Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene. Alkenes can decolourise bromine water, but alkanes cannot. The slideshow shows this process.

A dilute solution of bromine is added to samples of heptane and cyclohexene ... To Conduct Demonstration: Pour two samples of heptane and two samples of cyclohexene into separate test tubes in a ...

A halogen cannot displace itself from a solution of one of its salts, eg potassium bromine would not displace bromine water, so these combinations are not done.