The properties of a solvent are changed when a solute is dissolved. Osmotic pressure, freezing point, boiling point, and vapor pressure are colligative properties. For ideal solutions, the extent of the change in the property does not depend upon the nature of the solute but only on the number of solute particles in solution. When determining the number of particles, nonelectrolytes count molecule for molecule, while solutions of strong electrolytes depend upon the number of ions. For each sodium chloride unit dissolved, there will be two ions produced -- a sodium cation, and a chloride anion. Weak electrolytes, those partially dissociated, fall in between. For a dilute solution, the dominant factor is the solvent property -- a water solution freezes near the freezing temperature of pure water.

For additional reading, consider the very interesting paper by Hammel.