It is observed experimentally that as the concentration of a solution increases, the freezing point of the solution decreases and hence the depression in the freezing point (ΔTf) increases.
The depression in the freezing point of a solution is directly proportional to the molal concentration (expressed in mol kg-1) of the solution.
Thus,
ΔTf ∝ m
Where m is the molality of the solution.
∴ ΔTf = Kfm, where Kf is a constant of proportionality.
If m = 1 molal,
ΔTf = K.
Hence Kf is called the cryoscopic constant or molal depression constant. Kf is characteristic of the solvent.
