CHAPTER X 

 COMMON CHEMICAL PROCESSES 



260. Combustion. Combustion is a chemical union 

 which takes place rapidly, giving off light and heat. The 

 word fire is commonly used instead of combustion. Two 

 things are necessary in order that combustion may take 

 place a substance to burn (called a combustible) and 

 a substance with which it may unite. The latter sub- 

 stance is said to support the combustion. We have 

 learned that the things commonly burned as fuels con- 

 tain the elements C and H ( 259) ; also that the great 

 supporter of combustion in the air is O (222). With 

 these facts in mind, it will be seen that the most com- 

 mon fires are simply the rapid union of carbon and 

 hydrogen with oxygen. The compounds formed by this 

 union will be carbon dioxide (CO 2 ) and water vapor 

 (H,0). 



Now it is well known that in order to make any 

 substance burn, heat must be applied. In other words, 

 oxygen will not easily unite with other substances 

 unless their temperature be raised. The temperature 

 at which different substances will burn in air varies 

 greatly ; carbon, for example, needs a greater degree of 

 heat than hydrogen, while matches containing phos- 

 phorus may be sufficiently heated by simply rubbing 

 them. When we wish to start a fire, however, we do 

 not heat the whole mass that is to be burned, but only 



221 



