DECOMPOSITION OF COMPOUNDS. 53 



above, frequently the tendency of splitting up the molecules of 

 compound substances. 



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 solid state, because the 

 molecules do not come in sufficiently close contact to exchange 

 their atoms. The free motion of the molecules in liquid or 

 gaseous substances facilitates such a close contact, and conse- 

 quently chemical action. It is often sufficient to have but one of 

 the acting substances in the gaseous or liquid state, whilst the 

 second one is a solid. (By converting two solids into extremely 

 fine powder and mixing them together thoroughly, 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 ele- 

 ments or groups of elements : 



1. A -f B = AB. 



2. AB + C = AC + B. 



8. AB -f CD = AC + BD. 

 4. AB + 2C = AC + BC. 



As instances illustrating the above, may be mentioned the 

 following chemical reactions : 



1. H + Cl = HCl. 



Hydrogen. Chlorine. Hydrochloric 

 acid. 



The formula here given for the formation of hydrochloric 

 acid is not entirely correct, because the action between hydrogen 

 and chlorine does not take place between free atoms, but be~- 

 tween the molecules of the two atoms, each molecule containing 

 two atoms. The more correct way of writing the formula would 

 therefore be : 



HH + C1C = 2HC1, 

 Or 



2H + 2C1 = 2HC1. 



2. Hydrochloric acid and sodium form chloride of sodium and 

 hydrogen : 



HCl + Na = NaCl + H. 



