THE CHEMISTRY OF COMBUSTION 79 



pounds are known which, like calcium carbide, take in energy 

 during the process of their formation. Every chemical change 

 is accompanied by either the liberation or the absorption of 

 energy as well as the formation of one or more new sub- 

 stances. 



DEFINITION. Chemical changes are those changes in matter 

 in which one or more new substances are formed and in which 

 energy is produced or absorbed. 



88. Chemical Compounds. Two or more chemical elements 

 may unite to form a new substance with the accompanying 

 chemical change. Thus the elements, copper and sulphur, will 

 unite to form a new substance, a COMPOUND, COPPER SULPHIDE, 

 which is black in color. Heat energy is liberated at the same 

 time. 



Exercise 24. Union of Elements to Form Compounds 



Clean a piece of copper foil with emery or sand paper until the 

 surface of the metal is bright. What is the color of the copper? 

 Now hold the cleaned foil with a pair of tongs and sprinkle a thin 

 layer of powdered sulphur on the surface of the copper. What is the 

 color of the sulphur? Are the substances copper and sulphur elements 

 or compounds? (See Art. 84, list of chemical elements.) The copper 

 and sulphur are undergoing chemical change at ordinary temperatures, 

 but very slowly. The rate of union may be increased by heating 

 them. By means of the tongs hold the copper and sulphur in the flame 

 of the burner and watch changes. If too much sulphur has been used, 

 it may be burned off from the surface of the metal. Remove the foil 

 from the flame and examine the surface of the copper. What is its 

 color now? This is a new substance produced by the union of the 

 copper and the sulphur. It is known as COPPER SULPHIDE. Not all of 

 the copper was used in the change. Only that portion on the outside, 

 and next to the sulphur really underwent chemical change. Heat 

 was liberated as the elements united but it was not noticeable in the 

 flame. 



DISCUSSION OF THE EXERCISE. This experiment illus- 

 trates a number of chemical combinations that may be made 

 by the union of different chemical elements. Copper and oxy- 

 gen, a gas, may be united to form COPPER OXIDE. Sodium and 

 chlorine may be united to form SODIUM CHLORIDE, common table 



