How do impurities generally affect boiling point and melting point?

How do impurities generally affect boiling point and melting point?

• A. Impurities lower boiling point and raise melting point
• B. Impurities lower both boiling and melting points
• C. Impurities do not affect melting or boiling points
• D. Impurities raise boiling point and lower melting point

Answer: (D)

Impurities affect phase-change temperatures by disrupting how particles interact in a substance. In a solid, impurities create defects in the crystal lattice, weakening the overall binding. That makes it easier for the solid to lose its ordered structure and melt at a lower temperature. In a liquid, dissolved particles lower the solvent’s tendency to escape into the vapor phase at a given temperature, so you must add more heat to reach the external pressure and boil it; this raises the boiling point. So, impurities generally raise the boiling point and lower the melting point. A familiar example is salt in water: it raises the boiling point a bit and lowers the temperature at which ice would melt.

Impurities affect phase-change temperatures by disrupting how particles interact in a substance. In a solid, impurities create defects in the crystal lattice, weakening the overall binding. That makes it easier for the solid to lose its ordered structure and melt at a lower temperature. In a liquid, dissolved particles lower the solvent’s tendency to escape into the vapor phase at a given temperature, so you must add more heat to reach the external pressure and boil it; this raises the boiling point.

So, impurities generally raise the boiling point and lower the melting point. A familiar example is salt in water: it raises the boiling point a bit and lowers the temperature at which ice would melt.