Formation of alcohols from the reaction between alkyl chlorides and aqueous KOH is an example of simple nucleophilic substitution.But when aqueous KOH is replaced by alcoholic KOH, alkenes are formed instead of alcohols due to elimination of HCl from an alkyl halide.This can be exaplained if we consider the size of the nucleophile in the two reactions. In the aqueous medium the $\overset{\ominus}{\mathrm{Nu}}$ is $\overset{\ominus}{\mathrm{O}}\mathrm{H}$ which is relatively small while in the alcoholic medium the $\overset{\ominus}{\mathrm{Nu}}$ is $\mathrm{C}_2\overset{\ominus}{\mathrm{H}}_5$ which is bulky in nature. Thebulky $\overset{\ominus}{\mathrm{Nu}}$ will always find it easier to abstract a proton than to attack a tetravalent carbon to produce a substitution product.If $\mathrm{C}_2\overset{\ominus}{\mathrm{H}}_5$ was to attack the carbon carrying halogen, steric repulsions would hinder the attack and prevent substitution product.