278 GENERAL SCIENCE 



rocks and ores to determine their composition and value. 

 To-day nearly all mining companies and manufacturing 

 companies maintain their own laboratories for examining 

 the materials in which they are interested. 



To " test " a substance for the purpose of rinding out 

 whether a certain element or compound is present or not 

 we simply make an examination of the substance, neglect- 

 ing so far as possible other elements or compounds present. 

 No two substances have exactly the same properties, and 

 so " tests " may be devised for distinguishing any certain 

 substance. 



We may test for acids as a class by using litmus paper 

 or litmus solution, but we shall not be able by this test 

 to tell whether the acid present is hydrochloric, sulphuric, 

 or any one of a number of other acids. 



In testing a salt we need to determine : (a) what metal 

 is present and (b) what acid was used in producing it. If 

 sulphuric acid was used, we call the salt a sulphate ; if 

 hydrochloric acid was used, a chloride ; if nitric acid was 

 used, a nitrate ; if phosphoric acid was used, a phosphate ; 

 and so on. 



Iron. If solutions containing iron salts are heated 

 with a little concentrated nitric acid and boiled for a few 

 minutes, and a few drops of potassium thiocyanate are 

 added, a beautiful blood-red solution results. 



Copper. If copper is present in a solution, the addition 

 of ammonia gives a deep blue color. 



Sodium. Sodium salts give a bright yellow color when 

 held in a colorless gas flame. 



Sulphates. To test for sulphate salts, we add dilute 

 nitric acid to a water solution of the salt and then add a 

 few drops of barium chloride solution. If the salt is a 

 sulphate, there will be a white precipitate formed. 



