In aq. solution, KOH is almost completely ionised to give OH^- ions which being a strong nucleophile brings about a substitution reaction to form alcohols. Further in aq. solution, OH^- ions are highly solvated (hydrated).
This solution reduces the basic character of OH^- ions which fail to abstract a hydrogen from the -carbon of the alkyl halide to form an alkene.
However an alcoholic solution of KOH contains alkoxide (RO^-) ions which being a much stronger base than OH^- ions preferentially abstracts a hydrogen from the carbon of the alkyl halide to form alkene.

Iodide ion is a powerful nucleophile and hence reacts rapidly with RCl to form RI.
KI ——— K⁺ + I^-; R — Cl + I^- ——— R — I + Cl^-
Further I^- is a better leaving group than CI^- ion, therefore, RI is more rapidly hydrolysed than RCl to form ROH.
HO^- + R — I ——— R — OH + I^-

(b) Because by products of the reaction, i. e., SO₂ and HCl being gases escape into atmosphere leaving behind pure alkyl chloride.

(a) First convert phenol to benzene by heating with Zn dust.
(b) Nitration of benzene with conc. nitric acid in presence of conc. sulphuric acid.

(a) This is because pcc is a mild oxidising agent and can oxide methanol to methanal only while KMnO₄ being strong oxidising agent oxidises it to methanoic acid.
(b) Because esterification rxn is reversible and presence of base (pyridine) neutralises HCl produced during reaction thus promoting forward reaction.