COMPOUNDS MIXTURES VALENCE, ETC. 19 



inserting the bonds, we have A1"'(SO4)", and applying 

 the rule, the correct formula would be A1 2 (SO4)3. 



20. Equations. Chemical reactions are expressed by 

 equations which show in a condensed form the kind and 

 the quantities both of the substances entering into the 

 reaction and of the products formed. Equations may be 

 placed in four classes: 



(1) Combination. When two or more substances unite 

 to form a new combination, as, by example, 



Fe+S = FeS. 



This equation is read, one atom of iron unites with 

 one atom of sulphur to produce one molecule of iron sul- 

 phide. 



It also indicates that one atomic weight of iron, 56, 

 is uniting with one atomic weight of sulphur, 32, and pro- 

 ducing a molecular weight of iron sulphide, 88. 



This may be translated, one molecule of hydrogen 

 weighing 2, unites with one atom of oxygen weighing 16, 

 and produces one molecule of water weighing 18. 



(2) Decomposition. When a substance is separated into 

 two or more substances, the analysis may be expressed 

 as follows: 



= H 2 O+C0 2 . 



This may be read, one molecule of carbonic acid, weight 

 62, is decomposed into one molecule of water, weight 18, 

 and one molecule of carbon dioxide, weight 44. 



Fe 2 3 = 2Fe+3O. 



One molecule of ferric oxide yields two atoms of iron 

 and three atoms of oxygen. 



It is known that oxygen in a free condition exists as 

 molecules of two atoms each. The number of atoms in 



