What observation occurs when aluminum ions are treated with aqueous ammonia?

• A. White precipitate insoluble in excess.
• B. White precipitate soluble in excess.
• C. Colorless solution with no precipitate.
• D. Red precipitate.

Answer: (A)

When aluminum ions meet aqueous ammonia, the OH- ions from the ammonia raise the pH and cause aluminum to form aluminum hydroxide as a white gelatinous precipitate. This happens because Al3+ hydrolyzes in water and readily combines with OH- to precipitate as Al(OH)3. In excess ammonia, the precipitate does not dissolve because aluminum hydroxide doesn’t form a stable soluble ammine complex under these conditions; dissolving it would require a much stronger base to convert it into aluminate. So you observe a white precipitate that remains solid even when more ammonia is added.