Question #95468

1 Answer
May 13, 2017

This is a comples-forming reaction


At first you will see copper hydroxide precipitating:
#Cu^(2+) (aq) +2OH^(-) (aq) ->Cu(OH)_2 (s)#
The #OH^-# coming from the ammonia-solution. There is an equilibrium reaction between ammonia and water:
#NH_3+H_2O rightleftharpoonsNH_4^+ +OH^-#

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 #Cu(NH_3)_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).