Chromium(III) ions reacting with aqueous ammonia yield what?

• A. Grey/green precipitate; insoluble in excess.
• B. Green precipitate; soluble in excess.
• C. Blue precipitate; dissolves in excess.
• D. No precipitate.

Answer: (A)

When chromium(III) ions meet the hydroxide ions provided by ammonia, they form chromium(III) hydroxide, Cr(OH)3, which appears as a grey-green solid. This precipitate is insoluble in excess ammonia under ordinary qualitative analysis conditions because Cr(OH)3 does not readily dissolve in a weak base; dissolving it would require a stronger base to form a soluble hydroxo complex. So the observed result is a grey/green precipitate that does not dissolve when more ammonia is added.