The test for unsaturated compounds
3 marks · NormalA student wants to test whether an unknown colourless liquid is an alkane or an alkene. The student adds a few drops of bromine water to the liquid in a test tube and shakes it. After a short time, the orange colour disappears and the mixture becomes colourless. Explain what conclusion the student should make about the liquid and the reason for this result.
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The liquid is an alkene. Bromine water becomes colourless when an alkene reacts with it, showing it is unsaturated due to a carbon–carbon double bond.
Common mistakes: calling it an alkane; using 'clear' instead of 'colourless'
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Marking points
M1 · 1 mark · Identifies the unknown as an alkene (or unsaturated compound).
M2 · 1 mark · Explains that alkenes react with bromine water causing decolourisation.
M3 · 1 mark · Mentions the presence of a carbon–carbon double bond or unsaturation as cause for this test result.
Worked solution
The liquid is an alkene. Alkenes are unsaturated and will react with bromine water, causing its orange colour to disappear. This happens due to the presence of a carbon–carbon double bond, which allows addition of bromine across the bond.