Which observation occurs when iron(II) ions are treated with aqueous sodium hydroxide?

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

Answer: (A)

When a metal ion is treated with hydroxide ions, it often forms a metal hydroxide precipitate whose color depends on the oxidation state of the metal. Here, iron is in the +2 state (Fe2+). Adding sodium hydroxide provides OH− ions that combine with Fe2+ to form iron(II) hydroxide, Fe(OH)2. Fe(OH)2 is an insoluble solid and appears green, so you observe a green precipitate. It does not dissolve in excess NaOH because Fe(OH)2 is not amphoteric, unlike some other hydroxides that dissolve in excess base to form complex ions. If the iron were Fe3+, you’d expect a brown precipitate of Fe(OH)3 instead.