LAWS OF CHEMICAL COMBINATION. NOTATION. 37 



other. This is called the law of equivalent or reciprocal 

 proportions. 4. The molecular weight of a compound is 

 the sum of the atomic weights of its constituents. This 

 is the law of molecular weights. 



33. Chemical Equations. Returning to the symbols, there 

 is a further great advantage to be derived from their use 

 which we have not yet mentioned. By writing formulae in 

 a particular way, they place before the eye in small com- 

 pass the exact changes which occur when chemical action 

 takes place between two substances. We will give only 

 very few examples here, but you will become familiar with 

 them as you progress. 



The formula for water, you know, is H 2 O; now potassium 

 is simply K. In Chapter XVI. you will learn that potassium 

 decomposes water, forming potassium hydrate, and setting 

 half the hydrogen free ; this is expressed in symbols thus : 



Potassium and Water yield Potassium hydrate and Hydrogen 

 K + II 2 O KHO + H 



The plus sign between potassium and water signifies 

 " mixed with " or " in contact with," or " acted upon by," 

 and not " combined with." The sign of equality means 

 " yields " or " produces," and has not the precise meaning 

 of " equal to " as when used in algebra. Notice particu- 

 larly that you have the same elements on both sides of the 

 equation, viz., O, H 2 , and K,but they are arranged different- 

 ly. K+H 2 O expresses the condition of the substances be- 

 fore they come in contact, and KHO-fH their condition 

 after the chemical action. 



The most important meaning conveyed by the chemical 

 equations remains still to be explained. Bearing in mind 

 that the symbols stand for definite weights of the bodies 

 they represent (referred to hydrogen as unity), the equa- 

 tions give the actual proportion by weight of the elements 



