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. P o 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. P o P = P + P = X. P o + X. P o = (1-X). P o + X. P o

Vapor Pressure Lowering Changes the Boiling and Freezing Points

Equations for Boiling Point Elevation and Freezing Point Depression  T b = T b – T o b and  T b = K b. c m  T f = T f – T o f and  T f = -K f. c m