Bromine test for alkene. Two test tubes containing an organic layer floating on top of an aqueous, or water, layer. The aqueous layer contains bromine (orange), which has been used to test the organic chemicals for a reactive carbon double bond which reacts with bromine, decolourizing the water. The organic layer at right (cyclohexene), decolourized the water because cyclohexene contains a reactive double bond. The organic layer at left (benzene), didn't decolourize the water (and so did not react with the bromine) because benzene has a specially stable structure that is called aromatic. This is why benzene is less reactive than cyclohexene.