I can't do this chemistry!?

Question

Aqueous ammonia is added to Cu2+ and a precipitate is formed. If further addition of aqueous ammonia to the solution dissolves the precipitate, what happened and what is the product?

Answer 1

Ammonia reacts with water to form a basic solution:

NH3 + H2O ----> NH4+ + OH-

Because cupric hydroxide is insoluble, the hydroxide ion will precipitate with the Cu2+ ion:

Cu2+ + 2OH- ----> Cu(OH)2

Adding more ammonia leads to the formation of a complex ion, which in this case is soluble.  It will form a dark blue solution:

Cu(OH)2 + 4NH3 ----> [Cu(NH3)4]2+ + 2OH-

The link provides some more info on complex ions.