20 CHEMISTRY OF FARM PRACTICE 



a molecule of metallic iron has not been determined. There- 

 fore, by doubling the equation above, it may be so written 

 as to meet these conditions. 



2Fe 2 3 = 4Fe+3O 2 . 



This is read, two molecules of ferric oxide will produce 

 four atoms of iron and three molecules of oxygen. 



(3) Substitution. Frequently an element may be sub- 

 stituted for a portion of a compound, provided it is chem- 

 ically equivalent to the displaced portion. 



Mg+2HCl = MgCl 2 +H 2 . 



One atom of bivalent magnesium here substitutes for 

 two atoms of monovalent hydrogen. 



The formula for water, H 2 O, may be written HOH, 

 in which OH is a radical. Metals such as sodium react 

 with water as shown by the substitution equation. 



Na+HOH = H+NaOH. 

 This equation doubled, 



2Na+2HOH = H 2 +2NaOH, 



would be read, two atoms of sodium plus two molecules 

 of water will produce one molecule of hydrogen and two 

 molecules of sodium hydroxide. 



When iron is exposed to steam the reaction would be 

 expressed, 



Fe+3HOH = Fe(OH) 3 +3H, 

 or better, 



2Fe+6HOH=2Fe(OH) a +3H 2 . 



(4) Double Decomposition. When acids or bases or 

 salts react with one another, there is a simple exchange 

 between the parts of the decomposing substances. This 

 is the most common sort of chemical equation. 



