78 NON-METALS AND THEIR COMBINATIONS. 



11. HYDKOGEN. 

 H = l. 



History. Hydrogen was obtained by Paracelsus in the 16th cen- 

 tury; its elementary nature was recognized by Cavendish, in 1766. 

 The name is derived from Mup (hudor), water, and yew&u (gennao), to 

 generate, in allusion to the formation of water by the combustion of 

 hydrogen. 



Occurrence in nature. Hydrogen is found chiefly as a component 

 element of water ; it enters into the composition of most animal and 

 vegetable substances, and is a constituent of all acids. Small quanti- 

 ties of free hydrogen are found in the gases produced by the decom- 

 position of organic matters (as, for instance, in the intestinal gases), 

 and also in the natural gas escaping from the interior of the earth. 



Preparation. Hydrogen may be obtained by passing an electric 

 current through water previously acidified with sulphuric acid, by 

 which it is decomposed into its elements^"" 



H 2 O = 2H + O. 



A second process is the decomposition of water by metals. Some 

 metals, such as potassium and sodium, decompose water at the ordi- 

 nary temperature ; whilst others, iron, for instance, decompose it at a 



red heat : 



K + H 2 O = KOH + H ; 

 Fe + H 2 O = FeO + 2H. 



A very convenient way of liberating hydrogen is the decomposition 

 of dilute hydrochloric or sulphuric acid by zinc or iron : 



Zn + 2HC1 = ZnCl 2 + 2H ; 



Zinc 

 chloride. 



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

 Ferrous 

 sulphate. 



Hydrogen may also be obtained by heating granulated zinc or 



oxygen. 96. Explain the terms combustion, slow combustion, combustible 

 substance, and supporter of combustion. 97. Mention some oxidizing agents. 

 98. What is ozone, and how does it differ from common oxygen? 99. Under 

 what circumstances is ozone formed? 100. State the molecular weight of 

 oxygen and ozone. 



