206 NON-METALS AND THEIR COMBINATIONS. 



The following four oxides are known : Sulphur sesquioxide, S 2 O 3 ; 

 sulphur dioxide, SO 2 ; sulphur trioxide, SO 3 ; sulphur heptoxide, S 2 O 7 . 

 The three last named are acid oxides, which, on combining with one 

 molecule of water, form sulphurous, sulphuric, and persulphuric acid 

 respectively. 



Sulphur dioxide, SO 2 = 63.59 (Sulphurous anhydride, improperly 

 also called sulphurous acid), is formed when sulphur or substances con- 

 taining it in a combustible form (H 2 S, CS 2 , etc.) burn in air. It is 

 generated also by the action of strong sulphuric acid on many metals 

 (Cu, Hg, Ag, etc.), or on charcoal : 



2H 2 SO 4 + Cu = CuSO 4 +-2H 2 O + SO 2 . 

 2H 2 SO 4 + C = CO 2 + 2H 2 O + 2SO 2 . 



Sulphur dioxide is a colorless gas, having a suffocating, disagreeable 

 odor; it liquefies at a temperature of 10 C. (14 F.), and solidifies 

 at 75 C. ( 103 F.) ; it is very soluble in water, forming sulphur- 

 ous acid ; it is a strong, deoxidizing, bleaching, and disinfecting 

 agent ; when inhaled in a pure state it is poisonous ; when diluted 

 with air it produces coughing and irritation of the air-passages. 



Sulphurous acid, Acidum sulphurosum, H 2 SO 3 , SO 2 .H.OH. This 

 acid, similar to carbonic acid, is not known in a pure state, but is 

 believed to exist in aqueous solution, which decomposes into water 

 and sulphur dioxide when attempts are made to concentrate it. One 

 volume of cold water absorbs about 40 volumes of sulphur dioxide, 

 equal to about 11 per cent, by weight. The official acid must contain 

 not less than 6 per cent, by weight, equal to about 2000 volumes of 

 gas dissolved in 100 of water. According to the U. S. P. the acid 

 is made by generating sulphur dioxide from charcoal and sulphuric 

 acid in a flask, and passing the gas through a wash-bottle containing 

 water, inta distilled water for absorption. 



Experiment 14. Use an apparatus as shown in Fig. 42. Place in the flask 

 about 20 grammes of charcoal in small pieces, cover it with sulphuric acid, 

 apply heat, and pass the generated gas first through a small quantity of water 

 contained in the wash-bottle, and then into pure water, contained in the 

 cylinder. 



The solution, sulphurous acid, may be used for the tests mentioned below ; 

 when the neutral solution of a sulphite is required, make this by adding solu- 

 tion of sodium carbonate to a portion of the sulphurous acid until litmus-paper 

 shows neutral reaction. Examine also the contents of the wash-bottle by 

 means of the tests given below for sulphuric acid ; most likely some of the 

 latter will be found. How much carbon and how much H 2 S0 4 are required to 

 make 100 grammes of a 6.4 per cent, sulphurous acid? 



