Question

Explain, why zinc sulphate does not give a precipitate with excess of NH₄OH.

Answer 1

1. Zinc sulphate (ZnSO₄) reacts with NH₄OH to form zinc hydroxide (Zn(OH)₂) as a precipitate initially.
2. On adding excess NH₄OH, zinc hydroxide dissolves because it forms a soluble complex ion, tetraamminezincate(II), [Zn(NH₃)₄]²⁺.
3. This complex keeps zinc in solution, so no precipitate appears with excess NH₄OH.
4. This happens due to the formation of coordination compounds where NH₃ acts as a ligand bonding with Zn²⁺ ions.

Thus, zinc sulphate does not give a precipitate with excess ammonium hydroxide because of complex ion formation.