The test for unsaturated compounds
3 marks · EasyA student tests samples of pentane and pent-1-ene with bromine water.
State what is observed when bromine water is added to pent-1-ene.
State what is observed when bromine water is added to pentane.
Explain why these observations allow you to distinguish between pentane and pent-1-ene.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Bromine water is decolourised / orange colour disappears / solution becomes colourless.
(b) Bromine water stays orange / no change.
(c) Pent-1-ene is an alkene so it reacts with bromine water, decolourising it, whereas pentane does not, as it is an alkane and lacks a double bond.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Bromine water decolourised/colourless/orange colour disappears (pent-1-ene)
B1 (b) · 1 mark · Bromine water stays orange/no change (pentane).
B1 (c) · 1 mark · States that pent-1-ene is an alkene (unsaturated, reacts) and pentane is saturated (does not react), which allows them to be distinguished.
Worked solution
(a) When bromine water is added to pent-1-ene, the orange colour disappears and the solution becomes colourless.
(b) When bromine water is added to pentane, there is no change and the bromine water remains orange.
(c) Pent-1-ene is an alkene, so it reacts with bromine water, causing it to decolourise. Pentane is an alkane and does not react with bromine water, so the colour stays the same. This difference allows us to distinguish between them.