Freezing Point Depression and Boiling Point Elevation

The temperature at which the vapor pressure of a liquid equals the atmospheric pressure is known as its boiling point.

Since adding a non-volatile solute lowers the vapor pressure of a solvent, a solution requires a higher temperature to increase its vapor pressure to a point that equals the atmospheric pressure. Thus, the boiling point of a solution is greater than that of a pure solvent.

These changes in vaporization can be examined over a range of temperatures and pressure using a phase diagram.

A solution has a lower vapor pressure than the pure solvent at all temperatures. So, the vaporization curve of the solution would lie below that of the solvent.

At 1 atm, the curve corresponds to a temperature higher than the boiling point of the pure solvent.

The increase in the boiling point of the solution compared to that of the pure solvent is known as boiling point elevation.

The boiling point of a solution is a colligative property. The temperature increase, or ΔT_b, is directly proportional to the concentration of solute and can be calculated by multiplying the molality of the solute and the molal boiling point elevation constant.

The boiling point elevation constant has the units