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”.

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

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.

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

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

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

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

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

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

Vapor Pressure Lowering Changes the Boiling and Freezing Points

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