What is observed when iron(III) ions are treated with aqueous ammonia?

• A. Red/brown precipitate insoluble in excess.
• B. White precipitate soluble in excess.
• C. Green precipitate.
• D. No reaction.

Answer: (A)

When a base is present, metal ions react with hydroxide to form metal hydroxide precipitates. For iron(III) ions, OH− from aqueous ammonia combines to give iron(III) hydroxide, Fe(OH)3, which is a reddish-brown, gelatinous solid. This precipitate is sparingly soluble and does not dissolve in excess ammonia, so you still see the brown solid even when more ammonia is added. This behavior—brown Fe(OH)3 forming and remaining insoluble in excess base—fits the observation described. The other possibilities don’t match: a white precipitate would come from different ions, a green precipitate isn’t characteristic of Fe(III) with ammonia, and no reaction would ignore the formation of Fe(OH)3.