What is the boiling point elevation?

1 Answer
Jul 4, 2014

Whenever a substance is dissolved in a solvent, the boiling point of the solvent is raised slightly compared with the b.p. of the pure solvent.

This occurs because the thermodynamic activity of the liquid solvent is dependent on its concentration (which is slightly depleted in the presence of a solute), whereas the activity of the vapor phase solvent is dependent only on its partial pressure (which is 1 atm at the boiling point, by definition).

When a solute is present, the rate of evaporation of the solvent is slightly reduced, but the rate of condensation of the vapor is unaffected. Therefore, it is necessary to raise the temperature of the solution slightly to maintain an equilibrium vapor pressure of solvent at 1 atm, which defines the normal boiling point.

Boiling point elevation is called a colligative property because at low solute concentrations the identity of the solute doesn't matter. The boiling point elevation is the same regardless of what type of molecule is dissolved; the magnitude of the b.p. elevation is dependent only on the concentration of the solute.

At higher concentrations, this behavior is more complicated and depends at least slightly on the nature of the solvent.