Question

Give reasons for the following. Give the chemical equations wherever required.

Blue precipitate obtained on adding NH₄OН to Cu²⁺ ion dissolves in excess of NH₄ОН.

Answer 1

Dropwise addition of ammonium hydroxide precipitates copper(II) hydroxide, which dissolves in excess NH₄OН to form a deep blue soluble tetraamminecopper(II) complex.

\[\ce{Cu^{2+} (aq) + 2OH- (aq) -> Cu(OH)2 ↓ (blue ppt.)}\]

\[\ce{Cu(OH)2 + 4NH4OH (excess) -> [Cu(NH3)4](OH)2 (deep blue solution) + 4H2O}\]