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Plan the smallest useful sequence of tests for the candidate organic compounds. The examples develop alcohol, aldehyde, alkene and carboxylic-acid tests, then extend to haloalkanes, phenols and carbonyl evidence. Record observations on labelled fresh portions; select conditions that make a negative result meaningful.
Video by Royal Society of Chemistry. Use the demonstration alongside this lesson’s specification reference and written explanations.
Viewing focus: Plan tests using fresh portions and appropriate controls
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is the observation when dichromate(VI) is reduced in the usual organic test?
Orange to green. The colour change accompanies reduction of chromium(VI) to chromium(III).
After watching 2: Which result most directly distinguishes a typical aldehyde from a ketone?
A silver mirror with freshly prepared Tollens’ reagent. Both aldehydes and ketones have carbonyls; the aldehyde reduces Tollens’ reagent.
What you'll be able to do
Plan the smallest useful sequence of tests for the candidate organic compounds. The examples develop alcohol, aldehyde, alkene and carboxylic-acid tests, then extend to haloalkanes, phenols and carbonyl evidence. Record observations on labelled fresh portions; select conditions that make a negative result meaningful.
Alkenes decolourise bromine water under the usual test conditions because bromine adds across C=C. This observation is useful within an appropriate candidate set, but other reactive substances can also consume bromine. Avoid claiming that every decolourisation proves an isolated alkene group in an unknown with no other information.
Acidified potassium dichromate(VI) changes from orange to green when a primary or secondary alcohol is oxidised under suitable warming conditions. A tertiary alcohol gives no corresponding oxidation under the normal test conditions. Aldehydes and other reducing substances can also reduce dichromate, so a positive result is not uniquely an alcohol test.
Tollens’ reagent distinguishes a typical aldehyde from a ketone: warming an aldehyde gives metallic silver, often a silver mirror. Use freshly prepared teacher-supplied reagent and the specified immediate disposal; do not store residues. If a carbonyl screen with 2,4-DNPH is included, a yellow/orange precipitate supports a carbonyl, then a selective aldehyde test can narrow the identity. Never allow the reagent or residues to dry outside the approved procedure.
A carboxylic acid reacts with carbonate or hydrogencarbonate to release carbon dioxide; limewater can confirm the gas. An alcohol normally does not give this response. Universal indicator alone establishes acidity rather than a unique carboxylic-acid identity.
Use a water bath and the laboratory’s ventilation arrangements for volatile compounds and oxidising or toxic reagents. A negative result can reflect low concentration, wrong temperature or a degraded reagent. A known reference tested under the same conditions helps check reagent performance. Combine structural possibilities, positive tests and meaningful negatives before selecting an identification.
A haloalkane can be warmed with aqueous silver nitrate in a suitable ethanol–water mixture: hydrolysis releases halide, then a silver-halide precipitate forms. Keep solvent composition, temperature and concentration comparable. A negative room-temperature observation does not exclude a slowly hydrolysing chloroalkane. This is different from adding silver nitrate to an inorganic solution already containing free halide ions.
Phenol can give a violet colour with neutral iron(III) chloride under the appropriate conditions. It also decolourises bromine water and forms a white precipitate of 2,4,6-tribromophenol. Thus bromine-water decolourisation alone cannot uniquely distinguish phenol from an alkene. Carboxylic acids generally release carbon dioxide with hydrogencarbonate whereas phenol does not under the usual test conditions.
Aldehydes and ketones both form a carbonyl derivative with 2,4-DNPH. A selective oxidation test then distinguishes a typical aldehyde from a ketone. The iodoform test can support a methyl-carbonyl or a corresponding oxidisable alcohol group when required by the candidate structures: a pale-yellow precipitate is evidence for that structural feature, not a unique molecular identity. Use teacher-prepared reagents and the specified safe warming and disposal methods.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Another unknown reduces acidified dichromate and gives a silver mirror with Tollens’ reagent. Why does the first test alone not establish an alcohol?
The combined results support an aldehyde, rather than identifying an alcohol from dichromate alone.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.