Page:Text-book of Electrochemistry.djvu/75

 6o

��BOILING AND FREEZING POINT.

��CHAP.

��which appears to be the most evident. If the deviation in benzene solutions, in which the molecular lowering of the fi-eezing point is too small, is to be explained by assuming that the dissolved molecules are greater than is expressed by the chemical formula, then the deviation in aqueous solutions, where the lowering of the freezing point is greater than that calculated, may be assumed to be due to the dissolved molecules being smaller than indicated by the chemical formula. In the first case we imagine that a combination of simple molecules to a molecular complex takes place, and in the second case we have to assume that the simple molecules split up into smaller parts. As we shall see later, this assumption is quite justified. In the case of certain salts, as, for instance, sodium chloride (NaCl), there can be no doubt what the parts are because only one kind of split- ting seems possible, namely, into Na and CI. In order to receive general credence, this assumption must be sup- ported by other experimental evidence; for it does appear strange at first sight that in a solution of salt this substance has always the same constant composition, although the constituents Na and CI in the solution are separated from each other.

Range of Validity of van't Hoff 's Law.— The following values for the molecular lowering of the freezing point, obtained as mean values from experiments with a large number of dissolved substances, were used by van't Hoff in support of his theory : —

��Solvent.

��Water. . Acetic acid. Formic acid. Benzene. . Nitro-benzene

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��The van't Hoff law is only valid for dilute solutions, for in more concentrated solutions forces come into play which

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