SULPHUR AND HYDROGEN SULPHIDE 



253 



Preparation. Hydrogen and sulphur combine so slowly that 

 at 310 the completion of the union requires seven days. A trace 

 may be obtained in a few minutes by leading hydrogen over 

 sulphur, melted in a bulb (Fig. 68). A strip of paper, dipped in 

 lead acetate solution and placed in the wide part of the tube, is 

 darkened by the formation of insoluble lead sulphide PbS (black), 

 while acetic acid is also formed: 



2HCO 2 CH 3 . (== C^-y \ 



Pb(C0 2 CH 3 ) 2 + H 2 S -> PbS 



Laboratory Method. The gas is commonly 

 made by double decomposition, using a sulphide to get the S 

 radical, and an acid for the H radical. ex- 

 gus_sulpliij[te r made by^Jigjting iron filings 

 and sulphur, is the cheapesTTsulphide, and it 

 interacts easily with hyb!rochlonc__acid-or sul- 

 phuric acid: 



FeS + 2HC1 <= FeCl 2 + H 2 S t 



The action, like all double decompositions, is 

 reversible. But use of an excess of hydro- 

 chloric acid forces it forward, and the escape 

 of the gaseous hydrogen sulphide reduces the 

 backward action almost to zero. The gas can 

 be made in a flask fitted like that in Fig. 25 

 (p. 52), or in a Kipp's automatic generator 

 (Fig. 69). Hcan be collected by upward dia- 

 j^lacement. 



FIG. 69 



Physical Properties. Hydrogen sulphide is a colorless gas 

 with an pd.nr recalling rotten eggs. It is rather easily liquefied^ 

 and the liquid boils at about 60 and freezes at 83. The 

 density, implied in the formula H 2 S, shows that 22.4 liters weigh 

 32+2 or 34 g. r so that the density is only one-sixth greater than 

 that of air (of which 22.41. weigh 28.95 g.). The gas is moder- 



