ACIDITY 301 



Such paired ions, held solely by electric attraction, are referred 

 to as io7iic doublets. 



Other similar problems in connection with dissociation con- 

 stantly come up, some of them rather disturbing in regard to 

 accepted theories concerning the hydrogen ion. Lowry says that 

 there is no such thing as a hydrogen ion in solution because 

 there is no obvious reason why one and not both of the two 

 hydrogen atoms of water should become detached. It is now a 

 rather generally accepted hypothesis that the following reaction 

 takes place when an acid is added to water : 



H2O + HCl ±^ H3O+ + Cl- 



A number of chemists, including J. N. Bronsted and M. 

 Kilpatrick, have contributed to the newer concept of acidity. 

 On the basis of their work, we no longer define an acid as a com- 

 pound that can split off hydrogen ions but as a hydrogen com- 

 pound that can split off protons (the hydrogen ion is a proton). 

 In accepting this definition, we discard the requirement that an 

 acid should combine with a metal hydroxide to form a salt and 

 water and say merely: 



Acid ;^ base + proton 



From this point of view, water is an acid, for 



H2O ^ OH- + H+ 

 so also are certain anions such as 



H2PO4- ^ HPO4-- + H+ 

 and cations: 



NH4+ ^ NH3 + H+ 



The newer concept of a base also discards the older definition, 

 which requires that a base must give off hydroxyl ions, and 

 defines it as a substance that can take up protons; water is thus 

 a base. 



The equation H2O + HCl ;=± H3O+ + CI", which shows water 

 to be a base, and the equation H2O ^ OH" + H+, which shows 

 water to be an acid, also indicate the amphoteric character 

 (see page 483) of water. 



