Question

Why does ethylene decolourise bromine water, while ethane does not do so?

Answer 1

Bromine water is an orange solution of bromine. It becomes colourless when shaken with an alkene. Alkenes can decolourise bromine water, while alkanes cannot

The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction. The bromine is decolourised because a colourless dibromo compound forms. For example:

ethene + bromine → dibromoethane

C2H4 + Br2 → C2H4Br2

Answer 2

This is because when ethylene reacts with bromine water, it the forms dibromoethane which is colorless. But ethane doesn't react with Br.