The actual identity of the solute molecules is unimportant to the lowering of vapor pressure. Since theirs is a "spoiling" role in decreasing the frequency with which solvent molecules approach the surface, only their numbers matter. If a substance such as a salt dissociates into two particles or ions in solution, then it is doubly effective. One mole of NaCl lowers the vapor pressure of water by twice as much as a mole of glucose, because it yields twice the number of particles in an aqueous solution.

Example. At 35° C the vapor pressure of water is 42.2 mm Hg. What is the vapor pressure of an aqueous solution of glucose that has one glucose molecule for every 100 water molecules?

Solution

Example. The elemental abundance table in Chapter 8 shows that ocean water can be considered as a solution with 330 NaCl "molecules" for every 33,000 water molecules. The vapor pressure of pure water on a hot summer day (35° C) is 42.2 mm Hg. What is the vapor pressure of water in the middle of the ocean at that temperature?

Solution

Lowering of the equilibrium vapor pressure of a liquid by ions or molecules of a solute is known as a colligative property (meaning "collective" or "joint") because the size of the effect depends only on the total number of solute molecules or ions, and not on their identity.

There are three other common colligative properties of solutions: boiling point elevation, freezing point lowering, and osmotic pressure. In all four cases, adding solute molecules or ions decreases the escaping tendency of solvent molecules from the liquid. Therefore some adjustment in temperature or pressure must be made to restore equilibrium between the liquid and the other phase