Alkanes are unreactive and are mainly used as fuels .
Alkenes are the starting material for many different chemicals .
They are extremely useful to the chemical industry . An alkene may be distinguished from an alkane by shaking the hydrocarbon with bromine water . Bromine water is brown , and will lose its colour with an alkene but not with an alkane . Bromine adds across the double bond of an alkene to form a colourless dibromo alkane . This is an example of an addition reaction .