SOME CHEMISTRY OF EVERYDAY LIFE 195 



more or less pure metallic state by heating them to a high 

 temperature along with carbon (coal, or coke) with the 

 formation of the gases CC>2 and CO. 



However, the steps taken in extraction of the different 

 metals from their ores, whether as carbonates or other 

 compounds, vary widely owing to the character of the earthy 

 material mixed with the metallic compound, the physical 

 condition of the ores, and the relative expense of material 

 and labor involved in the processes. 



The term limestone is often loosely used to include not 

 only the sulphate of calcium (OaSO*) as well as the carbonate, 

 but the carbonates and sulphates of the element magnesium 

 (MgSC>4, and MgCOs). These latter are sometimes spoken 

 of as magnesium limestone. The rock CaSO^H^O with 

 more or less of earthy admixture is known as gypsum. 

 When relatively pure it can be heated to drive off part of 

 the water of crystallization, and then ground to form 

 " plaster of Paris" whose uses for making plaster casts and 

 stucco work are so well known. 



SUMMARY 



In the industrial world the varied uses of sodium carbonate, and of 

 the numerous compounds prepared from it, make its production in 

 enormous quantities and at a relatively low cost of utmost importance. 

 In the Solvay process of soda manufacture, the raw material is salt, 

 ammonia, and carbon dioxide all cheap materials. The chemical 

 process involved is relatively a simple one, and the market price of 

 both the carbonate and the bicarbonate in large quantities is low. 



When the limestone rock CaCO 3 is quarried and heated intensely 

 hot in kilns (kils), the valuable building material known as lump or 

 "quick" lime (CaO) is produced. This is ''slaked" by the addition 

 of sufficient water, and the chemical change yields the finely powdered 

 white solid CaO 2 H 2 . This when mixed with sand and water makes 

 mortar such as used in laying walls of brick and stone. 



The calcium carbonate of the enormous deposits of limestone rock 

 may be considered as once having been in solution in sea water. It 



