IONIZATION 193 



has escaped either as a precipitate or as a gas. All of the peculiar 

 properties of the original components (such as their action on 

 litmus) disappear, and we are left with a solution of common salt. 



The explanation, according to the ionic hypothesis, is very 

 simple. Before mixing the two solutions, we have our acid and 

 base almost entirely broken up into the four ions H + , Cl~, Na~*~ 

 and OH~. But as soon as mixture is effected, practically all 

 H + and OH~ ions withdraw from the solution, combining to form 

 water. The minute amounts of undissociated HC1 and NaOH 

 continue to break up, attempting to regain equilibrium with 

 their respective ions, until they are both eliminated, and only 

 chloride ions Cl~ and sodium ions Na + , in equilibrium with un- 

 dissociated Nad, remain in the solution. 



The reaction may be written in expanded ionic form as follows: 



HC1 ^ H+ + Cl- 

 NaOH ^ OH- + Na+ 



1 11 



H 2 Nad 



The ionization of water is so minute that it may be neglected. 

 When either the acid or the base employed is weak, however, 

 the tendency of water to break up into H + and OH~~ must be 

 taken into consideration (see hydrolysis, p. 369). 



We may note here a third method of driving reversible reactions 

 between electrolytes to completion. Besides removing one 

 product as a precipitate or as a gas (see pp. 126-7), we may ar- 

 range the conditions so that one of the substances formed is prac- 

 tically non-ionized. This is a procedure which is very frequently 

 employed by chemists in carrying out reactions. 



The Part Played by the Solvent in Ionization. So far, 

 we have regarded water as playing merely a physical role in the 

 ionization of electrolytes. The ions in a solution of hydrogen 

 chloride, for example, have been regarded as H + and Cl~, the 



