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A phase diagram combines the plots of a pressure versus the temperature for a liquid-gas, solid-liquid, and solid-gas phase-transitions equilibria of a substance.

A temperature is a substance's boiling point at which the vapor pressure of a liquid equals the atmospheric pressure of the liquid鈥檚 environment.

According to the phase diagram of a pure solvent and a solution of that solvent, we have to increase the temperature to equal a solution鈥檚 temperature to the outer atmospheric pressure as there is a lowering of the vapor pressure for a solution than a pure solvent. So, a solution will show a higher boiling point. The gas-liquid line for a solution lies below that of a pure solvent at any temperature and to the right of it at any pressure.

Similarly, a freezing point is a temperature at which the vapor pressure of a substance in its liquid phase is equal to the vapor pressure in a solid phase.

The vapor pressure of a solution is lower than that of a solvent at any temperature, and a solution freezes at a lower temperature than a solvent. The solid-liquid line for a solution lies to the left of a pure solvent at any pressure.

Hence, the main difference between the phase diagram of a pure solvent and a solution of that solvent is the boiling point elevation and the freezing point depression.