Which observation helps distinguish iron(II) from iron(III) when reacting with sodium hydroxide?

Which observation helps distinguish iron(II) from iron(III) when reacting with sodium hydroxide?

• A. Fe2+ forms green precipitate; Fe3+ forms orange-brown precipitate
• B. Fe2+ forms blue precipitate; Fe3+ forms green precipitate
• C. Fe2+ forms white precipitate; Fe3+ forms blue precipitate
• D. Fe2+ forms no precipitate; Fe3+ forms white precipitate

Answer: (A)

When you add sodium hydroxide to solutions of these iron ions, you’re looking at the different iron hydroxide solids that form because the two oxidation states make different compounds. Fe2+ precipitates as Fe(OH)2, which is a pale green solid. Fe3+ precipitates as Fe(OH)3, which appears orange-brown. So the color of the precipitate reveals the oxidation state: green means iron(II), orange-brown means iron(III). (Keep in mind Fe2+ can slowly oxidize to Fe3+ in air, so the green precipitate may darken over time if left exposed.)

When you add sodium hydroxide to solutions of these iron ions, you’re looking at the different iron hydroxide solids that form because the two oxidation states make different compounds. Fe2+ precipitates as Fe(OH)2, which is a pale green solid. Fe3+ precipitates as Fe(OH)3, which appears orange-brown. So the color of the precipitate reveals the oxidation state: green means iron(II), orange-brown means iron(III). (Keep in mind Fe2+ can slowly oxidize to Fe3+ in air, so the green precipitate may darken over time if left exposed.)