ELECTROLYTIC SOLUTIONS 25 



that which theory demands, is due to an increase in the 

 number of the gaseous molecules present. Some of the 

 vapour particles are dissociated into two or more fragments, 

 each of which plays the part of a single molecule. 



Arrhenius, in 1S85, advanced the hypothesis that the 

 apparent increase in the number of molecules of an electrolytic 

 solution was also due to dissociation. This interpretation 

 at once threw a flood of light on a number of phenomena 

 hitherto obscure. 



Coefficient of Dissociation. — We have seen that in order 

 to obtain values which accord with experiment we have to 

 multiply the number of gramme-molecules of the solute 

 by the coefficient i, which is called the Coefficient of Dis- 

 sociation. 



This coefficient of dissociation, i, may be found by observing 

 the lowering of the freezing point of a normal solution, and 



dividing it by 1 '85. i = _— -. 



The coefficient of dissociation varies with the degree of 

 concentration of the solution, rising to a maximum when the 

 solution is sufficiently diluted. 



If we know /, the coefficient of dissociation for a given 

 solute, contained in a solution of a definite concentration, 

 we can find 11', the number of particles present in a solution 

 containing 11 gramme-molecules of the solute per litre, since 

 n' = in. On the other hand, if from a consideration of its 

 freezing point and other constants we find that an electrolytic 

 solution appears to contain ri gramme-molecules per litre, 

 the real number of chemical gramme-molecules in one litre 



of the solution will be only - —>i. 



' 1 



Very concentrated solutions do not conform to these laws. 

 In this they resemble gases, which as they approach their 

 point of condensation tend less and less to conform to the 

 laws of gaseous pressure. 



Electrolysis. — If we take a solution of an acid, a salt, or a 

 base, and dip into it two metallic rods, one connected to the 

 positive and the other to the negative pole of a battery, we 



