PRINCIPLES OF CHEMISTRY 133 



141. Oxygen. Oxygen exists in a free state in the 

 air, which is a mixture composed of 20% oxygen and 80% 

 nitrogen. Oxygen is a colorless, odorless gas, and may 

 be prepared by decomposing a compound rich in oxygen, 

 like KC1O 3 (potassium chlorate), with heat, or it can be ex- 

 tracted from the air. In this latter case the gas is collected 

 over water. 



142. Oxidation. Oxygen unites readily with other ele- 

 ments, particularly metals, and forms compounds called 

 oxides. For example: When iron is exposed to moisture 

 and to the air which contains oxygen, it oxidizes and rusts. 

 When the oxygen combines with a carbon, as coal, it gives 

 off considerable heat and light. This process is called com- 

 bustion. A substance or element which burns is called a 

 combustible, and a substance or element that does not burn 

 is called a non-combustible. Oxygen is called a supporter 

 of combustion. Elements will not combine until a definite 

 temperature, called a kindling point, is reached. 



Ozone is a form of oxygen, and is a powerful oxidizing 

 agent. When an electric spark passes through the air, it 

 changes oxygen into ozone. Three atoms of oxygen form 

 a molecule of ozone. 



Questions 



1. Why is a knowledge of the chemical "make up" of certain 

 substances, such as iron, important? 



2. What is the difference between a mixture and a compound? 



3. Name a number of mixtures; compounds. 



4. Give the names of five common elements with their chemical 

 symbols. 



5. Why is it desirable to abbreviate chemical names into 

 svmbols? 



