What is the test for a primary or secondary alcohol?

Warm with acidified potassium dichromate(VI): orange to green.

What is the test for an aldehyde with Tollens' reagent?

Warm with Tollens' reagent: a silver mirror forms.

What is the test for an aldehyde with Fehling's solution?

Warm with Fehling's solution: blue to a red precipitate.

How do you distinguish an aldehyde from a ketone?

Aldehydes give a silver mirror or red precipitate; ketones give no change.

What is the test for a carboxylic acid?

Add sodium carbonate solution: effervescence of CO₂.

Why does dichromate(VI) not tell an alcohol from an aldehyde?

Both are oxidised, so both turn the mixture orange to green.

How is a halogenoalkane tested for its halogen?

Warm with NaOH(aq), acidify with dilute HNO₃, then add AgNO₃(aq).

Why is nitric acid added before the silver nitrate?

To remove excess OH⁻, which would form a precipitate with Ag⁺.

Test-tube reactions for functional groupsAQA A-Level Chemistry: Flashcards

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What is the test for an alkene, and the positive result?

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What is the test for an alkene, and the positive result?: Add bromine water: orange to colourless.
What is the test for a primary or secondary alcohol?: Warm with acidified potassium dichromate(VI): orange to green.
What does a tertiary alcohol do with acidified dichromate(VI)?: Nothing: it stays orange.
What is the test for an aldehyde with Tollens' reagent?: Warm with Tollens' reagent: a silver mirror forms.
What is the test for an aldehyde with Fehling's solution?: Warm with Fehling's solution: blue to a red precipitate.
How do you distinguish an aldehyde from a ketone?: Aldehydes give a silver mirror or red precipitate; ketones give no change.
What is the test for a carboxylic acid?: Add sodium carbonate solution: effervescence of CO₂.
How can the gas from the carbonate test be identified?: It turns limewater milky.
Why does dichromate(VI) not tell an alcohol from an aldehyde?: Both are oxidised, so both turn the mixture orange to green.
How is a halogenoalkane tested for its halogen?: Warm with NaOH(aq), acidify with dilute HNO₃, then add AgNO₃(aq).
What are the colours of AgCl, AgBr and AgI?: White, cream and yellow.
Why is nitric acid added before the silver nitrate?: To remove excess OH⁻, which would form a precipitate with Ag⁺.

Exam questions on Test-tube reactions for functional groups

A technician has four unlabelled bottles. Each contains one of hex-1-ene, hexan-1-ol, hexanal and hexanoic acid. She labels them W, X, Y and Z and plans simple test-tube reactions to identify each liquid.
Write an equation for the reaction of hexanoic acid with aqueous sodium carbonate.2 marks
A student is given an organic liquid that might be a halogenoalkane. She warms it with aqueous sodium hydroxide, cools the mixture, acidifies it with dilute nitric acid and then adds aqueous silver nitrate. A cream precipitate forms.
Name the type of reaction between the halogenoalkane and aqueous sodium hydroxide, and explain how this reaction leads to the cream precipitate.2 marks
An analyst has an unlabelled liquid with the molecular formula C₃H₆O. It could be propanal, propanone or prop-2-en-1-ol, CH₂=CHCH₂OH.
Describe the observations when each of the three compounds is warmed separately with acidified potassium dichromate(VI) and with Tollens' reagent.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).