AIR AND WATER COMPARED 161 



By means of it (as in burning) energy is released. This 

 energy may take the form of heat or of motion. It gives 

 us what we call the manifestations of life. The substances 

 of our bodies, that are thus reduced through oxidation and 

 their energy released, become of no further use to us; 

 they are eliminated as excretions. Thus we see how very 

 fundamentally chemical changes are a part of the processes 

 of life. 



QUESTIONS 



1. What is the great chemical difference between air and water? 



2. What is a molecule? 



3. What is meant by "analysis" and "synthesis"? 



4. What is an atom, and what relation does it hold to a molecule ? 



5. How is water decomposed? 



6. Explain the difference between a physical change and a chem- 



ical change, giving examples of each. 



7. Explain the chemical changes involved in the burning of wood. 



8. Explain the rusting of iron. 



