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 ammonia solution to give an azure blue solution. Explain with equation.

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Solution

The Copper (II) sulphate has $C u^{+ 2}$ ion. To add ammonia in water to Copper ion, it gives a precipitation of Copper(II) hydroxide, it is light blue colour.

$C u^{+ 2} (a q) + 2 N H_{3} (a q) + 3 H_{2} O (l) ⇌ C u (O H)_{2} (s) + 2 N H_{4}^{+} (a q)$

To add excess Ammonia, the Copper (II) hydroxide forms a complex compound, that is deep blue colour (azure).

$C u (O H)_{2} (s) + 4 N H_{3} (a q) ⇌ [C u (N H_{3})_{4}]^{+ 2} (a q) + 2 O H^{-} (a q)$

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