(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

First a white precipitate of `Zn(OH)_2` is formed which on further addition of NaOH forms soluble zincate `NaZnO_2`.
(b). `NH_4OH` 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_4Cl`, the ionisation of `NH_4OH` is further suppressed and sufficient `OH^(ɵ)` are not available to cause precipitation.