“Which test tells alkanes and alkenes apart?” is one of the most reliable marks in the carbon compounds chapter, if you know the observation and the conclusion. Students often name the right chemical but describe the colour change wrongly, or forget to explain why it happens. This guide gives you the tests, the exact observations, and the reason each one works, so you never mix them up.
Why they behave differently
The whole difference comes down to one bond. Alkanes are saturated, every carbon is joined by single bonds only, general formula CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆, propane C₃H₈). Alkenes are unsaturated, they contain at least one C=C double bond, general formula CₙH₂ₙ (ethene C₂H₄, propene C₃H₆).
That double bond is reactive. It lets alkenes undergo fast addition reactions at room temperature, while alkanes are far less reactive and only substitute in ultraviolet light. Every distinguishing test below is really testing for the presence of that double bond.
Test 1, Bromine water (the standard answer)
This is the test the examiner expects first.
- What you do: Add a few drops of the hydrocarbon to reddish-brown bromine water and shake, at room temperature and in the absence of strong sunlight.
- Alkene: the brown colour is decolourised quickly (turns colourless). The alkene adds across the double bond, ethene forms 1,2-dibromoethane:
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br
- Alkane: the brown colour remains (no fast change in the dark). Alkanes have no double bond to add across.
State both the observation and the reason. A full answer says: “The alkene decolourises bromine water rapidly because it is unsaturated and undergoes an addition reaction; the alkane does not.” You can revise this reaction on our addition reactions of alkenes page.
Test 2, Acidified potassium manganate(VII)
A second accepted test uses purple acidified potassium manganate(VII), KMnO₄.
- Alkene: the purple colour is decolourised. The double bond is oxidised, so the manganate(VII) is reduced and loses its colour.
- Alkane: the purple colour stays, because there is no double bond to be oxidised.
This test and the bromine-water test give the same logic: only the unsaturated alkene reacts. If a question asks for two tests, bromine water and potassium manganate(VII) are the safe pair.
Test 3, Sootiness of the flame
Alkanes and alkenes both burn, but they burn differently because of their carbon-to-hydrogen ratio.
- Alkene: burns with a yellow, sooty (smoky) flame with more soot, because it has a higher proportion of carbon and tends towards incomplete combustion.
- Alkane: burns with a cleaner, less sooty flame and produces less soot.
This is a supporting observation rather than a definitive chemical test, but it is worth knowing, and it appears in questions comparing fuels. You can see the clean-burning case on our combustion of alkanes page.
A comparison table to memorise
| Test | Alkane | Alkene |
|---|---|---|
| Bromine water (brown) | No change | Decolourised quickly |
| Acidified KMnO₄ (purple) | No change | Decolourised |
| Flame | Less sooty | More sooty (smoky) |
| Bond | C–C single only | C=C present |
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
Worked example
Two colourless gases, X and Y, are hexane vapour and hexene vapour in some order. When each is shaken with bromine water, X decolourises it within seconds while Y does not. Identify X and Y.
Answer: X decolourises bromine water, so X is unsaturated, X is hexene (C₆H₁₂). Y does not react, so Y is saturated, Y is hexane (C₆H₁₄). The reason: hexene has a C=C double bond and undergoes an addition reaction with bromine, whereas hexane has only single bonds.
Notice how the answer always ends with the reason, not just the identity. That is where the second mark lives.
Common mistakes to avoid
- Getting the colour change backwards. It is the alkene that decolourises bromine water, because it is the reactive one.
- Saying “clear” instead of “colourless”. Clear means see-through; colourless means no colour. Examiners want colourless.
- Forgetting the condition. The bromine-water test is done at room temperature; in strong sunlight an alkane can slowly react by substitution, which muddies the result.
- Naming only the observation. Add the reason, unsaturated, addition reaction.
Lock it in with practice
Telling alkanes from alkenes is a two-part answer every time: the colour change, then the reason. If your child can name the test but keeps losing the explanation mark, that is a quick thing to fix. Our online one-to-one teachers drill these observation-and-reason answers against real SPM questions, in English, from RM50 an hour with a paid one-hour trial. See how it works to try a lesson.
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