CHEMICAL EQUATIONS, TYPES OF CHEMICAL CHANGE, ETC. 113 



following equations, in which the letters stand for elements or groups 

 of elements : 



1. A -|- B = AB direct combination or addition. 



2. (a) AB = A + B i 



(6) ABC = AB -|- C > simple decomposition. 

 (c) ABC = AC f BCJ 



3. AB + C =-- CB + A displacement, 



4. AB -f- CD AD + CB double decomposition or metathesis. 



The following concrete examples will serve to illustrate the above 

 types of change : 



1. Mg -f O = MgO. 



When magnesium metal is heated to the ignition point it unites 

 with oxygen of the air, and gives a white ash known as magnesium 

 oxide. 



2. (a) HgO = Hg + O. 



Mercuric oxide, when heated to a sufficient temperature, decom- 

 poses into its elements mercury and oxygen. 



(6) KC1O 3 = KC1 + 30. 



When potassium chlorate is heated sufficiently high and long it de- 

 composes into the compound potassium chloride and the element 

 oxygen. 



(c) CaCO 3 = CaO + CO 2 . 



When calcium carbonate is heated to redness it is decomposed into 

 two new compounds namely, calcium oxide and carbon dioxide. 



3. Fe + 2HC1 = FeCl 2 + 2H. 



When a solution of hydrochloric acid is poured upon some iron, a 

 brisk evolution of hydrogen gas takes place ; and a new compound, 

 ferrous chloride, remains in the solution. 



Although in a sense there is a displacement of one element by 

 another in every chemical action between two substances in which 

 two new substances result, by custom the term displacement is used 

 in those cases where the element displaced is left in the free or uncom- 

 bined state. 



4. HC1 + AgNO 3 == AgCl + HNO 3 . 



When a solution of hydrochloric acid is added to a solution of 

 silver nitrate, silver chloride is obtained as a white precipitate, and 

 nitric acid is left in solution. This type of change, known as double 

 decomposition or metathesis, is one of the most frequently occurring 

 kinds of chemical change in analysis and chemical industry. 



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