ALKENES, C=C will delcolourise bromine water (which is red). The double bond in the alkene breaks and a bromine atom bonds to the C on each side. ALKANES do not react -- so the red of the bromine persists.
Cool experiment if you have bromine water: Add it to a solution of ketchup -- as the bromine reacts the red of the ketchup turns to other funky colours.
