538 SMITH'S INTERMEDIATE CHEMISTRY 



Group H. If no precipitate appears on addition of hydro- 

 chloric acid, hydrogen sulphide is led into the solution. The sul- 

 phides insoluble in active acids, namely, HgS, CuS, PbS, Bi 2 S 3 , 

 CdS, As2S 3 , Sb 2 S 3 , SnS, are therefore thrown down. The first 

 four are black or dark brown, the next two are yellow, and the 

 last two are orange and brown respectively. If too much HC1 

 is present, the precipitation of several of these sulphides will be 

 incomplete. On the other hand, if too little HC1 is used, zinc 

 sulphide may be partially precipitated (see pp. 461-3). 



This group is readily sub-divided. The last three sulphides 

 pass into solution when warmed with yellow ammonium sul- 

 phide, for they give soluble complex sulphides. The first five 

 sulphides will be unaffected. 



Group Ha. HgS, CuS, PbS, Bi 2 S 3 , CdS. A yellow precipitate 

 insoluble in ammonium sulphide indicates cadmium. To dis- 

 tinguish between the remaining four sulphides, boil with HN0 3 ; 

 HgS alone does not go into solution. Dilute the solution, and 

 add H 2 S04. Lead gives a white precipitate of PbS0 4 . If no 

 precipitate is obtained, add NH 4 OH till alkaline. Bismuth gives 

 a white precipitate of Bi(OH) 3 , copper a blue solution. 



Group lib. As 2 S 3 , Sb 2 S 3 , SnS. The color distinction is not 

 always certain. Reprecipitate the sulphides by adding HC1, 

 and boil the precipitate with concentrated HC1. As 2 S 3 does not 

 dissolve. If the precipitate does dissolve, cool the solution and 

 place in it a piece of bright tin. A black deposit forming on the 

 surface (by displacement, see p. 54) indicates Sb. 



Group Ilia. If no precipitate is obtained with H 2 S, the solu- 

 tion is boiled, and a few drops of concentrated HNO 3 added to 

 oxidize any ferrous salt to the ferric state. Ammonium chloride 

 is added, and then ammonium hydroxide in excess. A white, 

 gelatinous precipitate of A1(OH) 3 indicates aluminium, a bluish- 



