16 GETTING ACQUAINTED WITH THINGS 



of the blade. Some of the molecules are scraped off by 

 mechanical means, but they are still iron molecules. 

 Such a change is physical. Physical changes are illus- 

 trated by writing on paper, boiling water, bending a wire, 

 grinding corn, and plowing soil. The composition of the 

 molecules is not changed by a physical change, but is 

 changed by a chemical change. 



We Use the Factors of the Environment. What do 

 these facts about chemistry and physics have to do with 

 us? What is the meaning of chemical and physical 

 changes in the world about us? All we need to know 

 now is that such changes are continually going on and 

 as a result of such chemical and physical changes we are 

 able to use our environment. Take, for example, the 

 burning of coal. Energy or power to do work is locked 

 up in the coal. It is unused until the coal is burned, then 

 heat is released and this heat may make water boil, turn 

 the wheels of a locomotive, draw cars and passengers, 

 and cook our food. How does this energy get out? It 

 gets out simply because the elements, hydrogen and 

 carbon, in the coal unite chemically with the oxygen of 

 the air, forming new substances and releasing the heat 

 which may be transformed by machines into work. 

 Chemical changes of this sort are constantly going on in 

 nature ; rocks are crumbling and breaking down into 

 soil ; the soil itself is uniting with oxygen and breaking 

 into still finer pieces ; foods in the bodies of plants and 

 animals are being combined with oxygen or oxidized to 

 release energy. 



Life Is a Series of Physical and Chemical Changes. 

 And physical changes are going on as well. Wind and 

 water break down and wear away solid rocks, and water 

 turns wheels to transform their energy into power, perhaps 

 in the form of electricity. Ice is formed from water. We 

 may see it in the form of a glacier moving slowly down a 



