32 Our Surroundings 



the gas from a bottle on to the flame of a burning splinter and 

 observe what happens. 



That carbon dioxide is formed in the process of breathing 

 may be shown by forcing the gases from the lungs through the 

 mouth into a glass of lime water, by means of a tube. The lime 

 water will assume a milky appearance showing the presence of 

 carbon dioxide. 



SUMMARY 



The molecular theory of matter assumes that every substance 

 is composed of minute particles called molecules. A molecule 

 is the smallest particle of matter that can exist alone and still 

 keep the same properties as the substance from which it came. 



The atomic theory assumes that molecules of substances are 

 composed of particles, much smaller than molecules, called atoms. 



An element is a substance which cannot be broken up into 

 anything simpler. 



The four most important elements are oxygen, carbon, hy- 

 drogen and nitrogen. 



The most important characteristic of oxygen is its power to 

 support combustion. 



Carbon is an essential part of all living things. At a high 

 temperature, it unites readily with oxygen, releasing energy in the 

 form of heat. It is tasteless and odorless. 







The most important characteristic of hydrogen is that it will 

 unite with oxygen to form water. It is a colorless, odorless and 

 tasteless gas. 



The striking characteristic of nitrogen is its inactivity. It 

 is a colorless, odorless and tasteless gas. It forms a part of the 

 air we breathe, of most organic matter, and of many other sub- 

 stances. 



A compound is a chemical union of two or more elements. 

 Water and carbon dioxide are important compounds. 



A mixture is a physical union of two or more substances and 

 does not change the character of any of them. 



Water is a solvent. It is not combustible. It is tasteless, 

 odorless ar. \ in thin layers, is colorless. It is widely distributed. 

 It forms a large part of all living things. 



