INTRODUCTORY EXERCISES 17 



Remove the tube from the flame. When the stick ceases 

 to glow remove it and substitute for it a drop of limewater 

 on the end of the glass tube. What happens? What does 

 this indicate? As the tube cools what do you see on the 

 sides of the tube? Do you know the name of this sub- 

 stance? 



EXPLANATION. Oxide of mercury is a compound of mer- 

 cury (quicksilver) and oxygen. Heat decomposes this into 

 oxygen and mercury. In what form were these two sub- 

 stances given off in the above experiment? We have al- 

 ready learned that when charcoal burns it forms a gas called 

 carbonic acid gas. How was this formed in the above 

 exercise? We can express the above actions in the form of 

 equations as follows: 



(1) Oxide of mercury + heat = oxygen and mercury. 



(2) Oxygen + carbon + heat = oxide of carbon + heat. 

 In chemical language the process illustrated in (1) is 



analysis, or the separation of a compound into its parts. 

 The process illustrated in (2) is synthesis, or the union of 

 parts to make a compound. All chemical actions may be 

 grouped under one or the other of these processes. 



The special kind of compound that results from the union 

 of oxygen with a substance is called a compound of oxida- 

 tion, and the actual formation is called oxidation. When 

 oxidation takes place rapidly, light and heat are produced 

 at the same time and the process is called rapid oxidation 

 or combustion. Give examples from your experience of both 

 kinds of oxidation the slow and the rapid. Why does the 

 exclusion of air from a fire cause the fire to go out? What 

 is the precise action of water or sand when thrown on a 

 flame, in the light of the above explanation? 



