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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.
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Solution
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.
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