IONIZATION 191 



Solutions of weak electrolytes, such as acetic acid and am- 

 monium hydroxide, exhibit practically normal results, because 

 only an insignificant fraction of the solute is broken up into ions. 



Careful measurements show a close agreement in respect to 

 extents of ionization, as determined by the two independent meth- 

 ods, conductivity ratio and freezing-point depression, through- 

 out the whole list of hundreds of electrolytes. Minor divergences 

 exist in some cases which have not yet been entirely accounted for, 

 but the agreement in general is so remarkably close that it can- 

 not be a chance coincidence. We have here, in fact, very strong 

 confirmation of the truth of the ionic hypothesis. 



The Properties of Acids. The properties of acids (p. 131) 

 are now seen to be properties of hydrogen ion H + . An acid is a 

 substance which contains hydrogen as a positive radical and, 

 in solution, gives hydrogen ion. Strictly speaking, only the 

 conducting solutions of such substances are acids, but for con- 

 venience we extend the term sometimes to include the pure sub- 

 stances. For example, HN0 3 is usually called nitric acid, not 

 hydrogen nitrate. 



Many substances, such as sugar C^H^On, contain hydrogen, 

 but their solutions lack all of the properties of hydrogen ion. 

 They therefore do not contain hydrogen as a radical, and are not 

 acids. 



A strong acid is one which is highly ionized in solution, and 

 therefore shows the properties of hydrogen ion very markedly. 

 A weak acid is one which is very little ionized in solution, and 

 consequently exhibits the properties of hydrogen ion only feebly. 

 Solutions of very weak acids (such as boric acid) scarcely affect 

 blue litmus. Water, a still weaker acid, contains just as much 

 hydroxyl ion OH~ as hydrogen ion H + , and does not change the 

 color of either blue or reddened litmus. Its acid properties are 

 still evident, however, in its action on the most active metals, 

 such as sodium: 



Na + 2H+ 4 2Na+ + H 2 T- 



