This allows us to tell alkenes apart from alkanes using a simple chemical test. 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.

When alkenes react with bromine water the colour changes from?

An alkene will turn brown bromine water colourless as the bromine reacts with the carbon-carbon double bond. In fact this reaction will occur for unsaturated compounds containing carbon-carbon double bonds. An alkane undergoes no reaction with bromine water and therefore there is no colour change.

What is the product of alkene and bromine water?

Alkenes decolourise bromine water. This is complicated by the fact that the major product isn't 1,2-dibromoethane. The water also gets involved in the reaction, and most of the product is 2-bromoethanol.