OXIDES AND OXYGEN ACIDS OP SULPHUR 271 



The reducing action of HBr and HI on sulphuric acid has already 

 been noted (pp. 202, 204). The more active metals, like zinc, 

 reduce it to hydrogen sulphide, the less active, like copper, give 

 sulphur dioxide. Hydrogen is not liberated, because practically 

 no hydrogen-ion is present in concentrated sulphuric acid. Gold 

 and platinum alone are not attacked. 



4Zn + 5H 2 SO 4 -4ZnS0 4 + 4H 2 + H 2 S. 

 Cu + 2H 2 S0 4 - CuS0 4 + 2H 2 O + SO 2 . 



Uses of Sulphuric Acid. The acid has innumerable applica- 

 tions, some of which will be taken up in detail in later chapters. 



It is employed in almost every chemical industry, something 

 like 6,000,000 tons being produced yearly in the United States 

 alone. It is used in the manufacture of sulphates, hydrochloric 

 acid, nitric acid, sodium carbonate, etc., in making fertilizers and 

 dyes, in bleaching, electroplating and so on. Its dehydrating 

 power is especially valuable in making explosives (pp. 481-2). 



Other Oxygen Acids of Sulphur. Many other oxygen 

 acids of sulphur exist, such as hyposulphurous add H 2 S 2 4 and 

 persulphuric acid H 2 S 2 O 8 . The acids themselves are very unstable, 

 and cannot be isolated in the pure state. Some of the salts, 

 however, are in common use, and will be dealt with later under 

 their several positive radicals. 



When acid sulphates, such as NaHS0 4 , are heated, water is 

 given off and a pyrosulphate (Greek prefix, fire) remains. 



The pyrosulphates are salts of oleum, or fuming sulphuric acid, 

 which has already been mentioned. Oleum possess all of the 

 dehydrating and oxidizing powers of sulphuric acid in an accen- 

 tuated form, and is widely used in the industries on account of 

 these properties. 



