When excess Ammonium hydroxide is added to the Zinc hydroxide precipitate, the hydroxide ions from Ammonium hydroxide react with the Zinc ions to produce Zinc hydroxide. However, the excess Ammonium hydroxide can then react with the Zinc hydroxide to form the tetraamminezinc(II) ion. This complex ion is soluble in water, which is why the white precipitate dissolves when excess Ammonium hydroxide is added.
The reaction between Zinc hydroxide and Ammonium hydroxide can be represented as follows:
Zn(OH)2(s) + 4NH4OH(aq) -> [Zn(NH3)4]2+(aq) + 4H2O(l)
The tetraamminezinc(II) ion is a stable complex ion because it has a high charge-to-size ratio. This means that the positive charge of the Zinc ion is evenly distributed over the four Ammonia molecules, which makes it less likely to react with other ions in the solution.
