Page:Outlines of Physical Chemistry - 1899.djvu/230

 212 OUTLINES OF PHYSICAL CHEMISTBY

which it is arrived at, and as the indirect methods have been applied to an enormously large number of substances, we may say that we possess sufficient data to prove that the

The degree of dissociation depends on several circum- stances, but chiefly on the two following :

1. The nature of the dissolved substance. Strong acids, strong bases, and salts which contain a strong acid or a strong base are very easily dissociated. On the contrary, weak acids (such as H 2 S, S0 2, C 2 H 4 2 , &c), and weak bases (such as 1JH 3, C 6 H 6 NH 2 , &c), and salts which contain neither a strong acid nor a strong base, do not readily dissociate.

2. The concentration of the solution. For substances which dissociate easily the limiting value of x can be practically attained. A dilution of one gram-equivalent of substance in 1,000 litres of water is generally sufficient to produce quasi-complete dissociation. For more concen- trated solutions the value of x is fractional, as was shown by Kohlrausch from the molecular conductivities of solutions of salts, acids, or bases (pages 201 and following).

As to the natwre of salt dissociation we can only express some suppositions.

Sv. Arrhdnius in 1887 declared in favour of the idea of the absolute fbeedom of the ions, and his theory is accepted and expounded by Ostwald in his excellent treatise on theoretical chemistry and by Nemst in the theoretical introduction which he wrote for Dammer's book.

If this theory be accepted, then it is easy to understand the meaning of such terms as * the rate of migration of the

1 For the same solution we can find the degree of dissociation by three different methods : electric conductivity, boiling-point, freezing- point. If these methods could all be carried out at the same tem- perature, there is no doubt but that they would all give the same value for x. This, of course, is impossible, and, as the dissociation is a function of the temperature, the values found for x will not be quite the same,

�� �