HYDROGEN. 145 



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 ; while others, iron, for instance, decompose it at a 



red heat : 



K + H,0 == KOH + H ; 

 3Fe + 4H 2 = Fe s O 4 + 8H. 



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 S0 4 = FeSO, + 2H. 

 Ferrous 

 sulphate. 



Hydrogen may also be obtained by heating granulated zinc or 

 aluminum with strong solutions of potassium or sodium hydroxide, 

 in which case the decomposition is explained thus : 



Zn -f 2KOH == K 2 ZnO 2 -f 2H; 



Potassium 

 zincate. 



Al + 3NaOH = Na 3 AlO 3 + 3H. 



Sodium 

 aluminate. 



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. 



Experiment 3. Place a few pieces of granulated zinc (about 10 grammes) in 

 a flask of about 200 c.c. capacity, which is arranged as shown in Fig. 38. Cover 

 the zinc with water, and pour upon it through the funnel tube a little sulphuric 

 acid, adding more when gas ceases to be evolved. Notice the effervescence 

 around the zinc. Collect the gas in test-tubes over water and ignite it by taking 

 the test-tube (with mouth downward) to a flame near by. Notice that the first 

 portions of gas collected, which are a mixture of hydrogen and atmospheric 

 air, explode when ignited in the test-tube, while the subsequent portions burn 

 quietly. Pour the contents of one test-tube into another one by allowing the 

 light hydrogen gas to rise into and replace the air in a test-tube held over the 

 one filled with hydrogen. Take two test-tubes completely filled with the gas; 

 hold one mouth upward, the other one mouth downward : notice that from the 

 first one the gas escapes after a few seconds, while it remains in the second 

 tube a few minutes, as may be shown by holding the tubes near a flame to 

 cause ignition. 

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