Alkene and bromine water. Test tubes showing the initial (left) and final (right) appearance of the reaction between an alkene and bromine water (orange). Bromine is an example of a halogen. The alkene is immiscible with water and forms the colourless upper layer at left. The double bond in the alkene reacts with the bromine when the test tube is shaken. After being allowed to stand, the layers separate again (right). The bromine water has been decoloured by the reaction to form water (lower layer at right). The bromine has added to the alkene to form the corresponding bromoalkane (upper layer at right).