COMPOSITION OF MATTER 29 



ments do not unite in haphazard proportions, they may unite 

 in different, though definite, proportions to form different 

 substances. The behavior of nitrogen and oxygen are instruc- 

 tive in this connection. Various combinations of these two 

 elements form nitric oxide (NO), nitrous oxide (N 2 0), nitro- 

 gen peroxide (N0 2 ), nitrogen trioxide (N 2 3 ), and nitrogen 

 pentoxide (N 2 5 ). The first two are colorless gases the third 

 is a red-brown gas, the fourth is a bluish-green liquid, and the 

 last, a white solid, though all are made of different proportions 

 of the same two colorless gases. 



34. Chemical Affinity. The force which causes various ele- 

 ments to combine, and holds them in their compounds, is called 

 chemical affinity. This property is probably electrical in its 

 nature. Chemical affinity does not act with equal force in all 

 combinations, for in some the atoms are so weakly held to- 

 gether that they separate if the compound is left standing for 

 any length of time. In this way, hydrogen peroxide (H 2 2 ) 

 loses one of its atoms of oxygen and becomes merely water 

 (H 2 0). This is the reason that peroxide of hydrogen, as it is 

 called, spoils if left exposed to the air for any length of time. 

 In other gases, one chemical *may displace another in a com- 

 pound when associated with it. For instance, when hydro- 

 chloric acid (HC1) is poured on limestone (CaC0 3 ), carbon di- 

 oxide (C0 2 ) is liberated, and the other atoms form chloride of 

 lime and water, according to the equation 2HC1 + CaCO 3 = 

 C0 2 + CaCl 2 + H 2 0. When hydrochloric acid is poured 

 on zinc, the chlorine unites with the zinc, forming zinc chloride 

 (ZnCl 2 ) and the hydrogen is released as a gas. By a similar 

 process, iron is reduced from its ores. Ordinary iron ores are 

 oxides combined with iron. When such ores are mixed with 

 carbon and heated, the carbon unites with the oxygen leaving 

 the iron free. When the molecules of a substance are acted 

 upon in such a way as to make one or more new substances by a 

 rearrangement of the atoms, we speak of the change as a chem- 



