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

When ammonia (NH₃) is added to water, it forms ammonium hydroxide (NH₄OH).

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

Copper (II) sulphate (CuSO₄) reacts with ammonium hydroxide to form a pale blue precipitate of copper (II) hydroxide (Cu(OH)₂).

\[\ce{CuSO4 + 2NH4OH -> \underset{(Pale Blue ppt.)}{(NH4)2SO4} + Cu(OH)2↓}\]

The precipitate is a pale blue colour.

The pale blue precipitate of Cu(OH)₂ dissolves in excess ammonium hydroxide to form a deep blue (azure) solution of tetraammine copper (II) sulphate.

\[\ce{Cu(OH)2 + 4NH4OH -> [Cu(NH3)4]SO4 + 4H2O}\]

The product is a deep blue complex ion. [Cu(NH₃)₄]²⁺ known as tetraammine copper (II).

The deep blue coloration confirms the formation of this complex.