HYDROGEN 55 



Preparation by Electrolysis of a Dilute Acid. A 



convenient way of obtaining pure hydrogen is by passing a 

 current of electricity through a dilute acid (Fig. 27). The gas 

 is liberated at the negative wire (cathode) and collects in 

 the tube (also filled with the dilute acid). The direct, 110-volt 

 current, passing through a 16-c.-p. lamp placed in series with 

 the electrolytic apparatus, may liberate 50 c.c. of hydrogen in 

 7-8 minutes. Every acid contains hydrogen, combined with 

 other elements. The other elements are carried to the positive 

 plate (anode) and therefore do not interfere with the collection 

 of pure hydrogen. What may be liberated at the positive plate 

 depends upon the acid used. With hydrochloric acid, it is chlor- 

 ine; with sulphuric acid, oxygen comes off and sulphuric acid is 

 regenerated. 



Hydrochloric acid (in Aq. Soln.) > Hydrogen (neg. plate) -f- 

 Chlorine (pos. plate). 



The process is called electrolysis, from the Greek, meaning 

 decomposed by electricity. 



Physical Properties of Hydrogen. The gas is colorless, 

 odoilesSy and tasteless. It is, bulk for bulk, the lightest known 

 gas, the density of air being about 14.5 times as great. It can 

 be liquefied by compression below 234 (its critical temperature). 

 It dissolves in water to the extent of 1.8 volumes in 100 volumes of 

 water at 15. 



The lightness of the gas may be shown by pouring it upwards 

 from one jar to another, or by balancing an inverted beaker with 

 shot, and allowing hydrogen to flow in and displace the air. 



Several metals can adsorb (" occlude ") hydrogen gas. Iron 

 takes up about 19 times its own volume, platinum 50 volumes, 

 and palladium from 500 to 800 volumes. 



Liquid hydrogen, when allowed to evaporate rapidly, freezes to 

 a colorless solid, which melts again at 260. 



