When chromium(III) ions are treated with dilute sodium hydroxide, what is observed?

• A. Green precipitate that is soluble in excess
• B. Blue precipitate that remains insoluble
• C. No precipitate forms
• D. Red solution forms

Answer: (A)

The concept here is how some metal hydroxides behave with bases and the idea of amphotericity. When chromium(III) ions meet dilute sodium hydroxide, they first form chromium(III) hydroxide, Cr(OH)3, which appears as a green precipitate. Cr(III) hydroxide is amphoteric, meaning it can react further with extra base to form a soluble complex ion. In excess NaOH, Cr(OH)3 reacts to give the soluble complex [Cr(OH)6]3−, so the green precipitate dissolves and a green solution remains. An accompanying simple equation is Cr(OH)3(s) + 3 OH−(aq) → [Cr(OH)6]3−(aq). This behavior—green solid forming and then dissolving in excess base—is why the observation described is a green precipitate that is soluble in excess.