What is observed when copper(II) ions are treated with dilute sodium hydroxide?

• A. Light blue precipitate forms, soluble in excess to give colourless solution
• B. White precipitate forms, soluble in excess
• C. No reaction occurs
• D. Gas is evolved

Answer: (A)

When copper(II) ions meet hydroxide ions, they form copper(II) hydroxide, Cu(OH)2, which is only sparingly soluble in water and appears as a pale or light blue precipitate. If you add more hydroxide ions in excess, the solid dissolves because copper(II) can form a soluble hydroxo complex, [Cu(OH)4]2-. This complex dissolves to give a blue solution. So the key observations are a light blue precipitate forming, then dissolving in excess to yield a blue (not colourless) solution.