GENERAL INTRODUCTION 175 



Mixtures are very common ; many are made in nature 

 and many are made by the work of man. Wood and 

 coal are mixtures, also the air. Various sorts of vege- 

 table and animal products cloth, paper, leather, and 

 many other mixtures are common. 



The difference^ between chemical compounds and mix- 

 tures is important. In compounds the elements unite to 

 form a new substance, which has a molecule of its own ; 

 that is, all its molecules are like each other and like the 

 mass. In a mixture the molecules are those of the ele- 

 ments or compounds that compose it ; thus they are of 

 different kinds, and the mixture, as a substance, has no 

 molecule of its own. For further explanation see 207. 



Experiment 110. Grind small quantities of sulphur and iron 

 filings together in a mortar till well mixed. Draw a magnet 

 through the mass. Is it a compound or a mixture ? 



QUESTIONS 



1. Of what, in general, does the science of chemistry treat? 



2. Show the difference between physical changes and chemical 

 changes. Give examples of each. Does wood suffer a physical 

 or a chemical change when burned? 



3. What is meant by chemical composition? Define decom- 

 position. Try to think of examples of each process. 



4. What is meant by an element ? How are elements gener- 

 ally obtained ? How many have been discovered ? 



5. Are masses ever formed of one element alone ? Name some 

 common elements. With how many are you familiar? 



6. What is a chemical compound? How does a compound 

 mass differ from an elementary mass ? Is the molecule of a com- 

 pound the same in its nature as the mass ? 



7. What is a mixture ? How do compounds differ from mix- 

 tures ? Name some common substances that are mixtures. 



