Adding aqueous ammonia to chromium(III) ions yields what?

• A. Grey-green precipitate that is insoluble in excess
• B. Clear solution
• C. Yellow precipitate that dissolves in excess
• D. Blue precipitate that dissolves in excess

Answer: (A)

When a base is added to a metal ion in solution, the metal often forms a hydroxide that precipitates because many metal hydroxides are not very soluble. For chromium(III) ions, the aqua complex [Cr(H2O)6]3+ reacts with the hydroxide ions from ammonia to form chromium(III) hydroxide, Cr(OH)3(s). This solid appears as a grey-green precipitate. In this situation, even with excess ammonia, the precipitate remains as a solid rather than dissolving, so you end up with a grey-green precipitate that is insoluble in excess.