CH. I.] HYDROGEN ION CONCENTRATION 15 



So (H) x (OH) = a constant x (H 2 O). 



The brackets indicate the concentration per litre of 

 ions or moles respectively. 



Since the mass of undissociated water is enormously 

 large compared to the mass of the free ions, it can be 

 regarded as a constant, so 



(H) x (OH) = a constant. 



This constant varies considerably with the tempera- 

 ture. 



At 21 C. it is or io~ 14 . 



1 00,000,000,000,000 



Since hydrions and hydroxidions are equal in number, 



each has a concentration of or io~ 7 per litre. 



10,000,000 



If an acid be added to distilled water the acid is partially 

 or completely dissociated into hydrogen ions, and the 

 negative ions characteristic of the acid employed. In 

 such a mixture the concentration of hydrogen ions per litre 

 at 21 C. is greater than io~ 7 , and the solution is " acid." 

 If (H) be increased to io~ 4 it follows that the concentration 

 of hydroxyl ions per litre must be decreased to io~ 10 . For 

 (H) x (OH) = io- 14 . If an alkali be added to distilled 

 water the base is dissociated into hydroxyl ions and certain 

 positive ions. The concentration of hydrogen ions per 

 litre at 21 C. is consequently less than io~ 7 , and the 

 solution is " alkaline." 



A " neutral " solution is one in which (H) at 21 C. = 

 io- 7 . 



An " acid " solution is one in which (H) at 21 C. is 

 greater than io~ 7 . 



An " alkaline " solution is one in which (H) at 21 C. is 

 less than io~ 7 . 



Acids differ markedly in the degree to which they are 

 ionised in solution. " Strong " acids, like HC1 or HNO 3 , 



