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 °C per molality, and is different for each solvent. For water, the constant is 0.512 °C per molal.

So, a 2.00 molal aqueous solution will elevate the boiling point of water by 1.02 °C to 101.02 °C.

The addition of a non-volatile solute also lowers the freezing point of the solution compared to that of a pure solvent.

At the triple point, the vapor pressures of the solid, liquid, and gaseous states are equal.

Because a non-volatile solute lowers the vapor pressure of the solution, the entire freezing curve, which extends upward from the triple point, shifts such that the solution freezes at a lower temperature.

This decrease in the freezing temperature of a solution compared to that of a pure solvent is known as freezing point depression.

Like the boiling point, the freezing point of a solution is also a colligative property.

The temperature decrease or ΔT_f is directly proportional to the concentration of solute and can be calculated by multiplying the molality of the solute and the molal freezing point depression constant.

The freezing point depression constant also depends on the solvent and has the units °C/m.

For water, the freezing point depression constant is 1.86 °C per molal.

Thus, a 0.5 molal glycol solution will lower the freezing point of water by 0.93 °C to −0.93 °C.