HYDROGEN 51 



The metals, of course, act upon a small part only of the whole 

 vesselful of water. In each case the metal displaces one-half of 

 the hydrogen from that part of the water upon which it acts: 

 Sodium (23) + Water (18) - Hydrogen (1) + Sodium hydroxide (40). 



Hydrogen Sodium 



Hydrogen Hydrogen 



Oxygen Oxygen 



The products are hydrogen, along with potassium hydroxide, 

 sodium hydroxide, or calcium hydroxide. The first two hydrox- 

 ides are very soluble in water, but most of the calcium hydroxide 

 is not dissolved, and may be seen suspended in the liquid. 



Magnesium will liberate hydrogen from water, provided the 

 latter is hot. If steam be passed through a heated tube con- 

 taining iron filings, a mixture of hydrogen, with unused steam, 

 issues at the other end. The magnetic oxide of iron, not hydrox- 

 ide, remains in the tube: 



Iron + Water > Hydrogen -f- Magnetic oxide of iron. 



Hydrogen Iron 



Oxygen Oxygen 



This is a method much used in making hydrogen for commercial 

 purposes. 



Silver, gold and platinum, which do not combine directly with 

 pure oxygen, and even copper and mercury, which do, are all 

 unable to form oxides and to liberate hydrogen when heated in 

 steam. 



Displacement. The foregoing actions, in which hydro- 

 gen is liberated, present us with a new the third variety of 

 chemical change. Here an element displaces one of the elements 

 from a compound, setting it free, and unites with the rest of the 

 constituents of the compound. Thus, calcium displaces part of the 

 hydrogen, and unites with the oxygen and the rest of the hydrogen. 



Preparation by the Action of Metals upon Dilute Acids. 



All the metals which displace hydrogen from water or steam will also 

 act upon cold dilute acids and displace the hydrogen they contain. 

 This action is the one most commonly employed in the laboratory 



