COMPOUNDS OF SODIUM AND POTASSIUM 369 



double decomposition has occurred. But such a solution will 

 contain a much higher concentration of OH~ from the highly 

 ionized NaOH than of H+ from the practically non-ionized H 2 C0 3 . 

 It will, therefore, react like an alkali. 



-H^drolysisjtfJSglts. This interaction of a salt with water is 

 callecThydrolysis (Greek, decomposition by water). All salts are 

 hydrolyzed, at least to a slight extent. The action is the reverse 

 of neutralization (p. 192), water and a salt giving, by double 

 decomposition, an acid and a base. The effect is noticeable, 

 however, only when the acid and base are of very unequal activity. 

 A salt which, by hydrolysis, gives an active base and a weak acid, 

 furnishes a solution the reaction of which is basic. 



Conversely, if the salt gives, by hydrolysis, a weak base and an 

 active acid, then the solution is acid in reaction. Thus, the solu- 

 tion of cupric sulphate is acidic because cupric hydroxide is a 

 feeble base. 



The extent of hydrolysis, even in cases where it is distinctly 

 observable, such as sodium carbonate and cupric sulphate men- 

 tioned above, is in general only small. Borax, a salt of an ex- 

 tremely weak acid (p. 363), reacts distinctly alkaline in water, 

 but the degree of hydrolysis in a 0.1 AT solution is only one-half 

 of one per cent. The reason, of course, lies in the fact that water is 

 very much less ionized even than exceedingly weak acids like 

 boric acid or exceedingly weak bases like cupric hydroxide. The 

 tendency towards the completion of neutralization, therefore, 

 preponderates considerably over the tendency towards the com- 

 pletion of hydrolysis. 



Sodium Nitrate NaNO 3 . This salt is prepared by _recrystal- 

 lizing Chile saltpeter (p. 308). When heated, it gives off oxygen, 

 leaving sodium nitrite NaNOsi, a compound much used as a source 

 of nitrous_acid in the manufacture of organic dyestuffs. 



Sodium nitrate is at present the chief source of nitric acid (p. 



