216 NON-METALS AND THEIR COMBINATIONS. 



combine with many metals to form insoluble compounds, the color 

 of which frequently is very characteristic : 



CuS0 4 + H 2 S = CuS + H 2 S0 4 . 



The sulphides that are insoluble in water fall approximately into three 

 groups: 



1. Those that are almost completely insoluble in water, such as the sul- 

 phides of lead, copper, bismuth, silver, mercury, arsenic, antimony, tin, and a 

 few others. These, moreover, are not dissolved by dilute acids, and hence are 

 precipitated from solutions of salts of the metals bypassing hydrogen sulphide 

 into them even when a little free acid is present : 



Pb(N0 3 ) 2 + H 2 S = PbS + 2HN0 3 , 

 Lead sulphide. 



or Pb" + 2(N0 3 )' + 2H- + S" = PbS + 2H- + 2(NO 3 )'. 



2. Those that are practically insoluble in water, yet more soluble than the 

 sulphides of group 1, and are dissolved by even very dilute active acids. The 

 metals iron, cobalt, nickel, manganese, zinc, and a few others form such sul- 

 phides. Some of these sulphides are so readily soluble in dilute acids that they 

 are prevented from being precipitated by hydrogen sulphide by the acid that 

 would be liberated in the reaction (see equation above). In the case of zinc 

 salts partial precipitation takes place until equilibrium is reached, but if some 

 free acid is present, no precipitation takes place. To form the sulphides of 

 this group, a salt of hydrogen sulphide, such as ammonium or sodium sul- 

 phide, is applied to neutral solutions of the salts of the metals, thus : 



FeS0 4 + (NH 4 ) 2 S = FeS + (NH 4 ) 2 SO 4 , 

 Iron sulphide. 



or Fe" + S0 4 " + 2(NH 4 ) + S" = FeS + 2(NH 4 )- -f SO 4 ". 



3. Those that are known only in the dry state, and although they are insol- 

 uble as such in water, yet they dissolve because they are hydrolyzed into solu- 

 ble products, thus : 



2CaS -f 2H 2 = Ca(OH) 2 + Ca(SH) 2 . 



The sulphides of barium, strontium, and calcium belong to this class. They 

 cannot be precipitated, either by hydrogen sulphide or ammonium sulphide. 

 They are generally made from the sulphate by heating with carbon (reduction 

 to sulphide). 



Solutions of normal sulphides, as Na 2 S, and acid sulphides, as NaHS, or 

 NH 4 HS, when used as reagents in precipitation of insoluble sulphides, give 

 the same results because of the presence of S /x ions, which unite with the ions 

 of the other metals. 



Tests for hydrogen sulphide or sulphides. 

 1. Hydrogen sulphide or soluble sulphides (ammonium sulphide 

 may be used) when added to soluble salts of lead, copper, mercury, 

 etc., give black precipitates of the sulphides of those metals. 

 ^ 2. From insoluble sulphides (ferrous sulphide, FeS, may be used) 

 liberate the gas by dilute sulphuric or hydrochloric acid, and test as 



