Copper(II) ions react with dilute sodium hydroxide to form what?

• A. White precipitate soluble in excess.
• B. Light blue precipitate; soluble in excess: colorless solution.
• C. No precipitate forms.
• D. Green precipitate soluble in excess.

Answer: (B)

When copper(II) ions meet a small amount of sodium hydroxide, they react to form copper(II) hydroxide, which appears as a light blue precipitate. If you add more base, the precipitate doesn’t just sit there; it dissolves because excess hydroxide ions form a soluble copperate complex with copper, giving a colorless solution. This shows how metal hydroxides can switch from insoluble to soluble in strong base due to complex formation. The other possibilities don’t match what happens: you do get a precipitate with dilute base, and it isn’t green, and the solution isn’t left colored in the presence of excess base.