Unsaturated hydrocarbons show addition reaction as
they contain multiple bonds in them (alkene or alkyne).They contain pie bond which is weaker than sigma bond and is easily broken down .Hence they show addition reactions.Eg. Catalytic reduction of alkene by hydrogen in the presence of catalyst such as nickle, platinum or palladium .
         Saturated hydrocarbon consists of only sigma bonds which are strong and single only hence they are not easily broken down and these compounds do not undergo addition reaction.
this is because saturated hydrocarbons have single co valent bond but unsaturated hydrocarbons have double or triple bond in them