The test for unsaturated compounds
4 marks · NormalDescribe the result you would see when bromine water is shaken with a liquid alkene.
Describe what you would see if bromine water is shaken with a liquid alkane, under the same conditions.
Explain how you can use this test to distinguish between an alkene and an alkane.
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(a) The bromine water is decolourised / goes colourless.
(b) The bromine water stays orange (no colour change).
(c) The test is used to distinguish alkenes and alkanes because alkenes decolourise bromine water due to the presence of a double bond, while alkanes do not and the bromine water remains orange.
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Marking points
B1 (a) · 1 mark · Bromine water goes colourless / is decolourised with alkene.
B2 (b) · 1 mark · Bromine water stays orange (alkane does not react).
B3 (c) · 1 mark · Statement of the difference in observation between alkane and alkene.
B4 (c) · 1 mark · Explanation that decolourisation shows presence of a double bond/alkene (or equivalent).
Worked solution
(a) When shaken with a liquid alkene, bromine water is decolourised (goes colourless).
(b) When shaken with a liquid alkane, bromine water stays orange (no colour change).
(c) This test can be used to distinguish between the two because only alkenes decolourise bromine water, due to the presence of a carbon–carbon double bond. Alkanes do not react, so the bromine water remains orange.