CHAPTER III 

 ELECTRIC CONDUCTIVITY, DISSOCIATION, AND IONIZATION 



The osmotic pressure is not infrequently found to be considerably 

 greater than that expected from the strength of the solution. Although 

 A of a gram-molecular watery solution of cane sugar (342 gm. to the liter) 

 is 1.86 (see page 10), that of sodium chloride (58.5 gm. to the liter) is 

 considerably greater. If the hypothesis regarding the relationship of 

 molecular concentration to osmotic pressure is to hold good, it becomes 

 necessary to explain this apparent inconsistency; one must account for 

 a greater number of dissolved units than is represented by the actual 

 number of dissolved molecules (i.e., weight of dissolved substances). 



It was observed that the power to conduct the electric current— electric 

 conductivity — in the case of solutions (e. g., of sugar) which have an 

 osmotic pressure that corresponds to the weight of dissolved substances 

 is practically nil, whereas the conductivity of those solutions which give 

 higher osmotic pressure is quite pronounced. Arrhenius made the hy- 

 pothesis that the conductivity depends on the splitting of molecules into 

 two or more portions or ions, each of which carries either a positive or a 

 negative electric charge, and that it is only when such dissociation occurs 

 that the electric current can be conducted through the solution, the ions 

 serving as it were as floats carrying the electric current. When sodium 

 chloride is dissolved in water, it splits into Na carrying a positive charge 

 and CI carrying a negative charge, or Na + Cl-, as it is written; on the 

 other hand, when sugar is dissolved, the molecules remain unbroken and 

 no electric charges are set free. 



Substances which thus dissociate are called electrolytes, and those which 

 do not, nonelectrolytes. When the electric current is passed through a 

 solution of electrolytes, the ions which carry a positive charge move to 

 the electrode or pole by which the current leaves the solution — that is, in 

 the same directions as the current; and since this electrode is called the 

 cathode, these are called cations. Hydrogen and the metals belong to 

 this group. The ions carrying a negative charge go in the opposite direc- 

 tion, against the current — that is, towards the electrode by which the cur- 

 rent enters, or the anode; they are therefore called anions. They include 

 oxygen, the halogens and the acid groups, such as S0. 1; C0 3 , etc. 



It must be understood that this dissociation into ions is already present 

 in the solution before any electric current passes through it, the ions 



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