COMPOUNDS AND MIXTURES 97 



symbol or formula for carbon dioxide is CO 2 , which means 

 that its molecule is composed of one atom of carbon and 

 two of oxygen. 



34. Compounds and Mixtures. Water is a compound, 

 but air is not. Air is a mixture. It is composed of four 

 or more kinds of gases. Nitrogen, oxygen, carbon dioxide, 

 and water all exist in air as separate gases. There is no 

 such thing as a molecule of air. Air is made up of entirely 

 different kinds of molecules. It can be separated into the 

 different kinds of gases which compose it. The molecules 

 of these various gases tend to become equally spread 

 through the air, somewhat as the molecules of a solute 

 tend to become equally spread or diffused through a sol- 

 vent. There is a law known as the law of diffusion of 

 gases which is much like the law of solution. 



A solution is also a mixture. It is not a compound. 

 Thus in a cup of sweetened tea there are various kinds of 

 distinct molecules. There are molecules of sugar, of 

 water, and of the substance from the tea leaves which 

 gives the flavor. These molecules are evenly distributed in 

 this mixture in fulfillment of the law of solution, but they 

 do not unite. 



Another kind of mixture is called a mechanical mixture. 

 In it no law of solution or diffusion makes the molecules 

 tend to become equally distributed. You may have made 

 such a mixture. Did you ever make a salad dressing? 

 That is an excellent example of a liquid mechanical mixture. 

 In making it oil and vinegar must be vigorously stirred 

 together, for there is no tendency of their molecules to 

 become equally distributed, as is the case with sugar and 

 water. Oil is not soluble in vinegar, or even in water. 



