Vapour Pressure of Liquid Solutions

A liquid solution is one in which the solvent is a liquid. The solute that dissolves in it can be a gas, another liquid, or a solid — so liquid solutions cover a wide range of everyday mixtures.

Solutions of a gas in a liquid were already taken up while discussing solubility (and Henry's law). The focus now shifts to the other two families:

• liquid dissolved in liquid (for example, two miscible volatile liquids), and
• solid dissolved in liquid (for example, sugar or common salt in water).

What we restrict ourselves to

To keep the treatment clean, the discussion is limited to binary solutions — solutions built from exactly two components. In these solutions the liquid solvent is generally volatile, while the solute may or may not be volatile. That single distinction — whether the solute contributes its own vapour — controls how the vapour pressure of the solution behaves, and it is the thread running through the sections that follow. …