Question

Ammonia solution in water gives a blue precipitate when it combines with a solution of copper salt. The blue precipitate further dissolves in excess of an ammonia solution to give the azure blue solution. Explain with the equation.

Answer 1

\[\ce{NH3 + H2O <=> NH4OH}\]

\[\ce{CuSO4 + 2Nh4OH -> \underset{(Pale Blue ppt)}{[NH4SO4]} + Cu[OH]2↓}\]

The pale blue ppt. of copper hydrogen dissolves in excess of Ammonium hydroxide forming an Azure blue (deep blue) soluble complex salt. [Tetra amine copper II] Sulphate \[\ce{[Cu(NH3)4]SO4}\] is formed.

\[\ce{\underset{[Pale blue ppt.]}{Cu(OH)2} + \underset{(Amm. Hydroxide)}{(NH3)2SO4 + 2NH4OH} -> \underset{(Deep blue colouration)}{\underset{[Tetra amine copper II sulphate]}{[Cu(NH3)4SO4 + Cu(OH)2↓}}}\]