4 GENERAL AND PHYSICO-CHEMICAL. 



must also boil at the same temperature. The rise in the boiling-point 

 of a solution above the boiling-point of the solvent (elevation of the 

 boiling-point) is also, like the osmotic pressure, for dilute solutions pro- 

 portional to the concentration. 



Solutions have a lower freezing-point than the pure solvent, and as 

 in dilute solutions the solvent can be frozen out from the dissolved body, 

 then isosmotic solutions have the same freezing-point. The depres- 

 sion of the freezing-point is also proportional to the concentration. 



The determination of the elevation of the boiling-point for the esti- 

 mation of the osmotic pressure of animal fluids is applicable only in 

 exceptional cases, because on heating, precipitates often form. The 

 determination of the depression of the freezing-point has been found of 

 much greater use. This can be accomplished in an easy manner by aid of 

 the apparatus suggested by BECKMANN. In regard to the use of this 

 method we must refer to more complete works. 1 



The above rule that equimolecular solutions of different bodies have 

 the same osmotic pressure is only applicable to non-electrolytes. The 

 electrolytes (bases, acids, salts) show in aqueous solution a much greater 

 pressure (i.e., a much lower depression of the freezing-point) than equi- 

 molecular solutions of non-electrolytes. As is known, ARRHENIUS has 

 explained this lack of correspondence by the assumption that the mole- 

 cule of the electrolyte is divided or dissociated into so-called ions hav- 

 ing an opposed electric charge. An ion exerts upon the osmotic pressure 

 the same influence as the non-dissociated molecule. The larger the 

 number of dissociated molecules the more does the osmotic pressure 

 of the solution rise above the pressure of an equimolecular solution of a 

 non-dissociated body. The osmotic action of a dissociated body is equal to 

 that of a non-dissociated body which in a given volume contains as many 

 molecules as the dissociated body contains ions plus non-dissociated mole- 

 cules. If we assume that a is the degree of dissociation, i.e., the number of 

 the molecules that are dissociated, then 1 a is the number that is not 

 dissociated. If in the dissociation of a molecule n ions are formed 

 then the relation of the molecules present before the dissociation to the 

 ions + molecules present after the dissociation is 1:(1 a+na) or 

 = l:(l-f-[tt l]a). The expression (l+[w l]a) is generally denoted by 

 the letter i, and can be directly determined by estimating the freezing- 

 point of a solution of known molecular concentration. 



A gram-molecule aqueous solution (one that contains as many grams per 

 liter as the molecular weight of the substance) of any non-electrolyte freezes 

 at about - 1.86, or, the depression of the freezing-point A is = 1.86. For example, 



1 Ostwald-Luther, Hand- und Hilfsbuch zur Ausfiihrung physik.-chemischer 

 Messung, 3 Aufl., 1910. 



