CHAPTER XII 

 COMBUSTION AND OXIDATION 



67. Activity of Oxygen. Oxygen is the most active of the 

 chemical elements, and forms compounds with nearly all of 

 them. So great is its affinity for some, that it tears them from 

 other compounds in which they happen to be, and thus causes 

 many substances to break down or disintegrate. Most metals, 

 if left exposed to the air especially in damp surroundings, soon 

 begin to unite with oxygen. Thus iron rusts, and lead and 

 various other metals tarnish. Gold, silver, and platinum do 

 not readily unite with oxygen and thus are in great demand for 

 ornaments and the like. Other metals are often covered with 

 gold or silver to prevent tarnishing (173). A freshly quarried 

 stone or a newly sawed board soon loses its fresh look and be- 

 comes dull by the action of oxygen upon it. Our bodies are 

 kept warm by the slow union of oxygen with the carbon in 

 our tissues, and all ordinary burning is simply a more rapid 

 union of these two elements. It must not be assumed, how- 

 ever, that the union of oxygen and carbon is the only union 

 of this kind that produces great heat. One of the hottest 

 flames known is produced by the union of oxygen and hydrogen 

 and, strangely enough, the result of this union is water. 



68. Heat and Light from Oxidation. Whenever oxygen 

 unites with another substance, the process may be called oxi- 

 dation, though this term is commonly reserved for cases in 

 which the union is not accompanied by the emission of per- 

 ceptible light or heat, as when iron rusts. When sensible heat 

 and light appear, as in the burning of wood, coal, gas, and oil, 

 we usually call it combustion. The only difference, however, 



72 



