Which ion forms a blue precipitate when sodium hydroxide is added?

• A. Fe2+
• B. Cu2+
• C. Pb2+
• D. Zn2+

Answer: (B)

When a metal ion is treated with hydroxide ions, it can form a metal hydroxide solid that precipitates if it’s not soluble in water. The color of that precipitate helps identify the ion involved.

Copper(II) ions form a blue precipitate of copper(II) hydroxide, Cu(OH)2, when sodium hydroxide is added. The reaction is Cu2+ + 2 OH- → Cu(OH)2(s). That characteristic blue solid is a clear clue that copper(II) ions are present.

Other common ions would give different results: iron(II) would produce a pale green Fe(OH)2 (which can darken on exposure to air), zinc(II) would give white Zn(OH)2 (which is colorless in solution and can dissolve in excess base to form a soluble complex), and lead(II) would give white Pb(OH)2. The blue color specifically points to copper(II) ions.