Ammonia gas is passed in excess through copper nitrate solution.state one observation.

Dear student,
Upon a little addition of ammonia(NH3) to copper nitrate solution(Cu(NO3)2), cupric hydroxide(Cu(OH)2) which is a light blue precipitate and ammonium nitrate(NH4NO3) are formed which is a double displacement reaction. But when excess of ammonia is bubbled through copper nitrate solution, a blue solution of [Cu(NH3)4](NO3)2 complex is formed which is a complexation reaction.
Since passing ammonia gas through a aqueous solution (water as a solvent) results in the formation of ammonium hydroxide,
NH3(g) + H2O(l)           > NH4OH(aq)
2NH4OH(aq) + Cu(NO3)2(aq)        > Cu(OH)2(blue precipitate) + 2NH4NO3(aq)  (NH3 is in stoichiometric amount here)
NH4OH(aq)(excess) + Cu(NO3)2(aq)           > [Cu(NH3)4](NO3)2  (NH3 is in excess amount here)
Hence, upon excess bubbling of ammonia gas through copper nitrate solution results in the formation of a blue solution of copper tetramine complex.

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hi there
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A blue precipitate of cupric hydroxide initially appears which on the addition of excess ammonia dissolves to form a deep blue solution having the tetraamminecopper(II) ion, [Cu(NH3)4]2+.


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NH3 + Cu(NO3)2 ---------->Cu(NH3)4 + NO3 in case of ammonia gas.
NH3.H2O is aqueous ammonia, also NH4OH
NH4OH + Cu(NO3)2 ---------->Cu(OH)2 +NH4NO3
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