An unknown sample is treated with a small amount of elemental bromine in an organic solvent, being as dichloromethane or carbon tetrachloride. Presence of unsaturation and/or phenol or aniline in the sample is shown by disappearance of the deep brown coloration of bromine when it has reacted with the unknown sample. The formation of a brominated phenol (i.e. 2,4,6-tribromophenol) or aniline(i.e. 2,4,6-tribromoaniline) in form of a white precipitate indicates that the unknown was a phenol or aniline. The more unsaturated an unknown is, the more bromine it reacts with, and the less coloured the solution will appear.
Should the brown colour not disappear, possibly due to the presence of an alkane which doesn't react, or reacts very slowly with, bromine, the potassium permanganate test should be performed, in order to determine the presence or absence of the alkene. The iodine value is a way to determine the presence of unsaturation quantitatively.