• Halogens can be used to test if a molecule is?unsaturated?(i.e. contains a double bond)
• Br₂(aq) is an orange or yellow solution, called?bromine water?and this is the halogen most commonly used
• The unknown compound is?shaken?with the bromine water
• If the compound is unsaturated, an addition reaction will take place and the coloured solution will decolourise

The decolourisation of bromine water by an unsaturated compound as a result of an addition reaction