CHAPTER V 

 Hydrogen 



39. Occurrence. Hydrogen is found in nature in com- 

 bination with other elements, entering into the composi- 

 tion of water, animal and plant tissues, and some min- 

 erals. It is never found in a free state, except as given 

 off in traces with volcanic gases. Hydrogen is also found 

 in all acids and many other compounds. 



40. Preparation. In the laboratory, hydrogen is usu- 

 ally prepared by treating a metal with an acid, which 

 contains hydrogen ; the metal replaces the hydrogen of 

 the acid and the hydrogen is then liberated as a free gas. 

 When zinc and hydrochloric acid are employed, the re- 

 action which takes place is as follows : Zn -f- 2HC1 = 

 ZnCl 2 -j- 2H. Two molecules of hydrochloric acid are 

 required in the reaction because zinc has a valence of 2, 

 and whenever zinc enters into chemical combination, it 

 must take the place of two monovalent atoms. The 

 compound, ZnCl 2 , zinc chlorid, contains one atom of zinc 

 and two atoms of chlorin. 



Experiment 2. Arrange the apparatus as shown in Fig. 

 17. Use the small two-necked Woulff bottle, and in one of the 

 necks insert a tight-fitting cork with a thistle tube. In the 

 other neck insert a cork carrying a delivery tube. Place about 

 20 grams of zinc, Zn, and 25 cc. of water in the Woulff bottle. 

 The thistle tube should pass below the surface of the water to 

 prevent the escape of gas. Fill two or three cylinders with 

 water for collecting the gas. The corks carrying the delivery 

 tube and the thistle tube should fit tightly, otherwise the H is 



