Colligative properties

In chemistry, the term colligative property refers to any physical property of a dilute solution dependent on the number of solute particles present, but wholly independent of the solute's identity. Examples of colligative properties include vapor pressure, boiling point, freezing point, and osmotic pressure.

Vapor pressure of a dilute solution

The vapor pressure (P_i) of a dilute solution may be determined by Raoult's Law:

P_i=X_jP*_i

where X_j is the mole fraction of the solute and P*_i is the vapor pressure of the pure solvent. The vapor pressure of a dilute solution is a colligative property because the vapor pressure reduction is independent of the solute's identity.

Boiling point elevation

Concomitant with a reduction in vapor pressure is the elevation of a dilute solution's boiling point relative to the pure solvent. The relationship between boiling point elevation ($\Delta T$) and the molality (m) of a dilute solution is expressed as follows:

$\Delta T=K_{b}m$

where $K_{b}$ is the ebullioscopic constant of the solvent.

Freezing point depression

The relationship between freezing point depression ($\Delta T$) and the molality (m) of a dilute solution is expressed as follows:

$\Delta T=K_{f}m$

where $K_{f}$ is the cryoscopic constant of the solvent.

General References

Atkins, P.W. Physical Chemistry (5th ed.), New York: W.H. Freeman and Co, 1994.