# Question #95468

May 13, 2017

This is a comples-forming reaction

#### Explanation:

At first you will see copper hydroxide precipitating:
$C {u}^{2 +} \left(a q\right) + 2 O {H}^{-} \left(a q\right) \to C u {\left(O H\right)}_{2} \left(s\right)$
The $O {H}^{-}$ coming from the ammonia-solution. There is an equilibrium reaction between ammonia and water:
$N {H}_{3} + {H}_{2} O r i g h t \le f t h a r p \infty n s N {H}_{4}^{+} + O {H}^{-}$

On adding more, the ammonia forms a (soluable) complex ion with the copper ions, and even releases them from the precipitate.

What you will see is that the light blue pecipitate, will dissolve into a darker blue, clear solution.

Note : This complex $C u {\left(N {H}_{3}\right)}_{6}^{2 +}$ is easily reduced to a brown-coloured compound, by for instance glucose. That's why it was used, in the form of Fehling's or Benedict's reagens, for testing urine for glucose (diabetes).