ELECTROLYTIC SOLUTIONS 27 



An electrolytic solution contains three varieties of particles, 



positive ions or cations, negative ions or anions, and 1111- 



dissociated neutral molecules. The molecular concentration 



of such a solution, with the corresponding constants, depends 



on the total number of these particles, i.e. the sum of the ions 



and the undissociated neutral molecules. We may indicate 



an ion by placing above it the sign of its electrical charge, one 



+ 

 sign for each valency. Thus Na and CI indicate the two ions 



++ __ 



of a salt solution; Cu and S() 4 the two ions of a solution 



of sulphate of copper. A point is sometimes substituted for 



the + sign, and a comma for the — sign. Thus Na' and CT ; 



Cu' • and S0 4 ' '. 



My friend Dr. Lewis Jones has given a verv vivid picture 

 of the processes which go on in an electrolytic solution 

 when an electric current is passing. He compares an electro- 

 lytic cell to a ballroom, in which are gyrating a number of 

 dancing couples, representing the neutral molecules, and a 

 number of isolated ladies and gentlemen representing the 

 anions and cations respectively. If we suppose a mirror 

 at one end of the ballroom and a buffet at the other, 

 the ladies will gradually accumulate around the mirror, and 

 the gentlemen around the buffet. Moreover, the dancing 

 couples will gradually be dissociated in order to follow this 

 movement. 



Degree of Dissociatio7i. — The degree of dissociation is the 

 fraction of the molecules in the solution which have under- 

 gone dissociation. Let n be the total number of molecules of 

 the solute, and 11" the number of dissociated molecules. Then 



— =a will represent the degree of dissociation. Let k be the 

 11 



number of ions into which each molecule is split. Then 



a = -■-, i.e. the degree of dissociation is the ratio of the 

 nk ° 



number of ions actually present in a solution to the number 



which would be present if all the molecules of the solute were 



dissociated. 



Let n be the total number of particles present in a solution 



