SULPHUR AND ITS COMPOUND 257 



if the flame is cooled, the hydrogen alone completely burns and 

 the sulphur only partially, so that a quantity of sulphur is 

 deposited uncombined. 



EXPT. 272. Pass the gas through solutions of (1) copper 

 sulphate, (2) lead acetate, (3) an acid solution of arsenious 

 oxide (N.B. This is a very strong poison), and observe that in 

 each case a thick precipitate is formed (1) black, (2) black, (3) 

 yellow. 



These precipitates consist of the sulphides of the metals, thus 

 (1) is copper sulphide, (2) is lead sulphide, (3) is a sulphide of 

 arsenic. 



EXPT. 273. Filter off the precipitate (1) and evaporate the 

 solution, and hence observe that it consists of sulphuric acid. 



Double Decomposition. We thus find that from copper 

 sulphate and sulphuretted hydrogen we obtain copper sulphide 

 and sulphuric acid. As the sulphuretted hydrogen is hydrogen 

 sulphide, it hence appears that the sulphuric acid is the hydrogen 

 sulphate, that is, the copper sulphate is sulphuric acid Avith the 

 hydrogen replaced by the copper. Such a change as this is 

 called a double decomposition, both reacting compounds being 

 converted into new products by the exchange of some con- 

 stituents. 



In a similar manner the lead acetate and hydrogen sulphide 

 yield lead sulphide and hydrogen acetate (acetic acid). 



EXPT. 274. Pass some sulphuretted hydrogen into water, 

 and observe that the water smells like the gas itself and 

 behaves in the same way towards copper sulphate, &c. 



The gas is therefore soluble in water, and its solution is 

 frequently used in the chemical laboratory instead of the gas 

 itself, being employed in qualitative analysis for the separation 

 (or detection) of metals which, like copper, are precipitated as 

 sulphide, from those which are not so precipitated. 



EXPT. 275. Allow this solution to stand a few days, and 

 observe that it has become tinted and milky, and does not 

 smell (or at any rate not so strongly) of the gas. 



This change is due to the fact that the oxygen dissolved in the 

 water slowly replaces the sulphur of the sulphuretted hydrogen 

 ,Mid so forms water, while the displaced sulphur is precipitated 

 in the form of a white powder. 



