DECOMPOSITION OF COMPOUNDS. 55 



Mutual action of substances upon each other. As a general 

 rule, it may be said that no chemical action takes place between two 

 substances both of which are in the solidj3tae, because the molecules 

 do not come in sufficiently close proximity to exchange their atoms. 

 The free motion of the molecules in liquid or gaseous substances 

 facilitates such a proximity, and consequently chemical action. It is 

 often sufficient to have but one of the acting substances in the gaseous 

 or liquid state, while the second one is a solid. By converting two 

 solids into extremely fine powder and mixing them together thor- 

 oughly, chemical combination may follow, provided the affinity 

 between them be sufficiently strong. 



The action of substances upon each other may be represented by 

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

 groups of elements : 



1. A + B =AB = direct combination. 



2. AB -f C = AC -f B = direct decomposition. 



3. AB + CD = AC + BD = double decomposition. 



As instances illustrating the above, may be mentioned the fol- 

 lowing chemical reactions : 



1. H '+ Cl = HC1. 



Hydrogen. Chlorine. Hydrochloric acid. 



The formula here given for the formation of hydrochloric acid is 

 not entirely correct, because the action between hydrogen and chlo- 

 rine does not take place between free atoms, but between the mole- 

 cules of the two elements, each molecule containing two atoms. The 

 more correct way of writing the formula would therefore be : 



HH + C1C1 = 2HC1. 

 Or 



2H + 2C1 = 2HC1. 



2. Hydrochloric acid and sodium form sodium chloride and 



hydrogen : 



HC1 + Na = NaCl + H. 



The formula more correctly written would be : 

 2HC1 -f- 2Na = 2NaCl + 2H. 



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



Hydrochloric Silver Silver Nitric acid, 



acid. Nitrate. Chloride. 



This form of decomposition, known as double decomposition or 

 metathesis, is one of the common kinds of chemical changes met with, 

 in chemical operations. 



