Identifying Functional Groups by Chemical Tests
A concise revision guide to the characteristic reactions used to identify functional groups: the test-tube tests for alkenes, halogenoalkanes, alcohols, aldehydes and ketones, carboxylic acids, amines and esters, how to use them on a molecule with several groups, and how to predict the properties and reactions of an unfamiliar compound.
Before You Start
Every test on this page was met when its functional group was first taught. This page gathers them so that an unknown can be worked through.
- A positive test is an observation: a colour change, a precipitate, a gas or a smell. Always record what is seen, not what it means.
- The reagent and the conditions earn marks too: “warm with Tollens’ reagent” rather than “Tollens'”.
- Spectra and formulae: Deducing Formulae from Analytical Data. The test-tube work itself: Identifying Unknown Compounds and Required Practicals.
- A molecule with several groups shows every test that each group would show alone, which is how routes are checked in Planning a Synthetic Route.
GCSE Recap: Simple Chemical Tests
Three quick questions on the GCSE tests this page builds on.
The Test Table
Learn each row as reagent, conditions, observation and conclusion. The observation is the mark that students most often leave out.
Important: The PCl₅ and 2,4-DNPH rows, and the same tests in the figures and interactive checks on this page, are extension: neither test is on the AQA specification. AQA tests an alcohol with acidified potassium dichromate(VI) and an aldehyde with Tollens’ reagent or Fehling’s solution.
| Group | Reagent and conditions | Positive observation | What it shows |
|---|---|---|---|
| alkene, C=C | bromine water, shake at room temperature | orange to colourless | electrophilic addition across the double bond |
| halogenoalkane, C–X | warm with NaOH(aq), acidify with dilute HNO₃, add AgNO₃(aq) | white precipitate (Cl), cream (Br), yellow (I); the white one dissolves in dilute NH₃(aq), the cream in concentrated, the yellow not at all | a halide ion released by hydrolysis is precipitated as the silver halide |
| alcohol, O–H (extension) | PCl₅ at room temperature | misty fumes of HCl | an O–H group (water and carboxylic acids react too) |
| primary or secondary alcohol | warm with acidified potassium dichromate(VI) | orange to green | oxidation to an aldehyde, ketone or acid; a tertiary alcohol stays orange |
| aldehyde, CHO | warm with Tollens’ reagent (ammoniacal silver nitrate) | silver mirror | an aldehyde is oxidised; a ketone gives no change |
| aldehyde, CHO | warm with Fehling’s solution | blue solution to a brick-red precipitate | an aldehyde is oxidised and Cu²⁺ is reduced to Cu₂O; a ketone gives no change |
| aldehyde or ketone, C=O (extension) | 2,4-dinitrophenylhydrazine (2,4-DNPH) solution | orange precipitate | a carbonyl group; the melting point of the purified derivative identifies the compound |
| carboxylic acid, COOH | sodium carbonate or sodium hydrogencarbonate solution; universal indicator | fizzing, the gas turns limewater milky; indicator red | an acid strong enough to release CO₂ from a carbonate |
| amine, NH₂ | smell; damp red litmus; a drop of concentrated HCl held nearby | fishy smell; litmus turns blue; white smoke of the ammonium salt | a basic nitrogen lone pair |
| ester, COOR | smell; warm with NaOH(aq) then acidify | sweet fruity smell; the smell goes and a carboxylic acid forms | hydrolysis of the ester link |
The whole table as a poster: reagent, conditions, observation and conclusion for every group.
Exam focus: For the halogenoalkane test, say why each step is there: NaOH hydrolyses the C–X bond, nitric acid destroys the excess hydroxide (which would give Ag₂O), silver nitrate precipitates the halide.
Quick Check: Reading the Test Results
Pick the one accurate statement in each set, for compounds that are new to this page.
The Oxygen Groups in Detail
Most unknowns contain oxygen, so the alcohol, carbonyl and acid tests carry the most marks. Three distinctions matter.
- Aldehyde or ketone? Only an aldehyde gives the silver mirror with Tollens’ reagent or the brick-red precipitate with Fehling’s solution; a ketone gives neither.
- Which alcohol? Primary and secondary alcohols turn acidified dichromate(VI) green; a tertiary alcohol does not. Distilling the product and testing it (aldehyde or ketone) separates primary from secondary.
- Acid or alcohol? Both have O–H, but only the carboxylic acid fizzes with sodium carbonate and turns indicator red.
- Extension: the orange 2,4-DNPH derivative can be filtered, recrystallised and its melting point compared with data-book values to name the carbonyl compound: Reduction and Nucleophilic Addition.
The colour changes side by side: orange to colourless, orange to green, silver mirror, orange precipitate, fizzing and the three silver halide colours.
Common mistake: Writing “decolourised” on its own for bromine water. Give both colours: orange to colourless. “Clear” is not a colour.
Check: Tests and Observations
Match each reagent to its positive observation and conclusion, choose the test that separates two compounds, and explain the steps of the halide test.
An Unknown Compound: Which Test First?
Choose tests in an order that rules out the most each time. Start with cheap, quick tests and finish with the ones that confirm.
- Shake a little with water and add universal indicator. Red means a carboxylic acid; blue means an amine (fishy smell).
- Add sodium carbonate solution to an acidic unknown: fizzing that turns limewater milky confirms a carboxylic acid.
- Shake with bromine water: orange to colourless means an alkene.
- Warm with Tollens’ reagent or Fehling’s solution: a silver mirror or a brick-red precipitate means an aldehyde; a ketone gives no change.
- Warm with acidified dichromate(VI): orange to green means a primary or secondary alcohol (an aldehyde also reacts).
- Warm with NaOH(aq), acidify with HNO₃ and add AgNO₃(aq): a precipitate means a halogenoalkane; its colour gives the halogen.
The flow chart as yes / no decisions, from indicator and carbonate down to the silver nitrate test.
Remember: A negative result is evidence too. “No silver mirror with Tollens’ reagent, so no aldehyde” removes every aldehyde in one line.
Quick Check: Deducing an Unknown
Work out what each set of results shows, then flip the card to check.
A Molecule with Several Functional Groups
Each group on a molecule reacts as it would on its own. List the groups, then predict the test result for each in turn.
Worked example: 3-hydroxybutanal, CH₃CH(OH)CH₂CHO
Step 1. Identify the groups: an aldehyde, CHO, at carbon 1 and a secondary alcohol, OH, at carbon 3.
3-Hydroxybutanal (aldehyde, secondary alcohol) and glutamic acid (amine, two acids), each group boxed with the test that detects it.
- Glutamic acid fizzes with sodium carbonate (two COOH groups) and dissolves in dilute acid (the NH₂ group is protonated).
- Two groups can interact: an amine and a carboxylic acid on the same molecule form a zwitterion, so amino acids are high-melting solids soluble in water.
Exam focus: Say which group is responsible for each observation: “the silver mirror shows the aldehyde group; the broad infrared absorption at 3230–3550 cm⁻¹ shows the hydroxyl group”.
Predicting the Properties of an Unfamiliar Compound
An unfamiliar molecule is a set of familiar groups. Its physical properties follow from the groups, and so do its reactions.
| Group present | Intermolecular forces | Effect on boiling point and solubility |
|---|---|---|
| O–H (alcohol, acid) or N–H (amine, amide) | hydrogen bonding with itself and with water | high boiling point; small molecules dissolve in water |
| C=O (aldehyde, ketone, ester) or C–X | permanent dipole-dipole and London forces | moderate boiling point; C=O compounds hydrogen bond to water but not to themselves; halogenoalkanes do not hydrogen bond to water |
| hydrocarbon chain or ring only | London forces | boiling point rises with chain length; insoluble in water, soluble in non-polar solvents |
| COOH and NH₂ on the same molecule | ionic zwitterion in the solid | crystalline solid, soluble in water, dissolves in acid and in alkali |
Worked example: 2-hydroxypropanoic acid, CH₃CH(OH)COOH
Step 3. Reactions of the alcohol: oxidised by acidified dichromate(VI) to the ketone CH₃COCOOH (orange to green).
- Each group reacts in the way learnt for that group, with the same reagents and conditions.
- Order of acidity: carboxylic acid > alcohol. Only the acid reacts with carbonates and with NaOH; both react with sodium.
- An amine group makes the compound basic: it dissolves in dilute acid as a salt (Preparing Amines).
- A C=C bond will add bromine whatever else is on the molecule; a benzene ring will not.
Remember: Work group by group: name each group, predict its forces, predict its tests, predict its reactions. Then look for interactions such as the zwitterion.
Check: Several Groups and Unfamiliar Compounds
Predict which tests a molecule with two or three groups will give, explain the observations group by group, and predict solubility and reactivity from the groups present.
Common Exam Points
- Give reagent, conditions and observation together: “warm with Tollens’ reagent; a silver mirror forms”.
- Bromine water: an alkene turns it from orange to colourless by electrophilic addition.
- Halogenoalkane: warm with NaOH(aq), acidify with HNO₃, add AgNO₃(aq): white, cream or yellow precipitate for Cl, Br or I.
- Acidified dichromate(VI) turns green with primary and secondary alcohols and with aldehydes, never with tertiary alcohols or ketones.
- Tollens’ reagent or Fehling’s solution picks out an aldehyde; a ketone gives no change with either.
- Carboxylic acids fizz with sodium carbonate and turn universal indicator red; alcohols do neither.
- On a molecule with several groups, each group gives its own test result; explain every observation by naming the group responsible.
- Predict properties from the groups: O–H and N–H mean hydrogen bonding, a high boiling point and water solubility; an acid plus an amine means a zwitterion.
FAQs
Use these quick answers to check the test-tube tests for alkenes, halogenoalkanes, alcohols, aldehydes, ketones and carboxylic acids, and how to apply them to a molecule with more than one functional group.
How do I tell an aldehyde from a ketone?
Warm with Tollens’ reagent: an aldehyde gives a silver mirror and a ketone does not. Fehling’s solution turns from blue to a brick-red precipitate with an aldehyde only.
How do I test for a halogenoalkane and identify the halogen?
Warm the halogenoalkane with aqueous sodium hydroxide to release the halide ion, acidify with dilute nitric acid, then add silver nitrate solution. A white precipitate means chloride, cream means bromide and yellow means iodide; only the white one dissolves in dilute ammonia solution.
How do I tell a carboxylic acid from an alcohol?
Add sodium carbonate or sodium hydrogencarbonate solution: a carboxylic acid fizzes as carbon dioxide is released, and the gas turns limewater milky. An alcohol does not react with carbonate, but a primary or secondary alcohol turns warm acidified potassium dichromate(VI) from orange to green.
How can I tell a primary, a secondary and a tertiary alcohol apart?
Warm each with acidified potassium dichromate(VI). Primary and secondary alcohols turn it orange to green; a tertiary alcohol leaves it orange because it cannot be oxidised. Then test the product: the aldehyde from a primary alcohol gives a silver mirror with Tollens’ reagent.
What do I do when a molecule has more than one functional group?
Treat each group as if it were alone, so every test for a group present is positive. 3-Hydroxybutanal gives a silver mirror (aldehyde) and turns acidified dichromate(VI) green (secondary alcohol and aldehyde), but no fizzing with carbonate because there is no COOH group.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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