HOW MATTER CHANGES 139 



If mercuric oxide, a red powder, is heated slowly in a tube 

 to a high temperature, it does not behave like the red powder 

 described in 147. If a stick with a spark on the end is held 

 in the tube while the mercuric oxide is being heated, the 

 spark bursts suddenly into flame. This shows that there is 

 now something besides air in the tube. When the tube is 

 allowed to cool, a gray shining substance appears on the in- 

 side. This does not return to the form of a red powder, no 

 matter how long it stands. The mercuric oxide was a com- 

 pound that, on being heated, gave off a gas and mercury. 

 The gas made the wood blaze, and the mercury remained 

 in tiny globules on the inside of the tube. This change 

 was a chemical change called decomposition. In decom- 

 position, a compound separates into two or more different 

 substances. 



If the end of a thin piece of magnesium is heated in a 

 flame, it burns. When it is cooled, the substance remaining 

 is quite different from the original magnesium in color, tex- 

 ture, and composition. There has been a chemical change. 

 Oxygen from the air has united with the magnesium, making 

 a new substance, magnesium oxide. When two or more 

 substances unite to form new substances, there is a chemical 

 change called combination. 



Decomposition and combination include all kinds of chemi- 

 cal changes. Burning, decay, separation of metals from ores, 

 and digestion of food are chemical changes, and all involve 

 either decomposition or combination, or both. 



149. Oxidation. The most abundant element in the 

 earth is oxygen. It is a gas which exists as an element mixed 

 with other gases in air. The union of any other element 

 with oxygen is called oxidation, and the new substance or 

 product is always an oxide. Oxidation is the most common 

 chemical change. 



When a substance burns, it is because one or more of its 

 elements combines with the oxygen of the air. If the combi- 



