Add bromine in solution.
Alkane solution turns brown, alkene solution goes (brown then) colourless.
Bromine is brown; when it reacts with an alkene, the 2 bromines (from Br2) covalently bond with the two carbons in the alkene double bond (C=C). Since the bromine is now apart of another molecule, its brown colour is lost, so the alkene solution turns colourless.
Alkane solution turns brown, alkene solution goes (brown then) colourless.
Bromine is brown; when it reacts with an alkene, the 2 bromines (from Br2) covalently bond with the two carbons in the alkene double bond (C=C). Since the bromine is now apart of another molecule, its brown colour is lost, so the alkene solution turns colourless.