FREEZING POINT, the degree of cold at which a liquid freezes. It may be stated as accurately as the degree of heat at which a liquid congeals, since everything above absolute zero represents some degree of heat. In the case of water the freezing point is 32° F. or 0° C. The fusing or melting point of a metal is also its freezing point, the point where it changes from the liquid to the solid condition if the temperature is going down, or from solid to liquid if it is going up. But as the melting points of metals are regarded as hot, we use the word fuse, except in the case of mercury or quicksilver. The freezing or fusing points of familiar substances are: Sea water freezes at a lower point than fresh water, owing to the solution of salt and other minerals. Changes of pressure affect the freezing point as well as the boiling point. This is illustrated by the melting of glaciers from the bottom, which is the cause of their move ment of flow. This melting at the bottom is due to the great weight of ice, creating pres sure, which brings about melting though the temperature may be many degrees below 32° F.
To determine the molecular weights of sub stances, it is Important to know the freezing point. To ascertain this the common method is to make a solution of the substance. The freezing point of the solution is usually lower than that of the solvent, the difference being proportional to the amounts not only of volume but of weight. The curves obtained appear to be similar to those found by tests of the boil ing point to determine molecular weight. Beck
man's apparatus for determining the freezing point of solutions consists of a jar containing a liquid, which can be maintained at a tempera ture slightly below that of the solution, and a glass tube made double, that is, with inner and outer tube, the inner tube for the solvent and the outer for air space. A thermometer scale is marked on the inner test tube. With this apparatus the freezing point of the pure solvent can be found with a close approach to accuracy. Next a definite proportion of the substance to be dissolved is added to the solvent, through a side aperture provided in the tube, and the temperature of the solution is observed. The difference between the freezing point of the solvent and the solution is then apparent. Be cause of different results obtained by different experimenters with different apparatus it was learned that several conditions minutely affected the final temperature recorded. The size and shape of the vessel containing the cold mixture that accomplished the freezing, and the regular or irregular stirring of the contents, and the diameter of the test tube were all found to affect the ultimate temperature, as well as the temperature of the outside of the jar, due to the temperature of the room. To check such experiment therefore a boiling test and some times other tests are made for molecular weight, the average result being accepted. See MELTING POINT; SOLUTIONS.