AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Chemical tests to distinguish between aldehydes and ketones including…
Full specification wording
Aldehydes and ketones (A-level only): Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Both tests work by offering a mild oxidising agent that only an aldehyde can reduce.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Both tests work by offering a mild oxidising agent that only an aldehyde can reduce. Tollens' reagent is made fresh by adding aqueous ammonia to silver nitrate solution until the precipitate redissolves, giving the ion [Ag(NH₃)₂]⁺. Warmed in a water bath with an aldehyde, the silver ion is reduced to silver, which deposits as a mirror on the tube wall; a ketone gives no change. Fehling's solution contains copper(II) ions held in alkaline solution and is deep blue. Warmed with an aldehyde, copper(II) is reduced to copper(I) and a red precipitate of copper(I) oxide appears; with a ketone it stays blue. In each case the aldehyde is itself oxidised to the carboxylic acid or its salt.
What earns the marks
- Naming the reagent fully, Tollens' reagent or Fehling's solution, with warming in a water bath.
- The positive observation, a silver mirror or a red precipitate.
- Stating that the ketone gives no change.
- Saying the aldehyde is oxidised while the silver or copper ion is reduced.
Common mistakes and how to put them right
Cooling the tube instead of warming it, a method error AQA has specifically penalised.
Describing the Fehling's precipitate as brown or green rather than red.
Recording a "negative colour change" for the ketone instead of simply no change.
Giving the Tollens' product as silver oxide rather than silver metal.
Exam tips
- Always state the condition, warm in a water bath, alongside the reagent; it carries a mark of its own.
- Give the observation for both compounds, the aldehyde and the ketone.
- Say what happens to the metal ion as well as to the organic compound if the question says explain.
Try the lesson
The start of the Aldehydes and ketones (A-level only): Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent. lesson, made from checked teaching notes for this specification point.
Have a go first
Which reagent gives a silver mirror when warmed with an aldehyde?
Listen
Both tests work by offering a mild oxidising agent that only an aldehyde can reduce. Tollens' reagent is made fresh by adding aqueous ammonia to silver nitrate solution until the precipitate redissolves, giving the ion [Ag(NH₃)₂]⁺. Warmed in a water bath with an aldehyde, the silver ion is reduced to silver, which deposits as a mirror on the tube wall; a ketone gives no change. Fehling's solution contains copper(II) ions held in alkaline solution and is deep blue. Warmed with an aldehyde, copper(II) is reduced to copper(I) and a red precipitate of copper(I) oxide appears; with a ketone it stays blue. In each case the aldehyde is itself oxidised to the carboxylic acid or its salt. A strong answer earns marks for these points. naming the reagent fully, Tollens' reagent or Fehling's solution, with warming in a water bath. the positive observation, a silver mirror or a red precipitate. stating that the ketone gives no change. saying the aldehyde is oxidised while the silver or copper ion is reduced. Exam tip: Always state the condition, warm in a water bath, alongside the reagent; it carries a mark of its own.
See it
Key words
Say what each word means before you turn the card.
Tollens' reagent
Try answering aloud or in your head before turning the card.
Fehling's solution
Try answering aloud or in your head before turning the card.
silver mirror
Try answering aloud or in your head before turning the card.
red copper(I) oxide
Try answering aloud or in your head before turning the card.
See every definition
- Tollens' reagent
- Made fresh by adding aqueous ammonia to silver nitrate solution until the precipitate redissolves, giving the ion [Ag(NH₃)₂]⁺.
- Fehling's solution
- Contains copper(II) ions held in alkaline solution and is deep blue.
- silver mirror
- The deposit of silver on the tube wall when the silver ion is reduced by an aldehyde.
- red copper(I) oxide
- The red precipitate that appears when copper(II) is reduced to copper(I) by an aldehyde.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Aldehydes and ketones (A-level only): Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge