130 APPLIED SCIENCE 



its elements, oxygen and hydrogen, which can be collected 

 in separate tubes. Separating compounds into their elements 

 is called analysis. 



In the case of water, for -every volume of oxygen there 

 will be found to be just twice the volume of hydrogen, 

 Every molecule of water contains two atoms of hydrogen 

 and one atom of oxygen. The chemical formula expressing 

 this is H 2 O. In any formula the number of atoms is shown 

 by writing the number below the symbol of the element and 

 'at the right. 



135. Synthesis. As already stated, when the proper 

 proportions by weight of oxygen and hydrogen are mixed 

 and a spark passed through, water is formed. This change, 

 often called a reaction, may be written as follows: 



2H + H 2 O 



2 atoms of combined 1 atom of forms 1 molecule 



hydrogen with oxygen of water 



Abbreviating a reaction in this manner is called writing 

 a chemical equation. In a very concise form it shows: on 

 the left-hand side of the equation the substances (called 

 factors) which enter the reaction, on the right-hand side 

 the products, and also the exact amount of each that must be 

 taken or formed. Once the products are determined (usu- 

 ally by experiments), the equation may be written and 

 balanced by having the same number of atoms of the elements 

 on each side of the equation. Forming compounds by com- 

 bining elements is called synthesis. 



136. Molecular Weight. The molecular weight of a 

 compound is the sum of all the atomic weights in the com- 



