254 



SMITH'S INTERMEDIATE CHEMISTRY 



*aJ.iffcfa i water (290 vols. in 100 vols. water at 20), a prop- 

 erty which enables us to carry out many reactions of the gas upon 

 substances in solution. 



Physiological Properties. Care must be taken to allow as 

 little of the gas as possible to escape into the air, and all work with 

 it should be done in a well-ventilated hood. The proportion must 

 reach 1 part in 200 of air, however, before fatal results follow 

 breathing the mixture. The best antidote is very dilute chlorine. 



Chemical Properties. 1. The gas burns in the air, giving 

 water and sulphur dioxide: 



Skeleton: H 2 S + O 2 -> H 2 + S0 2 . 



Balanced: 2H 2 S + 30 2 - 2H 2 O + 2S0 2 . 



2. The compound is not very stable. When heated, for example, 

 in the interior of its own flame, it is partially decomposed into free 

 sulphur and hydrogen. A cold porcelain dish (Fig. 70) placed in 



the flame will condense some of the sulphur 



on its surface. 



3. On account of its instability, and the 

 ease with which it gives up hydrogen, the gas 

 is a. re.dum.na aaent. Thus, when jars of hydro- 

 gen sulphide and sulphur dioxide are placed 

 mouth to mouth, a deposit of sulphur gradu- 

 ally appears: 



SO 2 .+ 2H 2 S -> 2H 2 O + 3S. 



70 



Part of the free sulphur found in nature seems to be liberated by 

 the action of these gases, both of which are found in volcanic 

 regions. The gases must be moist, for, without water vapor as a 

 contact agent, no interaction occurs. 



In this action the sulphur dioxide loses its oxygen. We say that 

 the H 2 S was oxidized by the S0 2 , or that the SO 2 was reduced by 



