Recall that a solution is made up of a solvent and one or more solutes. Most of the properties of a solution are extremely dependent on the identity and properties of the solute. Four properties, however, are not.
These properties are called the Colligative properties. They are those properties of a solution that depend solely on the number of solute particles present, not on the identity of those solute particles. The four colligative properties of solutions are:
1. Vapor pressure lowering,
2. Boiling point elevation,
3. Freezing point depression, and
4. Osmotic pressure.
Recall that any volatile substance – that is, any substance that will evaporate – will have a vapor pressure. Nonvolatile substances do not have vapor pressures. Suppose you have a volatile solvent, such as water. In the absence of any solute the mole fraction of the water is 1.00. If you make a solution of a nonvolatile solute such as table sugar (sucrose) in the water the mole fraction of the water will be less than 1.00. Consequently, the vapor pressure of the water in the solution is less than that of the pure solvent because the mole fraction is less in the solution than in the pure solvent.
Freezing point and boiling point are affected in similar ways by the presence of a nonvolatile solute. In this experiment you will observe how freezing point changes once a nonvolatile solute is introduced. Sometimes it is possible to observe supercooling as a substance freezes. Supercooling is the rapid decrease in temperature of a substance to a temperature below the expected freezing point. It occurs because the first crystals of the frozen substance are slow to form, allowing the temperature of the liquid to decline precipitously.