Page:Text-book of Electrochemistry.djvu/73

 58 BOILING AND FREEZING POINT. chap.

cases be accounted for in a similar manner, leaving for the moment aqueous solutions out of account.

The existence of such double molecules, which, of course, are mixed with simple molecules and higher complexes, is by no means improbable. On the contrary, such relationships were formerly considered as the normal, and the difference between the liquid and gaseous conditions were attributed to them. More recent researches have, however, shown that at moderate dilutions it is only in exceptional cases that double molecules are formed. The substances which most easily form these double molecules belong to the classes already mentioned, namely, alcohols, phenols, and organic acids (particularly the fatty acids).

Molecule Complexes. — The formation of double, triple, etc., molecules of dissolved substances depends to a great extent on the nature of the solvent. It appears to take place very seldom in aqueous solution, although it does so in the case of some salts of cadmium and mercury, and with the sulphates of magnesium, zinc, and copper. (Tliis matter is discussed in more detail below.) The formation of these double molecules takes place more frequently when the solvent is acetic acid or formic acid, and still more so with benzene or other hydrocarbon.

The dielectric constant of the solvent has a great influence on the complex formation taking place in the solution. Liquids with a high dielectric constant have the power of decomposing the dissolved substance into simple molecules, and this power increases with the dielectric constant. These constants vary greatly with the chemical nature of the media ; of the solvents in common use water has the highest dielectric constant {BE), namely, 80 ; for formic acid DE = 57, for acetic acid DE = 65, for ethyl alcohol DE = 217, and for benzene DE = 2*2.

As we shall see later, the same holds good for the power of a solvent to dissociate an electrolyte into ions (5).

As the dilution increases the complex molecules become broken up into simpler ones, as the following results of

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