# 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.

> Eduqas A-Level Chemistry — 4.8 Organic synthesis and analysis · Explain · 5 marks

> 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.

## Mark scheme (5 marks)

1. The orange to green colour change with acidified potassium dichromate shows the compound has been oxidised / is oxidisable
2. No bubbles with sodium carbonate means the compound is not a carboxylic acid / does not contain the carboxyl (-COOH) group
3. The silver mirror with Tollens' reagent shows the compound can reduce the Tollens' reagent / contains an aldehyde group (-CHO)
4. The compound is therefore an aldehyde (contains the -CHO functional group)
5. 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

- [Tollens' reagent](https://www.gradenine.co.uk/glossary/tollens-reagent)
- [acidified potassium dichromate](https://www.gradenine.co.uk/glossary/acidified-potassium-dichromate)

## Related

- [Revision notes for Eduqas A-Level Chemistry](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-chemist-is-analysing-an-unknown-bc0d189e) · Published by Druglandscape Ltd.