PART II 



XII. BASIC OXIDES, ACID OXIDES, AND SALTS 



In studying air and water, it has been found that oxygen forms 

 an important constituent of each. This is also true of the crust 

 of the earth. The older rocks, such as granite ; the stratified 

 rocks formed by the disintegration of the older rocks, such as 

 limestone or chalk, clay and sand ; and soil formed by the dis- 

 integration and mixture of the foregoing, almost entirely consist 

 of oxygen compounds. It is therefore necessary in commencing 

 a study of the constituents of the earth's crust to examine the 

 oxides more closely. 



It has already been observed that certain oxides are soluble 

 in water, and that their solutions have acid properties ; for 

 they are sour in taste and turn blue litmus red. Prepare the 

 oxides of phosphorus and sulphur by burning the elements in jars 

 filled with air or oxygen (Fig. 7), and containing a little water 

 to dissolve the fumes of the oxides produced. Prepare the 

 oxides of the metals magnesium and potassium by heating in 

 the air (the latter in an iron spoon) till ignited. Dissolve the 

 oxide of potassium in water and filter the solution. Mix the 

 oxide of magnesium with a few drops of water in a dish, it being 

 only very slightly soluble. Compare the action of the four 

 liquids on blue and red litmus paper. Do they turn blue litmus 

 red or red litmus blue ? Are they acid or alkaline in reaction ? 



It will be found that of these four oxides, two dissolve in 

 water forming acid, two dissolve forming alkaline liquids. The 



61 



