256 THE REALITIES OF MODERN SCIENCE 



gives a hydrogen cation ; a base, one which gives an 

 hydroxyl anion (OH) ; and a salt is an electrolyte which 

 has neither of these ions. The mobility of the hydro- 

 gen ion is the highest of all, OH is next, and other ions 

 much less. In these individual mobilities, therefore, 

 we see the explanation of the physical fact that acids 

 are better conductors than bases and the latter better 

 than salts. 



Chemists are most generally interested in acids as 

 sources of the H ions which cause the characteristic 

 effects. For this reason they are accustomed to speak 

 of acids, like HC1, which, in a solution of one tenth 

 mole per liter, dissociate more than 70 per cent, as 

 " strong." If the dissociation of a decinormal solution 

 of the acid lies between 10 and 70 per cent it falls into 

 a second group of strong acids. On the other hand, 

 acids like acetic, which in similar concentrations have 

 dissociations of 1 to 10 per cent, are " weak, " and those 

 like carbonic acid, which have even smaller values, 

 are " feeble." In the same way the chemist divides 

 bases into two groups ; those like sodium hydroxide 

 with large dissociations are called " strong, " while 

 those which, in a decinormal solution, form hydroxyl 

 ions from less than two per cent of the molecules are 

 weak bases. Of the latter ammonium hydroxide is an 

 example. Salts may be formed by weak acids and 

 strong bases and vice versa; thus sodium acetate is 

 the salt of a weak acid and a strong base. 



