SULPHUR. 



207 



Sulphurous acid is a colorless acid liquid, which has the odor as well as the 

 disinfecting and bleaching properties of sulphur dioxide; it is completely 

 volatilized by heat. Sulphurous acid is a dibasic acid, the salts of which are 

 termed sulphites. Both the acid and its salts are easily oxidized by the air, 

 and hence almost always give the tests for sulphuric acid. In its chemical be- 

 havior sulphurous acid is very much like carbonic acid. It is a weak acid, 

 being displaced from its salts by all acids except carbonic and boric. The sul- 

 phites of the alkali metals are freely soluble in water, but the normal sulphites of 

 all other metals are insoluble or nearly so ; hence addition of a solution of an 

 alkali sulphite to a solution of salts of the other metals causes precipitates. A 



FIG. 42. 



Apparatus for making sulphurous acid. 



few sulphites are soluble in a solution of sulphurous acid, like carbonates, but 

 are precipitated on boiling. Acid sulphites of the alkali metals can be obtained in 

 the solid state. These show an acid reaction to litmus, but the normal sul- 

 phites of these metals have an alkaline reaction, due to hydrolysis by water 

 into bisulphite and free alkali. All the common sulphites are white. 



The ionic equations for the liberation of sulphurous acid from a sulphite, 

 and the ionization of normal and bisulphites in solution, are ir all respects like 

 those pertaining to the liberation of carbonic acid and the ionization of normal 

 and bicarbonate of sodium, which are given on pages 195 and 201. The reason 

 that bisulphites are slightly acid in solution, and not neutral as bicarbonates 

 are, is that an appreciable amount of hydrogen ions are formed, thus : 



NaHSO 3 ^ 



HS0 3 ' 

 SO 3 ". 



The second reaction takes place only to a small degree. 



