What happens when iron(III) ions are treated with aqueous sodium hydroxide?

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

Answer: (A)

When metal ions meet hydroxide in solution, they form metal hydroxide precipitates whose color and whether they dissolve in excess base help identify the ion. For iron(III), reacting with hydroxide gives iron(III) hydroxide, Fe(OH)3, which appears as a reddish-brown solid. This precipitate is not soluble in excess sodium hydroxide, so it remains as a precipitate even when more base is added. That combination of reddish-brown color and insolubility in excess base matches the observed result.

If the ion were iron(II), you’d expect a greenish Fe(OH)2 precipitate, which can oxidize to Fe(III). A white precipitate that dissolves in excess base would point to amphoteric hydroxides like aluminium or zinc, not iron(III) hydroxide. No reaction would mean no precipitate forms at all.