Freezing Point Depression of Electrolytes
Colligative properties are properties of solutions that depend on the concentrations of the
samples and, to a first approximation, do not depend on the chemical nature of the samples. A
colligative property is the difference between a property of a solvent in a solution and the same
property of the pure solvent: vapor pressure lowering, boiling point elevation, freezing point
depression, and osmotic pressure. We are grateful for the freezing point depression of aqueous
solutions of ethylene glycol or propylene glycol in the winter and are continually grateful to
osmotic pressure for transport of water across membranes. Colligative properties have been used
to determine the molecular weights of non-electrolytes.
Colligative properties can be described reasonably well by a simple equation for solutions
of non-electrolytes. The “abnormal” colligative properties of electrolyte solutions supported the
Arrhenius theory of ionization. Deviations from ideal behavior for electrolyte solutions led to the
determination of activity coefficients and the development of the theory of interionic attractions.
The equation for the freezing point depression of a solution of a non-electrolyte as a
function of molality is a very simple one (as you may remember from GEN CHEM):
Δ TF = K F m
The constant KF, the freezing point depression constant, is a property only of the solvent, as
given by the following equation, whose derivation is available in many physical chemistry texts1:
1000 Δ H F
In equation (2), R is the gas constant in J/K*mol, TFo is the freezing point of the solvent (K), ΔHF
is the heat of fusion of the solvent in J/mol, and the factor of 1000 is needed to convert from g to
kg of water for molality. For water, KF = 1.860 o/molal from the properties of pure water and
from experimental data on the freezing point depressions of dilute...