Select tests that separate the actual candidates, record visible changes precisely and distinguish observations from conclusions.
A useful test must give different outcomes
Start with the candidate structures and ask which functional group differs. A reagent that reacts with both candidates cannot distinguish them, even if its colour change is dramatic. Record reagent, conditions, observation for each candidate and the conclusion that follows.
Use separate fresh portions for different tests unless a prescribed sequence explicitly requires otherwise. Earlier reagents can change the sample or interfere with later tests. Compare appropriate controls and do not treat a failed test on an insoluble or inadequately heated sample as proof of absence.
The core RP6 tests
Name Tollens’ reagent or ammoniacal silver nitrate rather than writing only silver nitrate. Fehling’s is a specified alkaline copper reagent, not simply any blue copper sulfate solution. Use the named test under its conditions.
| Target | Reagent and conditions | Expected positive observation | Limit of conclusion |
|---|---|---|---|
| Alkene | Bromine water; shake at room temperature without UV | Orange/brown colour disappears | Other reactive groups may also decolourise bromine |
| Primary or secondary alcohol | Acidified potassium dichromate(VI); warm | Orange changes to green | Aldehydes can give the same result; tertiary alcohols normally do not |
| Aldehyde | Tollens’ reagent; warm gently | Silver mirror or silver deposit | A simple ketone gives no visible change |
| Typical aliphatic aldehyde | Fehling’s solution; warm | Brick-red Cu₂O precipitate from blue solution | A simple ketone remains blue without that precipitate |
| Carboxylic acid | Aqueous sodium carbonate or hydrogencarbonate | Effervescence; evolved CO₂ turns limewater milky | Other acids also react; use the candidate list |
Connect the observation to chemistry
In the dichromate test, chromium(VI) is reduced to chromium(III) while the organic compound is oxidised. With Tollens’, silver(I) is reduced to Ag metal. With Fehling’s, copper(II) is reduced to copper(I) oxide. The organic product of these alkaline aldehyde tests is a carboxylate ion.
Carbonate tests are acid–base reactions, not oxidation tests. With propanoic acid, sodium carbonate produces sodium propanoate, water and carbon dioxide. State bubbles/effervescence as the observation; “CO₂ produced” is an interpretation that can be confirmed with limewater.
Worked example: three labelled candidates
Suppose three samples are propanal, propan-2-ol and propanoic acid. A carbonate test identifies the acid by effervescence. On fresh portions of the other two, Tollens’ identifies propanal by a silver mirror. Propan-2-ol gives no silver mirror but reduces warm acidified dichromate from orange to green.
Testing all three with dichromate first would not distinguish propanal from propan-2-ol because both can be oxidised. AQA June 2023 Q01.1 and the report used this exact kind of reagent-selection issue; the lesson is to compare responses before choosing the test.
For an unknown outside this stated candidate list, a negative Tollens’ result is not enough to identify a secondary alcohol. Bring in other evidence such as an OH absorption and molecular formula.
Diagram placeholder
Test-selection decision diagram
Labels to include:
- fresh portions
- carbonate: acid effervescence
- Tollens’: aldehyde silver deposit
- dichromate: oxidisable alcohol
- confirm against candidate list
Begin from the three named candidates, mark which candidate each positive test identifies and state the contrasting negative outcomes. Use separate sample portions for the tests, and label the conclusion as conditional on the supplied candidate list.
Related analysis: a covalent halogen is not already a halide ion
A halogenoalkane must release X⁻ by hydrolysis before Ag⁺ can precipitate AgX. Warming with an appropriate aqueous silver nitrate/ethanol mixture can reveal this release: chloride gives white AgCl, bromide cream AgBr and iodide yellow AgI. A delay reflects the organic reaction as well as the detection step.
This differs from adding silver nitrate directly to a solution already containing halide ions. If an alkaline hydrolysate is tested instead, acidify suitably with dilute nitric acid before silver nitrate to remove hydroxide interference; do not use hydrochloric acid and introduce chloride.
RP6 is supervised practical work. Use small prescribed quantities, suitable heating for flammable liquids, and the laboratory waste procedures for dichromate and silver/copper reagents. Prepare Tollens’ fresh and dispose of it promptly; do not store it or let residues dry.
Quick checks
Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official AQA mark allocations.
Q1. Why does acidified dichromate not distinguish ethanal from ethanol?Show answer
Both are oxidised under suitable warm test conditions, so both can change orange dichromate to green chromium(III). Choose a test with different outcomes, such as Tollens’.
Q2. Describe a test distinguishing pentanal from pentan-2-one.Show answer
Warm separate portions with Tollens’ reagent. Pentanal gives a silver mirror/deposit; pentan-2-one gives no visible change. Include the reagent and both outcomes.
Q3. What observation and gas confirmation support a carboxylic acid in a carbonate test?Show answer
Effervescence occurs. Passing the gas into limewater gives a milky calcium carbonate precipitate, supporting CO₂ production.
Q4. An unknown gives no dichromate colour change. Does that prove it contains no OH group?Show answer
No. A tertiary alcohol can retain its OH group without reacting in this oxidation test. Check the structure, test conditions and other evidence.
Q5. Why should hydrochloric acid not be used before a silver nitrate test for chloride in an organic hydrolysate?Show answer
It introduces chloride ions and can itself cause AgCl precipitation, giving misleading evidence about the sample. Use the specified dilute nitric acid instead.
Sources
Sources and examiner guidance (reviewed 2 October 2026)
- Chemrevise: Organic Analysis — Coverage checklist, pp1–3; original examples and questions.
- AQA 7405 specification — 3.3.6.1–3.3.6.3 and required practical 6.
- AQA June 2023 AS Paper 2 mark scheme — Q01.1–01.2 pp11–12: reagent choice and contrasting observations.
- AQA June 2023 AS Paper 2 examiner report — Q01 p3: tests that do and do not distinguish the candidates.
- AQA June 2022 AS Paper 2 mark scheme — Q02 p14: sequence and combined evidence.
Finesse Tuition is not endorsed by AQA or Chemrevise. All explanations and examples here are our own.