AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Alkenes are more reactive than alkanes and react with bromine water, which…
Full specification wording
Cracking and alkenes: Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes.
Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes, whose molecules have only single bonds.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 5 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes, whose molecules have only single bonds. A key test uses bromine water, an orange-brown solution. When shaken with an alkene, the bromine water is decolourised, turning colourless, because the alkene adds bromine across its double bond in an addition reaction. An alkane does not react in this way, so the bromine water stays orange-brown. For example, ethene decolourises bromine water, while ethane does not. This test distinguishes unsaturated from saturated hydrocarbons. Remember that the colour change is from orange-brown to colourless, and that the test is carried out at room temperature by shaking the mixture.
What earns the marks
- Alkenes are more reactive than alkanes because they contain a carbon–carbon double bond.
- Bromine water is an orange-brown solution used to test for alkenes.
- An alkene reacts with bromine water and decolourises it, giving a colourless solution.
- An alkane does not decolourise bromine water, so the orange-brown colour remains.
- The reaction of an alkene with bromine is an addition reaction across the double bond.
Common mistakes and how to put them right
Saying bromine water turns clear rather than colourless.
Put it right by using the term colourless for the observed result.
Claiming alkanes also decolourise bromine water.
Put it right by stating that alkanes do not react and the colour persists.
Describing the test as adding bromine gas rather than bromine water.
Put it right by naming bromine water as the reagent.
Exam tips
- State the starting colour and the final colour: orange-brown to colourless.
- Mention shaking the test tube so the hydrocarbon and bromine water mix.
- Use the result to classify the hydrocarbon as unsaturated (alkene) or saturated (alkane).
Try the lesson
The start of the Cracking and alkenes: Alkenes are more reactive than alkanes and react with bromine water, which is used as a test for alkenes. lesson, made from checked teaching notes for this specification point.
Have a go first
Bromine water is added to a sample of an unknown hydrocarbon and the mixture turns colourless. What does this result show about the hydrocarbon?
Listen
Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes, whose molecules have only single bonds. A key test uses bromine water, an orange-brown solution. When shaken with an alkene, the bromine water is decolourised, turning colourless, because the alkene adds bromine across its double bond in an addition reaction. An alkane does not react in this way, so the bromine water stays orange-brown. For example, ethene decolourises bromine water, while ethane does not. This test distinguishes unsaturated from saturated hydrocarbons. Remember that the colour change is from orange-brown to colourless, and that the test is carried out at room temperature by shaking the mixture. A strong answer earns marks for these points. Alkenes are more reactive than alkanes because they contain a carbon–carbon double bond. Bromine water is an orange-brown solution used to test for alkenes. An alkene reacts with bromine water and decolourises it, giving a colourless solution. An alkane does not decolourise bromine water, so the orange-brown colour remains. A common slip: Saying bromine water turns clear rather than colourless. Instead, using the term colourless for the observed result. Exam tip: State the starting colour and the final colour: orange-brown to colourless.
See it
Key words
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Saturated and unsaturated hydrocarbons
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Addition reactions of alkenes
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Structure and reactivity
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Qualitative chemical tests
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See every definition
- Saturated and unsaturated hydrocarbons
- The bromine water test distinguishes unsaturated hydrocarbons, which decolourise it, from saturated hydrocarbons, which do not.
- Addition reactions of alkenes
- The alkene adds bromine across its double bond in an addition reaction, decolourising the bromine water.
- Structure and reactivity
- Alkenes contain a carbon–carbon double bond, which makes them more reactive than alkanes, whose molecules have only single bonds.
- Qualitative chemical tests
- Bromine water, an orange-brown solution, is used as a test for alkenes and turns colourless when shaken with one.
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