270 SMITH'S INTERMEDIATE CHEMISTRY 



as an add than is sulphurous acid, but is somewhat inferior in 

 this respect to hydrochloric acid HC1 and nitric acid HN0 3 . Like 

 other active, soluble acids, its solution turns litmus red, gives 

 hydrogen upon addition of active metals, and enters into double 

 decomposition with bases and salts. Thus, insoluble barium 

 sulphate is obtained as a white precipitate by the action of dilute 

 sulphuric acid on any soluble barium salt: 



BaCl 2 + H 2 S0 4 -> BaS0 4 j + 2HC1. 



Any soluble sulphate will, of course, give the same precipitate 

 with barium chloride, and the action is used as a test for this ion. 

 Some other salts of barium are also insoluble in water, but the sul- 

 phate is recognized by the fact that it is too insoluble to be acted 

 upon by dilute pure hydrochloric acid or nitric acid. The other 

 insoluble salts of barium interact with these acids and dissolve. 

 The addition of one of these acids is therefore part of the test for 

 S0 4 = ion. 



On account of its high boiling-point, the double decompositions 

 of the concentrated acid can be used for preparing more volatile 

 adds: 



Nad + H 2 SO 4 - NaHS0 4 + HC1 t (gas at room temp.). 



NaN0 3 + H 2 S0 4 -* NaHSO 4 + HNO 3 T (volatile at 86). 



3. Concentrated sulphuric acid combines with water to form a 

 stable hydrate H 2 SO 4 ,H 2 0. Hence it removes the elements of 

 water from many substances containing hydrogen and oxygen, 

 and is called a dehydrating agent. Thus, paper (cellulose), 

 moistened with the acid and warmed, turns black from the libera- 

 tion of carbon. Sugar (Ci 2 H 22 On) is decomposed even more easily: 



CiaH^On - 12C + 11H 2 O. 



4. Finally, concentrated sulphuric acid acts as an oxidizing 

 agent. Sulphur and carbon, boiled in it, are oxidized : 



2H 2 S0 4 + S -> 3S0 2 + 2H 2 0. 

 2H 2 S0 4 + C -> 2S0 2 -f 2H 2 + CO 2 . 



