An unsaturated hydrocarbon contains at least one carbon-carbon double or triple bond. It has fewer hydrogen atoms than the maximum possible.The alkenes are unsaturated hydrocarbons with the general formula CnH2n, for example ethene C2H4. A C=C double bond is made of one σ bond and one π bond.The π bond is a region of high electron density above and below the plane of the molecule. This makes unsaturated hydrocarbons much more reactive than alkanes.Unsaturated hydrocarbons undergo addition reactions. Adding hydrogen to ethene gives ethane, a saturated compound. Bromine water is decolourised from orange to colourless by an unsaturated compound.