260 THE REALITIES OF MODERN SCIENCE 



rium point the rate of disappearance of A and B 

 must be equal to the rate at which they are formed by 

 the disappearance of C and D. Therefore 



where K is the equilibrium constant of the reaction. 



This law is commonly known as the law of "mass 

 action " because originally the words "active mass" 

 were used in place of the modern term "concentration." 

 The equilibrium constant will depend upon the tem- 

 perature and may also depend upon pressure. For 

 this reason whenever an equilibrium constant is given 

 in numerical value it is necessary to state the other 

 conditions. For very many reactions, on the other 

 hand, no equilibrium point may be reached, for they are 

 not reversible. For example, there is no equilibrium 

 point for a liquid above its critical temperature, and 

 if this temperature is exceeded the reaction occurs in 

 one direction only and goes to completion. 



In the reactions of electrolytes this law of mass 

 action is of 'greatest convenience, since the ionization 

 constants may be easily measured and tables of them 

 prepared. These data, with tables of solubility, permit 

 the chemist to foretell the results of the reactions of 

 mixtures of electrolytes and hence to make efficient 

 use of them in the quantitative analysis of unknown 

 mixtures. 



If both components of such a mixture are soluble and 

 also have appreciable dissociations, chemical reactions 

 are possible, that is, neutral molecules may be formed 

 by the collisions of the anion of one component with the 



