Which statement best describes how purity affects melting and boiling behavior of a solid?

Which statement best describes how purity affects melting and boiling behavior of a solid?

• A. Impurities raise boiling point but do not affect melting point.
• B. A pure solid melts over a range of temperatures.
• C. Boiling point decreases with impurities.
• D. Pure solids have a sharp melting point; impure solids melt over a range, and impurities cause boiling point elevation and melting point depression.

Answer: (D)

When a solid is pure, its crystal structure is uniform, so there is a single temperature at which the lattice breaks apart and it becomes a liquid. That gives a sharp melting point. Introducing impurities disrupts that orderly lattice. Different impurities melt at different temperatures and they interfere with the crystal, so melting begins before the pure substance can fully melt and continues over a range. That’s why impure solids don’t have one melting point and instead melt over a span of temperatures. In the liquid phase, impurities lower the vapor pressure of the substance, which means you have to heat it more to reach boiling. This is boiling point elevation. So impurities make melting occur over a range and lower the melting point, while they raise the boiling point of the liquid. That’s why the statement describing a pure solid with a sharp melting point, impure solids melting over a range, and impurities causing both boiling point elevation and melting point depression fits best.

When a solid is pure, its crystal structure is uniform, so there is a single temperature at which the lattice breaks apart and it becomes a liquid. That gives a sharp melting point.

Introducing impurities disrupts that orderly lattice. Different impurities melt at different temperatures and they interfere with the crystal, so melting begins before the pure substance can fully melt and continues over a range. That’s why impure solids don’t have one melting point and instead melt over a span of temperatures.

In the liquid phase, impurities lower the vapor pressure of the substance, which means you have to heat it more to reach boiling. This is boiling point elevation. So impurities make melting occur over a range and lower the melting point, while they raise the boiling point of the liquid.

That’s why the statement describing a pure solid with a sharp melting point, impure solids melting over a range, and impurities causing both boiling point elevation and melting point depression fits best.