(a). The addition of NaOH solution to a solution of zinc chloride produces a white precipitate which dissolves on further addition of NaOH.
(b). The addition of `NH_4OH` to `ZnSO_4` solution produces white precipitate but no precipitate is formed if it contains `NH_4Cl`.

Explanation

(a). First a white percipitate `Zn(OH)_(2)` is formed which on further addition of NaOH forms soluble zincate `Na_(2)ZnO_(2)`.
(b). `NH_(3)OH` is a weak hydroxide. It is ionised slightly furnishing `OH^(ɵ)` ions which are sufficient to precipitate `Zn(OH)_(2)` because its low solubility product. However, in presence of `NH_(4)Cl` the ionisation of `NH_(4)OH` is further suppressed and sufficient `OH^(ɵ)` are not available. to cause precipitation.