MIXTURES 



97 



substance, or element. Hydrogen, oxygen, nitrogen, 

 carbon, iron, tin, mercury, etc., are elements (see Appen- 

 dix, Table IX). 



How complete the change is, when substances unite chemically, is 

 seen when we compare water, the compound, with the two elements 

 that make it up. The formation of carbon dioxide by the burning of 

 carbon is another common illustration of the same thing. No one 

 would suspect that this colorless gas (cf. 51), which puts out fire, 

 which is exhaled from our lungs, and is taken up by plants (cf. 58), 

 is really the black carbon of coal and charcoal, and of the "black lead" 

 of our pencils, combined with the active gas oxygen. Yet this is 

 the case. 



102. Mixtures. If, in the electrolysis of water, we 

 were to collect two test tubes of hydrogen and one of 

 oxygen, and were to mix them in a bottle over mercury 

 (Fig. 78), would they unite at once to produce water? 

 They would not. They would 

 remain in each other's presence 

 for a long time without any 

 noticeable change. It is neces- 

 sary for the temperature to be 

 raised to about 620 C. before 

 the two gases unite rapidly. 

 When they are mixed at the 

 ordinary temperature, they form 

 only a physical, or mechanical, 

 mixture. No heat is set free; 

 no new substance is formed. If we shake the mixture with 

 water, the hydrogen dissolves as if no oxygen were present, 

 and the oxygen dissolves as if no hydrogen were present. 



FIG. 78. 



A gas can be transferred from one 

 vessel to another under a liquid. 



