Which metal ion forms a white precipitate with sodium hydroxide that dissolves in excess sodium hydroxide?

• A. Copper(II).
• B. Chromium(III).
• C. Calcium(II).
• D. Iron(III).

Answer: (C)

When a metal ion is treated with sodium hydroxide, it often forms a hydroxide precipitate. The key clue here is the color of the precipitate and what happens when you add more base.

Calcium(II) forms calcium hydroxide, Ca(OH)2, which appears as a white precipitate. If you add a lot more sodium hydroxide, the conditions are very strongly basic and the calcium can form a soluble hydroxy complex (calciumate), so the solid dissolves. That combination—white precipitate that disappears in excess NaOH—matches calcium(II) in this set.

The other ions produce different behavior or colors: copper(II) gives a blue precipitate that can dissolve in excess base to give a blue solution, chromium(III) gives a green precipitate, and iron(III) gives a brown precipitate, none of which are the white precipitate that dissolves in excess base.