CHAPTER XIII 

 SOME COMPOUNDS OF COMMON ELEMENTS 



175. Chemical Formulas. A chemical formula is a col- 

 lection of symbols representing the kinds and the relative 

 amounts of the elements in a given compound. For example, 

 CO2 is the formula for carbon dioxide and indicates that a 

 molecule of this compound is made up of one atom of carbon 

 and two of oxygen chemically united. NaCl is the formula 

 for common salt. 



176. Chemical Analysis. It is often necessary to find 

 out what substances are present in food, medicine, explosives, 

 and construction materials. The process by which this is 

 done is called chemical analysis. A complete analysis dis- 

 covers what elements and compounds are present and in 

 what amounts. Laboratories are established by the govern- 

 ment, by scientific schools, and by private enterprises for 

 the purpose of learning the composition of different forms of 

 matter. Manufactories maintain their own laboratories to 

 examine raw materials purchased for their use. 



A test is an examination of a substance to find out if a 

 certain element or compound is present. A test is not 

 analysis but may be used to show whether the conclusions 

 drawn from the work of analysis are correct. 



To apply a test, one must know how the given element or 

 compound behaves under given conditions and then apply 

 those conditions to the substance under examination. For 

 example, if any compound of copper is present in a solution, 

 the addition of ammonia gives a deep blue color. Knowing 

 this fact, we may test a given solution to see if it contains 



158 



