ELEMENTS, COMPOUNDS, AND MIXTURES 247 



A piece of ice when heated changes to water. If the 

 heating is continued, the water is changed to an invisible 

 gas and passes into the air. By cooling the air the water 

 may be recovered. It has never ceased to be water in 

 some form. Are these changes physical or chemical 

 changes ? 



When a piece of iron wire is burned in oxygen, a new 

 substance, iron oxide, is formed. The red rust that 

 forms on iron when it is left out of doors or in a damp 

 place is this same new substance. When iron and hydro- 

 chloric acid are put into a flask together, they react to 

 form a new substance, iron chloride, and hydrogen is 

 liberated. These new substances have properties which 

 are quite different from those of iron. 



What kind of changes are these ? 



Oxidation. - - The union of any other element with 

 oxygen is called oxidation, and the new substance is called 

 an oxide. 



Oxygen is the most abundant of the elements, and 

 oxidation is the most common chemical change. 



The rusting of iron and the rotting of wood are ex- 

 amples of slow oxidation. When we say that a substance 

 oxidizes, we mean that one or more of its elements com- 

 bines with the oxygen of the air. If the oxidation takes 

 place rapidly, it is called combustion or burning. In 

 such cases measurable quantities of light and heat are 

 given off. Heat is developed in the decay of wood, but 

 the process is so slow that the change in temperature is 

 too small to be noticeable. 



Elements, Compounds, and Mixtures. Many sub- 

 stances are the result of a combination of two or more 

 other substances. For example, hydrogen and oxygen 

 combine to form water; iron and oxygen combine to 



