State one relevant observation for the reaction
Addition of excess ammonium hydroxide into copper sulphate solution.

Explanation

Addition of excess ammonium hydroxide into copper sulphate solution leads to formation of a deep blue coloured solution.
When ammonium hydroxide is added in the solution of copper sulphate drop-wise, a pale blue precipitate of copper hydroxide is obtained. The equation for this follows:
`CuSO_(4)+2NH_(4)OH to Cu(OH)_(2)+(NH_(4))_(2)SO_(4)+4H_(2)O`
When ammonium hydroxide is added in excess, the precipitate dissolves and gives a deep blue solution of tetraammine copper (II) sulphate. The equation for this follows:
`Cu(OH)_(2)+(NH_(4))_(2)SO_(4)+2NH_(4)OH to [Cu(NH_(3))_(4)]SO_(4)+H_(2)O`
Hence, the product formed is a complex named as tetraammine copper (II) sulphate.