SULPHUR AND HYDROGEN SULPHIDE t 255 



the H 2 S. As we have already noted, every reduction involves 

 also an oxidation. 



4. The metals, down to and including silver in the activity 

 series, quickly receive a coating of sulphide when exposed to the 

 gas: 



2Ag + H 2 S->Ag 2 S + H 2 t . 



The tarnishing of silver in the household is due to the presence of 

 a trace of hydrogen sulphide in the illuminating gas which escapes 

 from slight leaks in the pipes. 



Chemical Properties An Acid. The aqueous solution 

 is an acid, and hence the compound is frequently called hydro- 

 sulphuric acid. It turns faintly tinted litmus paper distinctly 

 pink. The poor conductivity of the solution shows the substance 

 to be little ionized, and therefore a weak acid. 



Like all acids, it enters into double decomposition with bases 

 and salts. A number of these actions are used in analytical 

 chemistry. Thus, with cupric sulphate solution, we get cupric 

 sulphide (black), and with antimony trichloride antimony tri- 

 sulphide (orange), both as precipitates: 



CuSO 4 + H 2 S * CuS 1+ H 2 S0 4 . 

 2SbCl 3 + 3H 2 S <=> Sb 2 S 3 j + 6HC1. 



Sulphides. Many sulphides of metals are found as minerals. 

 Most sulphides are insoluble, and can therefore be made by double 

 decomposition. They may also be prepared by reduction of 

 sulphates. Thus, when sodium sulphate is heated on a piece of 

 charcoal (such as a half -burnt match) the sulphide is formed: 



4C -> Na 2 S + 4CO. 



j^will be^observed that in sulphides. H 2 S, Na 2 S, ZnS, CuS, and 

 so forth, jsulyhur is invariably 



Carbon Bisulphide CS 2 . This compound is an important 

 solvent for sulphur, caoutchouc (rubber), and other substances 



