\ M i SULPHUR AND ITS COMPOUND 255 



The hydrogen that we should have expected to be liberated has 

 then abstracted some of the oxygen from this trioxide and so 

 liberated the dioxide. 



Sulphates. With alkalis sulphuric acid also forms salts, just 

 as was found in the case of sulphurous acid. 



EXPT. 267. To a little sulphuric acid, in an evaporating 

 dish, add drop by drop a solution of caustic soda, until the 

 liquid has only a faint acid action. Evaporate partially and 

 allow to .crystallise and observe that colourless crystals are 

 obtained. 



These crystals are known as sodium sulphate, the salts obtained 

 from sulphuric acid being called sulphates just as those from 

 sulphurous acid were called sulphites. We have already pre- 

 pared three sulphates, viz., sodium sulphate, zinc sulphate, and 

 copper sulphate, the last of these being distinguishable by its 

 fine blue colour. 



Manufacture of Sulphuric Acid. Tn the manufacture of 

 sulphuric acid on the large scale, sulphur dioxide is first obtained, 

 usually from iron pyrites (Expt. 255). The sulphur dioxide is 

 then oxidised, but for this purpose a compound of nitrogen and 

 oxygen is used known as nitrogen peroxide which readily 

 gives up its oxygen to the sulphur dioxide, being itself converted 

 to a compound of nitrogen and oxygen which contains less 

 oxygen than the peroxide, and is known as nitric oxide. This 

 nitric oxide, however, has the property of taking up oxygen 

 from the air and becoming reconverted to the peroxide, which can 

 again give up the oxygen to more sulphur dioxide, and thus acts 

 as a carrier of oxygen from the air to the sulphur dioxide. 

 Hence, in the presence of steam, sulphuric acid is formed. The 

 sulphur dioxide, oxides of nitrogen (obtained from nitric acid), 

 steam and air are passed into large leaden chambers where the 

 reactions indicated above take place, with the production of 

 sulphuric acid, which collects on the floor of the chambers and is 

 afterwards concentrated by distilling off the water from glass or 

 platinum retorts. 1 



Sulphides. We have already found that copper and sulphur 

 on being heated together unite to form a compound known as 

 copper sulphide. This experiment should be again performed, 



1 It may be here stated that in reality the reaction is more complex than that 

 idicated above, the formation of intermediate products having been neglected and 

 ly the final products considered. 



