a) why ethene decolourises bromine water , but ethane no.?

b) write down the relevant chemical equation involved in decolourisation.

Your friends are correct.
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.

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Hi Friend,

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.

Happy To Help!!

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