Bromine test for alkenes. Bromine water (orange) in a bottle next to a gas jar. The bromine water has been used to test the gas jar for the presence of an alkene. The decolourized bromine water seen at the base of the gas jar indicates that the gas is an alkene. Alkenes decolourize bromine water because they react with the bromine that it contains. It is this bromine that gives the water its orange colour. This test distinguishes between alkanes and most alkenes. This is because alkenes have double bonds which makes them more reactive than alkanes that are saturated with single bonds. Ethene is the simple alkene used here.