1. Colligative Properties
Depend on the concentration of solute particles, but not on chemical identity.
In the case of a solute that does not ionize, “concentration of solute particles” has the same meaning as “solute concentration”.

2. Some Colligative Properties
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure

3. Why Vapor Pressure Lowering?
Results when a non-volatile solute is dissolved in a volatile solvent.
Only some of the surface molecules have the ability to vaporize, compared to all of the surface molecules in the case of a pure solvent.
As a result, the vapor pressure of the solution is less than that of a pure solvent.

4. Molecular-Level Explanation for Vapor Pressure Lowering (image 1 of 3)

5. Molecular-Level Explanation for Vapor Pressure Lowering (image 2 of 3)

6. Molecular-Level Explanation for Vapor Pressure Lowering (image 3 of 3)

7. An Interesting Illustration of the Difference in Vapor Pressure Between Solvent and Solution

8. Raoult’s Law P = X . Po Obeyed exactly, only for ideal solutions
For non-ideal solutions, Raoult’s law is more closely followed for dilute solutions

10. Raoult’s Law Applied to Both Solvent and Solute (when both are volatile)
P = X . Po
P = P + P = X . Po + X . Po = (1-X) . Po + X . Po

12. Vapor Pressure Lowering Changes the Boiling and Freezing Points

13. Equations for Boiling Point Elevation and Freezing Point Depression
DTb = Tb – Tob and DTb = Kb . cm
DTf = Tf – Tof and DTf = -Kf . cm