Magnesium reacts with p-nitrobenzene resorcinol to form which color precipitate?

• A. Violet-red
• B. Blue lake
• C. Yellow precipitate
• D. Blue precipitate

Answer: (D)

When a metal ion meets a chelating reagent like p-nitrobenzene resorcinol, the resulting metal–ligand complex can absorb different wavelengths of light, producing a distinct color. In this test, magnesium ions form an insoluble complex with p-nitrobenzene resorcinol that appears as a blue precipitate under the reaction conditions. The blue color serves as a diagnostic signal for magnesium in this specific reagent system. Other metals or different conditions would typically yield a different color or no precipitate at all, so the blue precipitate is the expected result for magnesium in this setup.