Which property would you test to assess solvent purity by comparing to known values?

• A. Melting point
• B. Boiling point
• C. Melting or boiling points
• D. Density

Answer: (C)

Purity shows up in phase-change temperatures: a pure solid has a specific melting point, and a pure liquid has a specific boiling point when measured under standard conditions. Impurities disrupt the orderly structure or the vapor pressure, so the melting point occurs over a range and usually at a lower temperature, while the boiling point can shift and broaden as impurities change how easily molecules escape. Because of this, comparing the measured melting point or boiling point to the known values for the pure substance is a reliable way to assess purity. You can use either property depending on the substance: melting point for solids and boiling point for liquids. Density is less reliable for purity assessment since it can vary with temperature and composition and isn’t as sensitive to impurities as phase-change temperatures.