aldehydes are oxidised by dichromate and give a positive Tollens’ test, while ketones do not, the two positive tests together confirm the aldehyde identity and distinguish it from a ketone.
No effervescence with sodium carbonate means the compound does not release CO₂, so it is not a carboxylic acid (no –COOH group).
A silver mirror with Tollens’ reagent indicates the compound can reduce Ag⁺ to Ag, which is characteristic of an aldehyde (–CHO) group.
Thus the unknown is an aldehyde.
Because aldehydes are oxidised by dichromate and give a positive Tollens’ test, while ketones do not, the two positive tests together confirm the aldehyde identity and distinguish it from a ketone.
Examiner tips
- Use the command word ‘Explain’ – describe each observation and link it to the functional group. Show the logical sequence: oxidisable → not CO₂ → aldehyde. Mention the distinguishing feature that aldehydes oxidise and give Tollens’ test, unlike ketones.
Common mistakes
- Confusing the dichromate test with a carboxylic acid test. Assuming the silver mirror proves a ketone. Omitting the logical link that no CO₂ rules out –COOH.
Mark scheme (5 marks)
- The orange to green colour change with acidified potassium dichromate shows the compound has been oxidised / is oxidisable
- No bubbles with sodium carbonate means the compound is not a carboxylic acid / does not contain the carboxyl (-COOH) group
- The silver mirror with Tollens' reagent shows the compound can reduce the Tollens' reagent / contains an aldehyde group (-CHO)
- The compound is therefore an aldehyde (contains the -CHO functional group)
- Aldehydes (unlike ketones) are oxidised by dichromate AND give a positive Tollens' test, so both results together confirm the aldehyde identification / distinguishes it from a ketone
Key terms in this question
Tollens' reagent · acidified potassium dichromate
Related
- All Eduqas A-Level Chemistry revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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