260 SMITH'S INTERMEDIATE CHEMISTRY 



As already noted, concentrated solutions are very unstable. A 

 solution of sulphurous acid therefore smells strongly of sulphur 

 dioxide. 



Being rather easily convertible into sulphuric acid H 2 SO 4 , 

 sulphurous acid is a reducing agent. Thus oxygen from the air 

 acts slowly upon the solution : 



2H 2 SO 3 + O 2 -2H 2 S0 4 

 and iodine is turned into hydrogen iodide : 



H 2 S0 3 + H 2 + I, -* H 2 S0 4 + 2HI. 



SuJphites and Bisulphites. Sulphites are formed by 

 neutralization of sulphurous acid with a base: 



2NaOH + H 2 S0 3 - Na^SOa + 2H 2 O. 



With excess of sulphur dioxide passed into the solutions of the 

 bases, the acid sulphites are f ocmed : 



NaOH + H 2 S0 3 - NaHS0 3 + H 2 O. 

 Ca(OH) 2 + 2H2SO 3 -> Ca(HS0 3 ) 2 + 2H 2 0. 



Such acid salts are known in commerce as bisulphites, because, 

 the proportion of the metal being half that in a sulphite, the pro- 

 portion of the sulphite radical is, relatively, twice as great. 

 They are used extensively on paper manufacture (see p. 398). 



Dibasic Acids. Acids containing two atoms olJiy-drogeiL j.n 

 each molecule are called dibasic acids. H 2 C0 3 (p. 336), H 2 S, 

 H 2 S0 3 and H 2 S0 4 are such acids. Each molecule is able to 

 react with two molecules of a base like sodium hydroxide, as 

 may be seen in the first of the equations in the preceding section. 

 When half the quantity of the base is used, an acid salt (p. 192) is 

 produced, as the two other equations show. Phosphoric acid 

 H 3 PO 4 is a tribasic acid, and forms two series of acid salts, for 

 example NaH 2 PO 4 and Na^HPC^. Hydrochloric and nitric 

 acids HNO 3 are monobasic. 



