100 



SMITH'S INTERMEDIATE CHEMISTRY 



The formula is therefore 



Making Equations. The condensed statements of chemical 

 changes which we have been using can now be still further sim- 

 plified by using the formula in place of the names of the sub- 

 stances (p. 78). Thus 



. Sn + O 2 - SnO 2 



This is to be read: 119 parts (or 1 atomic weight) of tin, acting 

 chemically with 2 X 16 parts (or two atomic weights) of oxygen, 

 give 151 parts of stannic oxide. We may also read it thus: 1 

 atom of tin with 1 molecule of oxygen gives 1 molecule of stannic 

 oxide. 



In making an equation there are four stages or steps: 



1. Find out by experiment what the substances used and pro- 

 duced are. 



2. Learn the molecular formula of each substance. 



3. Set down the molecular formulae in the form of a skeleton 

 equation. Place the formulae of the initial substances on the left, 

 and those of the products on the right. 



4. Adjust, or balance the equation. 



For example: 



(1) When hydrogen and oxygen combine, water is formed. 



(2) The molecular formulas are H 2 , 2 , and H 2 O. . 



(3) Skeleton equation: H 2 + 2 - H 2 0. 



(4) In accordance with the law of conservation of mass, the 

 numbers of atomic weights (or atoms) of each element must be 



