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Freezing points are best determined by plotting a curve of temperature vs. time for a substance as it cools from a liquid (or molten) state to the solid state.This is caused by the tendency of the solvent to exclude solute as it freezes; that is, it tends to freeze pure, which causes the concentration of the solution to increase as solid forms.The cooling curve will often show a dip where the liquid becomes super cooled before crystallization begins.The freezing point is determined by extrapolating the temperature at the flat section of the graph where freezing is occurring back to the Y-axis.Once freezing actually begins, the temperature will rise due to the release of ?Hfus, the enthalpy of fusion.

Freezing points are best determined by plotting a curve of temperature vs. time for
a substance as it cools from a liquid (or molten) state to the solid state. The cooling
curve will often show a dip where the liquid becomes super cooled before crystallization
begins. Once freezing actually begins, the temperature will rise due to the release of
∆Hfus, the enthalpy of fusion. For pure substances, while the solid and liquid states are
in equilibrium, the temperature will remain constant at the freezing point until the liquid
is completely solidified before dropping again. For a mixture, the cooling curve will not
be constant, but will show a steady decline in the freezing temperature (see figure 5.1).
This is caused by the tendency of the solvent to exclude solute as it freezes; that is, it
tends to freeze pure, which causes the concentration of the solution to increase as solid
forms. The freezing point is determined by extrapolating the temperature at the flat
section of the graph where freezing is occurring back to the Y-axis.