Chromium(III) ions reacting with dilute sodium hydroxide yield what?

• A. Green precipitate; soluble in excess.
• B. Blue precipitate; soluble in excess.
• C. White precipitate; insoluble.
• D. No precipitate.

Answer: (A)

When a transition metal ion like chromium(III) reacts with hydroxide, it first forms a metal hydroxide precipitate. For chromium(III), this precipitate is Cr(OH)3, which is green. Cr(OH)3 is amphoteric, so in a large excess of base it reacts further to form a soluble hydroxy complex, [Cr(OH)6]3-. That means you’d see a green solid form with dilute NaOH, and that solid would dissolve when more NaOH is added. Hence the observed result is a green precipitate that is soluble in excess base.