70 NON-METALS AND THEIR COMBINATIONS. 



A second process is the decomposition of water by metals. 

 Some metals, such as potassium and sodium, decompose water 

 at the ordinary temperature, whilst others, iron for instance, 

 decompose it at a red heat : 



K + H 2 = KHO + H ; 



Fe + H 2 O = FeO + 2H. 



A very convenient way of liberating hydrogen is the decom- 

 position of dilute hydrochloric or sulphuric acid by zinc or 

 iron : 



Zn + 2HC1 ZnCl 2 -f 2H ; 



Zinc 

 chloride. 



Fe + H 2 SO 4 = FeSO 4 + 2H. 



Ferrous 

 sulphate. 



Whenever hydrogen is generated, care should be taken to 

 expel all atmospheric air from the vessel in which the generation 

 takes place, before the hydrogen is ignited, as otherwise an 

 explosion may result. 



Properties. Hydrogen is a colorless, inodorous, tasteless gas ; 

 it is the lightest of all known substances, having a specific 

 gravity of 0.0692 as compared with atmospheric air = 1. 



In its chemical properties, hydrogen resembles the metals 

 more than the non-metals ; it easily burns in atmospheric air, or 

 in pure oxygen, with a non-luminous, colorless flame, producing 

 during this process of combustion a higher temperature than 

 can be obtained by the combustion of an equal weight of any 

 other substance : 



H 2 + O = H 2 O. 



The formation of water by the combustion of hydrogen dis- 

 tinguishes it from other gases. 



Two volumes of hydrogen combine with one volume of 

 oxygen, forming two volumes of gaseous water. 



Water, H 2 = 18. Water is not found in nature in an abso- 

 lutely pure state. The purest natural water is rain-water, col- 

 lected after the air has been purified from dust, etc., by previous 

 rain. Comparatively pure water may be obtained by melting 

 ice, since, when water containing impurities is partially frozen, 

 these are mostly left in the uncongealed water. 



