Lugol's solution consists of what?

• A. 2% I2 in NaI + water
• B. 5% I2 in NaI + water
• C. 5% I2 in KI + water
• D. 7.5% I2 in KI + alcohol

Answer: (C)

Iodine doesn’t dissolve well in water on its own, but when iodide ions are present, iodine forms a soluble I3– complex. Lugol’s solution uses potassium iodide to provide those iodide ions, allowing iodine to stay dissolved in water. That’s why the description of 5% I2 in KI dissolved in water best matches Lugol’s solution, which is typically about 5% iodine with 10% potassium iodide in water. The other options differ by the iodide source, solvent, or iodine concentration, so they don’t describe Lugol’s solution correctly.