12 PROPERTIES OF SOLUTIONS 



The occurrence of electrolysis, as well as the unusually large effects of 

 electrolytes on the osmotic pressures of solutions (Chap. VIII), have led to 

 an explanation of the behavior of electrolytes in terms of the theory of electro- 

 lytic dissociation. This theory was first advanced by the Swedish chemist 

 Arrhenius who introduced it in essentially its present form in 1887. Accord- 

 ing to the Arrhenius theory when an electrolyte is dissolved in water some 

 of the molecules dissociate into two kinds of particles, one positively charged, 

 the other negatively charged. Each of these particles is called an ion. Ions 

 are supposed to be present in a solution whether any electrolysis occurs or not. 

 Two, three, four or even more ions may be formed from a single molecule. 

 The conduction of an electrical current by an electrolyte is due to the presence 

 of these ions. The dissociation of several typical electrolytes is indicated by 

 the following equations : 



NaCl ^ Na+ + Cl" 

 CaCl2 ^ Ca++ + CI- + Cl" 

 Na2S04 ^ Na+ -f- Na+ + SO4— 



The positively charged ions, which in electrolysis migrate towards the 

 cathode, are called cations; the negatively charged ions which migrate toward 

 the anode are called anions. A cation may carry one, two, three, or even four 

 positive charges; an anion may carry from one to several negative charges. 

 When an electrolyte dissociates the number of positive charges carried on the 

 cations is always equal to the number of negative charges carried by the 

 anions. 



The Arrhenius theory does not assume complete ionization of all of the 

 solute molecules in an electrolyte but that the molecules are continuously 

 dissociating into ions, while free ions in the solution are continuously reuniting 

 and forming molecules, both processes proceeding at an equal rate whenever 

 an equilibrium condition prevails. An equilibrium of this sort, which is 

 maintained by two opposing processes proceeding at equal rates is termed a 

 dynamic equilibrium. Electrolytes vary greatly in degree of dissociation 

 (Table 2). Those in which a large proportion of the molecules are main- 

 tained in a dissociated condition are termed "strong" electrolytes; those of 

 which the contrary is true, "weak" electrolytes. "Strong" electrolyte solutions 

 conduct electric currents better than "weak" electrolyte solutions of equal 

 concentration. In general the more dilute a given electrolyte solution the 

 larger the proportion of dissociated molecules present. In extremely dilute 

 solutions ionization is practically complete. Increase in temperature reduces 

 the dissociation of an electrolyte. 



