OUR ENVIRONMENT AND HOW WE USE IT 15 



it is at rest. When wood burns it produces factors of our 

 environment that did not exist before. These new factors, 

 such as light, heat, chemical action, electricity, and 

 mechanical action, are forms of energy. Matter without 

 energy would make a very different world from ours. 

 Energy is just as useful as matter and the two always are 

 to be found together and they are present in everything 

 in the universe as far as we can tell. 



What Is a Chemical Change ? When we burn a strip 

 of magnesium metal, we get a bright flame, and there is 

 left a white brittle compound called magnesium oxide. 

 Here we have combined oxygen from the air with the 

 magnesium and have an example of a chemical change 

 called oxidation. 



When magnesium is burned, oxygen combines with it 

 and a single new product results. The change is chemical. 

 The chemist expresses this change as follows : 



2 atoms of 

 Magnesium 



2Mg. 



2 atoms of 

 Oxygen 



2 



2 molecules of 

 Magnesium oxide 



2MgO 



When iron rusts, we have a similar chemical change taking 

 place : oxygen of the air unites with the iron, forming iron 

 oxide. Such changes are continually taking place in 

 nature. We shall see later that life itself depends upon 

 this process of oxidation. 



Physical Changes. When your knife gets dull and you 

 have to sharpen it, you do not change the composition 



Which of these changes are chemical and which physical? How many similar 

 changes can you list for your workbook ? 



