Colligative Properties


Colligative Properties of Solutions refer to the affects solutes have on the physical properties of their solvents. The word “colligative” means ‘depending on the collection’ which would therefore refer to the number or amount of solute particles in the solution or ‘collection’. There are 4 main Colligative Properties:

1. Lowering of Vapor Pressure

2. Elevating or Raising the Boiling Point

3. Lowering the Freezing Point

4. Changing Osmotic Pressure

Simply put, the solute particles in the solvent interfere or crowd out the solvent molecules space. With fewer solvent molecules near the surface, fewer molecules escape and the vapor pressure is lowered. Increasing the concentration of solute molecules or particles has a similar affect on boiling point, freezing point and osmotic pressure. More solute particles raise the boiling point, lowers the freezing point and affects osmotic pressure. This property is extremely useful in winter. Adding salt on the highway lowers the freezing point of the water. Adding “Antifreeze” to car radiator lowers the freezing point, preventing your engine block from freezing and cracking. The same principle applies to raising the boiling point in your car engine so that it does not overheat in summer.

Boiling Point Elevation and Freezing Point Depression are based on Molality: Moles of Solute/ Kilogram of Solvent. The equation is below.

The change in the Boiling Point Temperature, ΔTb = Kbm.

Kb represents the Molal Boiling Point Elevation Constant and m is molality.

The change in the Freezing Point Temperature, ΔTf = Kfm.

Kf represents the Molal Freezing Point Depression Constant and m is molality.