258 



oxygen, the prefix hydro implies none at all. The per-acid con- 

 tains the most oxygen. 



The names of compounds containing only two elements (the 

 binary compounds) end in ide: Zinc sulphide ZnS, magnesium 

 nitride Mg 3 N 2 , calcium carbide CaC 2 , sodium chloride NaCl, and 

 the oxides CaO, etc. 



SULPHUR DIOXIDE AND SULPHUROUS ACID 



Preparation of Sulphur Dioxide SO 2 . In commercial prac- 

 tice sulphur dioxide is obtained in three ways: 



1. By burning sulphur. 



2. By burning natural sulphides, such as pyrite: 



Skeleton: FeS 2 + O 2 -> Fe 2 3 + S0 2 . 



Balanced: 4FeS 2 + 1 10, -> 2Fe 2 3 + 8S0 2 . 



With fairly pure pyrite the combustion has only to be started, 

 the heat evolved in the reaction being sufficient to offset loss of 

 heat by radiation and to keep it going of its own accord. But 

 with some sulphides, like zinc blende ZnS, which is used as a 

 source of sulphur dioxide as well as of zinc, the air must be strongly 

 heated throughout to maintain the combustion : 



2ZnS + 3O 2 - 2ZnO + 2S0 2 . 



^Forced combustion of an ore, like this, is called roasting, or 

 x^alcimng, and is the first stage towards obtaining the metal. The 

 oxide is subsequently reduced by heating with coke. 



3. By dropping concentrated sulphuric acid into red-hot iron 

 retorts : 



2H 2 S0 4 -> 2H 2 + 2S0 2 + O 2 . 



4. In the laboratory a steady stream of the gas is easily obtained 

 by dropping hydrochloric acid upon crystals of sodium-hydrogen 

 sulphite (Fig. 25, p. 52) : 



NaHS0 3 + HC1 ^ NaCl + H 2 SO 3 , (1) 



H 2 S0 3 <=* H 2 + S0 2 1 . (2) 



