In the copper test, what happens to the light blue precipitate when excess reagent is added?

• A. It remains as a solid.
• B. It dissolves to form a deep blue solution.
• C. It changes to white.
• D. It sinks to the bottom without reaction.

Answer: (B)

The test hinges on how some metal hydroxide precipitates react with excess ligand to form a soluble complex. For copper, adding the reagent that acts as a ligand (ammonia) to the light blue copper(II) hydroxide precipitate causes it to dissolve and form a deep blue solution. This happens because ammonia binds to the copper ion to form the soluble complex [Cu(NH3)4]2+, replacing the hydroxide and pulling the solid into solution. So the correct outcome is that the precipitate dissolves to give a deep blue solution. The other possibilities don’t occur because the copper precipitate doesn’t stay solid, turn white, or sink without reacting when excess ammonia is added.