When testing Cr3+ with dilute sodium hydroxide, what happens to the precipitate when excess NaOH is added?

• A. It dissolves to form a colorless solution.
• B. It remains as an insoluble solid.
• C. It changes to a blue solution.
• D. It forms a white precipitate.

Answer: (A)

The main idea here is amphotericity. Chromium(III) hydroxide, Cr(OH)3, is an amphoteric substance: it can act as a base with acids and as an acid with bases. When you add dilute NaOH, you first get the green precipitate Cr(OH)3. If you add excess NaOH, this precipitate doesn’t just stay solid. The Cr(OH)3 reacts with more hydroxide to form a soluble complex ion, [Cr(OH)6]3-, which goes into solution. That’s why the solid disappears and the solution becomes colorless—the complex in basic solution does not absorb visible light to give a color, so you see a colorless solution.