PHYSICAL AND CHEMICAL CHANGES 53 



Break the test tube and examine the solid with a magnifying glass. 

 Can you now distinguish the iron from the sulphur? The solid is a 

 chemical compound called iron sulphide. , 



When water freezes it does not become a different sub- 

 stance ; it is still water, but water in a solid state. When 

 water is " boiled away " or evaporated by the heat of the 

 sun, it is still water, but water in a gaseous state. When 

 the iron used in Experiment 16 was pulverized it still re- 

 mained iron. Such changes as these, which do not affect the 

 nature of a substance, are called physical 

 changes. . ' , . 



But when molecules break up into 

 their atoms, or atoms unite to form 

 molecules, a chemical change is said to 

 occur. Such is the change that occurs 

 when hydrogen and oxygen unite to 

 form water ; or when the electrical cur- 



FIGURE 22 



rent breaks up the molecules of water 



into the two kinds of atoms composing them; or when 



sufficient heat is applied to an iron and sulphur mixture. 



One of the most common examples of chemical change 

 is the rusting of iron exposed to air. The atoms of oxygen 

 in the air and in the water of the air combine with the iron 

 to produce rust. A chemical change takes place and a 

 compound of the two elements is formed which is entirely 

 different in its nature from either. 



A chemical compound such as iron rust, made up of oxygen 

 and some other element, is called an oxide. 



Mixtures must be carefully distinguished from chemical 

 compounds. If we mix milk and water, neither the water 

 nor the milk is really changed in nature as the result of put- 

 ting them together in the same vessel. If we try to mix 



