Bromine test for a double bond. Initially the test tube contains orange bromine water Br2. To this is added colourless ethene C2H4, which contains a double bond. The liquids are shaken together to mix them, resulting in a colourless solution of 1,2_dibromoethane C2Br2H4. The bromine adds across the carbon_carbon double bond, with each carbon forming a new bond with a bromine atom. This reaction is used as a test for double bonds in organic chemistry.