Which ion is indicated by a green precipitate formed when sodium hydroxide is added?

Which ion is indicated by a green precipitate formed when sodium hydroxide is added?

• A. Fe3+
• B. Fe2+
• C. Cu2+
• D. Al3+

Answer: (B)

The test relies on how metal ions form insoluble hydroxides with added hydroxide ions, and the color tells you which ion is present. Iron(II) ions react with OH− to form iron(II) hydroxide, Fe(OH)2, which appears as a pale green precipitate. If iron were in the +3 oxidation state, you’d get Fe(OH)3, which is brown. Copper(II) would give blue Cu(OH)2, and aluminum(III) would give a white gelatinous Al(OH)3 that dissolves in excess alkali. So a green precipitate from adding sodium hydroxide points to Fe2+ ions.

The test relies on how metal ions form insoluble hydroxides with added hydroxide ions, and the color tells you which ion is present. Iron(II) ions react with OH− to form iron(II) hydroxide, Fe(OH)2, which appears as a pale green precipitate. If iron were in the +3 oxidation state, you’d get Fe(OH)3, which is brown. Copper(II) would give blue Cu(OH)2, and aluminum(III) would give a white gelatinous Al(OH)3 that dissolves in excess alkali. So a green precipitate from adding sodium hydroxide points to Fe2+ ions.