^lrt-lsyy\~>>9 



CHAPTER XXII 

 OXIDES AND OXYGEN ACIDS OF SULPHUR 



THERE are two familiar oxides, namely sulphur dioxide or sul- 

 phurous anhydride S0 2 , and sulphur trioxide or sulphuric anhy- 

 dride S0 3 . Each of these dissolves in water and combines with it 

 to form an acid. The former gives sulphurous acid H 2 0,SO? 

 or H 2 S0 3 , and the latter sulphuric acid H 2 0,S0 3 or H 2 SO 4 . 



Acidic and Basic Oxides. An oxide, like carbon dioxide 

 C0 2 (p. 336) or sulphur dioxide S0 2 , wMck-,cQrnbine&,with_water 

 to form an acid, is said to be the anhydride of the acid. The npidaR 

 of the n.f}n-m.p,^/i.c. elements, when they combine with water, in 

 so doing jn variably form acids. In the next chapters we shall 

 meet with other examples (e.g., N 2 5 , P 2 O 5 , SiO 2 , etc.). On the 

 other hand, the oxides of metallic elements, when they are able to 

 combine with water, generally give bases (e.g. Ca(OH) 2 from CaO). 

 For convenience, therefore, we shall often speak of an oxide as an 

 acidic oxide or a basic oxide, as the case may be. 



Nomenclature. The acids and salts within one group are 

 distinguished by the terminations of, and prefixes to, their names. 

 Thus we have: 



Hydrochloric acid HC1 Sodium chloride NaCl 



HypoMorous acid HOC1 Sodium hypochlorite NaOCl 

 Chlorous acid HC10 2 Sodium chlorite NaClO 2 

 Chloric acid HC1O 3 Sodium chlorate NaClOs 



Perchloric acid HC1O 4 Sodium perchlorate NaClO4 



The proportion of oxygen to the other elements is at the basis of 

 the system. The terminations ous and ite indicate less oxygen 

 than ic and ate. The prefix hypo (Greek, below) implies still less 



257 



