AIR AND WATER COMPARED 159 



Now it is evident that, by chemical change, complex 

 molecules may be reduced to simpler ones, or simple mole- 

 cules may be built up into more complex ones. Already 

 you have considered a number of cases of chemical changes. 

 It will be well to reconsider these, and to determine in 

 each case whether reduction or the building up of mole- 

 cules occurs. 



Perhaps the most familiar of chemical changes is the 

 process of burning. You have learned that, in the burn- 

 ing of wood or coal, gases are produced. Two of these 

 gases are carbon dioxide and water vapor. They result 

 from the union of oxygen of the air with carbon and hydro- 

 gen, which, before the burning occurred, were in the 

 molecules of the fuel. These fuel molecules are much 

 more complex than the simple ones of water and carbon 

 dioxide. Burning, therefore, is a case of chemical re- 

 duction. We shall consider it further in the chapter on 

 combustion. 



In Chapter V you learned that in washing your hands 

 with soap in hard water, the soap " unites" with the 

 mineral matter in the water, and that thereby a dirty 

 sediment is produced. The word unites, used in this way, 

 indicates that a chemical change occurs; that molecules, 

 or parts of molecules, of different substances have united 

 in the form of another kind of molecule. In this case the 

 molecule of the sediment is more complex than the mole- 

 cules from which it was derived, so this change is not a 

 reduction but a building up. When a chemical change (or 

 reaction) occurs which causes the production of an insolu- 

 ble substance in a liquid (as in this case of the reaction 

 between hard water and the dissolved soap), the insoluble 

 substance is called a precipitate. So we may say that the 



