Why ethene decolourises bromine water, but …

Bromine water decolourises unsaturated compounds.Ethene contains double bond and hence it is unsaturated.It reacts with Br to give colourless compound whereas ethane is saturated compound.

Bromine Water + Unsaturated Hydrocarbon -- Saturated Hydrocarbon

When bromine water is added to unsaturated hydrocarbon , it decolorizes bromine water.

but Ethane is a saturated hydrocarbon. So it does not Decolourise bromine water.

Ans. Ethene is an unsaturated compound.it has one double bond.When Bromine water(brown in colour) is added to ethene it reacts with ethene to form a new compound 1,2 dibromoethane which is colourless.The colour of bromine water is gone now.

Generally bromine water is decolourise by all the compounds that contain double or triple bond.
Ethane is a saturated compound with no multiple bond hence does not decolourise Br water.