Alkene test. Image 1 of 2. Test tubes containing bromine water (brown) to test for the presence of an alkene. Hexane (C6H14, an alkane) is being poured into the test tube on the left. No reaction will occur. The hex-1-ene (C6H12, an alkene) will be added to the tube at right. The colour will be lost (decolorized) as the hex-1-ene reacts with the bromine (Br2). The hexene has a double bond (it is unsaturated) and is more reactive than the hexane (which has formed all the bonds it can and is saturated). The bromine 'adds' to the double bond in what is termed an addition reaction. For the result of this test, see image A500/283.