CHAPTER XVII 

 Writing Equations 



147. Importance. A chemical equation expresses con- 

 cisely the changes which take place when two or more 

 compounds are brought together so as to react, or when 

 a material is acted upon by any agent which causes a 

 chemical change. When chemical equations are under- 

 stood by the student, they are of great assistance, as they 

 necessitate a knowledge of the laws of valence, of the 

 power of replacement, and of the properties of the ele- 

 ments and their compounds. 



148. Common Errors in Writing Equations. Some 

 of the more common errors in writing equations are : 



(i). Failure to use correct formulas. 



(2). Failure to use the correct number of parts of com- 

 pounds, radicals, or elements. 



(3). Failure properly to balance the equation. 



(4) . Failure to form reasonable compounds or products. 



If the correct formula, or the right number of mole- 

 cules is not used, the equation is incorrect, it cannot be 

 balanced, and the principle represented by the sign of 

 equality is violated.- There should be as many atoms of an 

 element on one side of an equation as on the other. In 

 order properly to balance an equation, as many mole- 

 cules of the compounds on the left of the equation should 

 be taken as are needed to satisfy the valences of the re- 

 acting elements and radicals. In the equation 

 AgNO 8 + HC1 = AgCl -f- HNO S , 



