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A-level Chemistry required practicals

Tests for organic functional groups

AQA 3.3.6.1 · RP6

A-level Chemistry (7405) · Required practical 6 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works

Use the minimum valid sequence of test-tube reactions to distinguish alcohol, aldehyde, alkene and carboxylic acid samples.

Apparatus

  • Clean test tubes, rack and labelled dropping pipettes
  • Water bath and test-tube holder
  • Bromine water for carbon–carbon double bonds
  • Freshly prepared Tollens' reagent for aldehydes
  • Fehling's solution for aldehydes
  • Aqueous carbonate and limewater for carboxylic acids
  • Acidified potassium dichromate(VI) for oxidisable alcohols where appropriate

Apparatus & techniques (AT)

AT b · controlled heating

Warm only the tests that require heating in a controlled water bath, keeping volatile organic samples away from naked flames.

AT d · titration/distillation/reflux/qualitative tests/filtration

Carry out small-scale qualitative reactions on fresh portions and record both positive and negative observations precisely.

AT k · safe handling

Handle bromine water, oxidising reagents, silver-containing reagent and volatile organics with appropriate controls and prompt disposal.

Safety

Hazard

Bromine water is harmful and irritating, and many organic samples are volatile or flammable.

Control

Use drop-scale quantities in good ventilation, wear eye protection and use a water bath rather than a naked flame.

Hazard

Acidified potassium dichromate(VI) is toxic, carcinogenic, corrosive and oxidising.

Control

Use the minimum quantity under the required local controls, avoid skin contact and collect chromium(VI) waste separately.

Hazard

Fehling's solution B is strongly alkaline and corrosive.

Control

Wear eye protection, use drop-scale quantities, warm only in a water bath and rinse any skin contact immediately with plenty of water.

Hazard

Tollens' reagent can leave hazardous silver residues if stored or allowed to dry.

Control

Prepare it freshly in small quantity, warm in a water bath, dispose of it immediately as instructed and never store the mixture or residues.

Method

  1. 1Label clean test tubes and place a fresh small portion of each unknown in each tube needed; use a separate pipette for every sample and reagent.
  2. 2Test for a carboxylic acid with aqueous carbonate, then pass any gas into limewater to confirm carbon dioxide rather than reporting bubbles alone.
  3. 3Test a fresh portion for a carbon–carbon double bond by adding bromine water and shaking gently; record whether its orange colour is removed.
  4. 4Test a fresh portion for an aldehyde with freshly prepared Tollens' reagent, warming in a water bath and looking for a silver mirror or grey silver deposit; alternatively, warm with Fehling's solution and look for a red or brick-red Cu2O precipitate from the blue solution.
  5. 5Only after a negative Tollens' or Fehling's result, warm a fresh portion with acidified potassium dichromate(VI); an orange-to-green change then supports a primary or secondary alcohol.
  6. 6Use the combined positive and precise negative results to identify the four samples, recognising that aldehydes as well as primary and secondary alcohols reduce acidified dichromate from orange to green.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 1: Editorial focus: follow reagent, condition and disposal instructions exactly, using a fresh portion for every independent test.
  • CPAC 3: Editorial focus: control carcinogenic oxidant, bromine, volatile-liquid and freshly prepared silver-reagent hazards.
  • CPAC 4: Editorial focus: record initial and final colours, gas confirmation and precipitate or mirror formation before assigning a functional group.

Results & processing

  • Build a comparison table containing reagent, condition, positive observation and negative observation for every fresh portion.
  • A carboxylic acid releases carbon dioxide with carbonate; confirm the gas by turning limewater cloudy.
  • An alkene removes the orange colour of bromine water, while an aldehyde gives a silver mirror or grey silver deposit with Tollens' reagent or changes blue Fehling's solution to a red or brick-red Cu2O precipitate on warming.
  • Acidified dichromate changes from orange to green with primary alcohols, secondary alcohols and aldehydes; use it to identify an alcohol only after a negative Tollens' or Fehling's result.

Analysis skills

  • Construct a decision tree that tests for an aldehyde before using acidified dichromate to assign a primary or secondary alcohol, using a fresh portion at each branch.
  • Distinguish a reagent observation from the identity inference and state which substance is being oxidised or reduced.
  • Use a second test only when it resolves a genuine ambiguity left by the first result.

Uncertainty

Sources

  • Cross-contamination, aged reagents and different sample quantities can cause weak or false observations.
  • Subjective colour descriptions and a dirty test tube can obscure decolourisation, a silver mirror or a faint deposit.

Calculations

  • This diagnostic practical normally has no numerical uncertainty calculation; assess the reliability of each decision-tree branch from controls, fresh portions and confirmatory observations.

Interpretation

  • Write the precise unchanged colour or absence of the named product rather than the vague phrase 'no observation'.
  • A negative dichromate result does not by itself exclude all alcohols because tertiary alcohols resist oxidation under these conditions.
  • An orange-to-green dichromate result does not distinguish an aldehyde from a primary or secondary alcohol without a prior Tollens' or Fehling's result.

Exam angles

  • Select one valid reagent and condition when the question asks for one test, rather than listing every remembered test.
  • State both positive and negative observations precisely and identify which compound reacts.
  • Order Tollens' or Fehling's before acidified dichromate so an aldehyde is not misassigned as an alcohol.
  • Confirm carbon dioxide rather than treating any effervescence as proof of a carboxylic acid.
  • Use a short decision sequence to identify an unknown from the four functional-group classes.

Where students lose marks

Naming dichromate without acid, or recording only that it changes colour without saying which sample is oxidised.

Fix: State acidified potassium dichromate(VI), warm if required, give orange to green, and link the positive result to oxidation of the organic sample.

  • Examiner report: C2-23 · PDF p. 4

Writing only 'effervescence' for a carboxylic-acid test.

Fix: Add carbonate and pass the gas into limewater; carbon dioxide is confirmed when the limewater turns cloudy.

  • Examiner report: C2-23 · PDF p. 5

Adding several unrequested tests or describing a negative result only as 'no observation'.

Fix: Choose the minimum test that answers the question and state the specific retained colour or absent named product.

  • Examiner report: C2-19 · PDF p. 5

Improve the method

  • Use fresh reagents, clean labelled tubes and dedicated pipettes so each result belongs to one sample and one test.
  • Run known positive and negative controls when an observation is weak or the reagent condition is uncertain.
  • Compare colours against a white background and use the same small sample and reagent quantities for parallel tests.
  • Plan the decision tree before starting to minimise hazardous reagent use and avoid redundant tests.

Source references

  • Specification: CSpec 3.3.6.1 · PDF p. 58

Try it — exam-style

Medium
ORIGINAL

An unknown organic liquid does not effervesce with aqueous sodium carbonate. It rapidly changes bromine water from orange to colourless and gives no silver mirror with Tollens' reagent. Identify the functional group and explain how the results distinguish it from an aldehyde.

[3 marks]

Total for this question: 3

Hard
ORIGINAL

A student adds aqueous carbonate to an unknown and sees bubbles. State the extra test needed to support identification of a carboxylic acid and write the general ionic equation using RCOOH.

[3 marks]

Total for this question: 3

Questions are written in the style of past AQA papers — never copied from them.

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