SM244     Molecular depression of freezing point    circa 1960    Heat

Solidification is the passage of a substance from a liquid to a solid state.   The phenomenon is explained with the following two laws:

  (a)       Every substance, under the same pressure, solidifies at a fixed temperature which is the same as that of fusion.

  (b)      From the commencement to the end of the solidification, the temperature of a liquid remains constant.

Pure water solidifies at 0^o C.   But, when it contains a dissolved salt, the freezing point is depressed below 0^o C.   Blagden’s law states that the amount of depression is proportional to the weight of salt dissolved.

After the work of Raoult on temperature of solidification of solutions we can state that if P is the weight of substance dissolved in 100 grams of solvent and C is the depression of freezing point observed, then

where A is called the coefficient of depression or the depression produced by one gram of substance in 100 grams of solvent.

If we consider the molecular weight of the substance, the coefficient of molecular depression  T,  becomes

Now it is found in many cases that T is constant for a large number of substances in the same solvent.   An apparatus that is used to obtain the molecular weight of a substance by the depression of freezing point consists of an outer vessel containing the freezing solution, and an inner tube having a side tube for introducing the sample.   There is also a means of stirring the contents of the inner tube and a sensitive thermometer to record the freezing point.   The inner tube is insulated by an air space from the outer vessel.

Van’t Hoff has shown that the coefficient of depression t may be calculated from

  where ω is the heat of fusion and T is the absolute temperature of fusion.