Treatment of alkyl halides with KOH …

The treatment of alkyl halide with aqueous KOH solution leads to the formation of  alcohols  because -

Aqueous KOH ionises to form hydroxyl ions ( OH^- ) . Since OH^- ion is a strong nucleophile  it brings about substitution reaction  with alkyl halides forming alcohols.

                     KOH (aq)   -------------->   K⁺ (aq)      +    OH^- (aq)

                       RCl   +   OH ^- _(aq) ------------>   R-OH      +    Cl^- _(aq)

but with alcoholic KOH solution ,an alkene is formed .

This is  due to formation of an  alkoxide ion (RO^- ) . The alkoxide ions are much stronger bases  than OH^- ions  and eliminates a molecule of HCl   from alkyl halide to form alkenes 

                 C₂ H₅ Cl   +   KOH (alcoholic)   ------------>    CH₂  =  CH₂   +   KCl      +   H₂O