What does a precise, sharp boiling point indicate about a solvent's purity?

• A. A precise, sharp boiling point indicates purity
• B. A high boiling point indicates purity
• C. Boiling point cannot indicate purity
• D. The solvent should not boil

Answer: (A)

A pure liquid has a single, well-defined boiling point at a given pressure. When a liquid is pure, all its molecules reach vaporization at roughly the same temperature, so the boiling point shows up as a precise, sharp temperature. If impurities are present, other substances with different boiling points are in the mixture, causing boiling to occur over a range of temperatures rather than at one exact point, and often raising the observed boiling point slightly. So a precise, sharp boiling point is a sign of purity, whereas a broad or shifted boiling range suggests impurities. The other statements either ignore the effect of impurities on the boiling range or mix up what a high boiling point alone implies.