How are the boiling point and freezing point of a solvent affected when a solute is added?

The boiling point increases and freezing point decreases on addition of non volatile solute in a solvent.

Explanation:

When we add a non volatile solute in a volatile solvent, the vapor pressure of the resulting solution is lower than the vapor pressure of pure solvent. Due to decrease in vapor pressure:

a) The temperature required to boil the solution increases (boiling point is the temperature at which the vapor pressure of a liquids is equal to atmospheric pressure). This is known as elevation in boiling point. It is a colligative property which depends upon number of solute particles.

b) The temperature required to freeze a liquid decreases and this is known as depression in freezing point, which is also a colligative property which depends upon number of solute particles.