AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to recall the colour change when bromine water…
Full specification wording
Cracking and alkenes: Students should be able to recall the colour change when bromine water reacts with an alkene.
Bromine water is an orange solution used to test for a carbon–carbon double bond.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 6 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Bromine water is an orange solution used to test for a carbon–carbon double bond. When shaken with an alkene such as ethene, the bromine adds across the double bond in an addition reaction, so the orange colour is decolourised and the mixture turns colourless. An alkane such as ethane has no double bond, so no addition occurs and the orange colour remains. To perform the test, add a few drops of bromine water to the sample, shake, and record the colour before and after. The test is assessed by asking you to state the colour change from orange to colourless for an alkene, and to explain that the double bond is responsible.
What earns the marks
- Bromine water is orange (or orange-brown) before the test.
- An alkene decolourises bromine water, turning it colourless.
- The colour change is from orange to colourless.
- The change occurs because bromine adds across the C=C double bond.
- An alkane gives no colour change and stays orange, so the test distinguishes alkenes from alkanes.
- The test is an addition reaction, not a substitution or combustion reaction.
Common mistakes and how to put them right
Saying the colour change is from colourless to orange.
Put it right: Bromine water starts orange and becomes colourless with an alkene.
Confusing bromine water with limewater.
Put it right: Limewater tests for carbon dioxide, while bromine water tests for alkenes.
Claiming alkanes also decolourise bromine water.
Put it right: Alkanes have no C=C bond and leave the orange colour unchanged.
Exam tips
- Learn the exact starting colour as orange, not just 'coloured'.
- State both the before and after colours to secure the colour-change mark.
- Link the colour change to the C=C double bond rather than to 'bromine reacting' vaguely.
Try the lesson
The start of the Cracking and alkenes: Students should be able to recall the colour change when bromine water reacts with an alkene. lesson, made from checked teaching notes for this specification point.
Have a go first
Bromine water is added to a test tube containing an alkene and the tube is shaken. What colour change is seen?
Listen
Bromine water is an orange solution used to test for a carbon–carbon double bond. When shaken with an alkene such as ethene, the bromine adds across the double bond in an addition reaction, so the orange colour is decolourised and the mixture turns colourless. An alkane such as ethane has no double bond, so no addition occurs and the orange colour remains. To perform the test, add a few drops of bromine water to the sample, shake, and record the colour before and after. The test is assessed by asking you to state the colour change from orange to colourless for an alkene, and to explain that the double bond is responsible. A strong answer earns marks for these points. Bromine water is orange (or orange-brown) before the test. An alkene decolourises bromine water, turning it colourless. The colour change is from orange to colourless. The change occurs because bromine adds across the C=C double bond. A common slip: Saying the colour change is from colourless to orange. Instead, bromine water starts orange and becomes colourless with an alkene. Exam tip: Learn the exact starting colour as orange, not just 'coloured'.
See it
Key words
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Bromine water
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Addition reaction
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Decolourised
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Alkane
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See every definition
- Bromine water
- An orange solution used to test for a carbon–carbon double bond, added to the sample and shaken, with the colour recorded before and after.
- Addition reaction
- A reaction in which bromine adds across the carbon–carbon double bond of an alkene, so the orange colour is decolourised and the mixture turns colourless.
- Decolourised
- Describes the change in which an alkene turns bromine water from orange to colourless, because bromine adds across the C=C double bond.
- Alkane
- A compound such as ethane that has no double bond, so no addition occurs and the orange colour remains, distinguishing it from an alkene.
Label the diagram
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3Box 3
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