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

Do alkanes react with bromine?

Alkanes are far less reactive than alkenes and will only react with bromine water in the presence of UV light. Under these conditions, alkanes undergo substitution reactions with halogens, and will slowly de-colourise bromine water.

What will Decolourise bromine water rapidly?

Ethene, butyne, and hexyne can decolourize bromine water.