MASS ACTION 107 



to take place so that n moles of A are capable of reacting 

 with m moles of B to form p moles of X and q moles of Y, 

 or vice versa if the number of moles of A, B, . . . X, Y, 

 . . . actually present in unit volume of the reacting mixture 

 are represented respectively by G, C 2 , . . . d h d 2 , . . ., 

 and further if sufficient time be allowed to permit the 

 system to come to equilibrium, then at a given temperature 

 the condition of the system is expressed by the equation 

 dS d 2 q . . . 



n m 



= a constant. 



This relation is readily understood when one considers 

 that a chemical reaction takes place as a result of very 

 minute particles (molecular or ionic) of the reacting ma- 

 terials coming into intimate contact with each other. 

 Obviously the amount of chemical action will depend on 

 the number of these particles present in a given volume. 

 Moreover, if one of these materials is in great excess in 

 the system, it would be expected that the substance with 

 which it tended to react would at equilibrium be nearly 

 all used up. 



In the case of solutions of inorganic salts, the reactions 

 are for the most part ionic and take place therefore with 

 great rapidity. It also frequently happens that one of 

 the reacting bodies is only slightly soluble, and this fact 

 predisposes the reaction in favor of its continued forma- 

 tion. But as no salt can be said to be completely insolu- 

 ble, it is quite possible for a reaction to take place, having 

 a so-called insoluble substance as one of the starting 

 materials. 



For example, consider the reaction 



Na 2 C0 3 + CaCl 2 1; 2 NaCl + CaC0 3 

 which normally proceeds from left to right on account of 



