Iodine test for double bonds. Iodine solution can be used to detect for the presence of carbon-carbon double bonds within unsaturated organic molecules. Iodine can add across the double bonds, in an addition reaction known as halogen-addition or halogenation. In this example, iodine solution is added to a saturated alkane, cyclohexane. Halogen addition does not occur and there is no visible colour change. There is a slight purple colour present in the organic layer as non-polar iodine molecules have dissolved in the non-polar cyclohexane.