Addition of aqueous ammonia to copper(II) ions produces what?

• A. Light blue precipitate that dissolves in excess to form a dark blue solution
• B. White precipitate that dissolves in excess
• C. No reaction
• D. Gas is evolved

Answer: (A)

When copper(II) ions meet aqueous ammonia, the first thing that happens is the formation of a light blue precipitate of copper(II) hydroxide, Cu(OH)2, as some of the OH- from the ammonia reacts with the copper ions. If you keep adding ammonia, this solid dissolves because copper(II) forms a soluble ammine complex. The ammonia molecules bind to the copper ion to give a complex such as [Cu(NH3)4(H2O)2]2+, and this complex is deep blue in solution. So you observe a light blue precipitate forming, which then dissolves in excess ammonia to give a dark blue solution. This behavior shows how ligands like ammonia can change the coordination environment of a metal ion, increasing solubility and changing color. The other options don’t fit because there is a reaction with formation and dissolution of a copper-containing species, not no reaction, not gas release, and not a white precipitate.