How can you check the purity of a collected solvent?

• A. By testing its melting point
• B. By testing its boiling point
• C. By testing its color
• D. By testing its density

Answer: (B)

Purity is best judged by the boiling point. A pure solvent has a single, sharp boiling temperature at a given pressure. If there are impurities, the liquid’s boiling behavior changes: it may boil over a range of temperatures instead of at one precise point, and the temperature at which it boils can shift away from the expected value. So, by measuring the boiling point and comparing it to the known value for the pure solvent, you can assess purity. A simple way is to gently heat the collected solvent with a thermometer and note the boiling temperature, ideally seeing a narrow range around the literature boiling point. If it boils consistently at the expected temperature, it’s likely pure; if the boiling occurs over a broad range or at a different temperature, impurities are present.

Melting point isn’t relevant for liquids, so it’s not a useful test here. Color can hint at impurities but isn’t a reliable or universal measure of purity. Density can vary with temperature and composition and doesn’t provide a definitive purity check.