When sodium hydroxide solution is added to zinc sulphate, a white gelatinous precipitate of zinc hydroxide is formed. On further addition of excess sodium hydroxide to the formed precipitate, soluble, colourless complex salt of sodium zincate is formed.
ZnSO4 + 2 NaOH → Zn(OH)2 + Na2SO4
Zn(OH)2 + 2 NaOH → Na2ZnO2 + 2H2O