166 



Sodium peroxide (" oxone "), made by burning sodium, is used 

 as a source of oxygen (p. 31). 



Preparation of Sodium Hydroxide. As we have seen, this 

 compound is formed by the action of water on sodium, and may 

 be obtained by evaporating the solution to dryness. But manu- 

 facturing it from an expensive substance like sodium is out of the 

 question. 



Much sodium hydroxide is made by boiling an aqueous solution 

 of sodium carbonate Na^COa with slaked lime (calcium hydroxide, 

 Ca(OH) 2 ): 



Ca(OH) 2 + Na-jCOs ^ 2NaOH + CaCO 3 J. 



The calcium carbonate, which is chemically the same substance 

 as lime-stone and chalk, is much less soluble than any of the other 

 substances taking part in the reaction, and is consequently pre- 

 cipitated out of the solution. This reaction is another example 

 of a double decomposition (p. 132). All double decompositions 

 are reversible, but here the precipitation of the calcium carbonate 

 prevents it from acting upon the sodium hydroxide and reversing 

 the action to any appreciable extent. After the precipitate has 

 settled, the solution of sodium hydroxide is drawn off and evap- 

 orated to dryness. 



Electrolytic Process. Sodium hydroxide is also manufactured 

 by electrolysis of sodium chloride solution (pp. 139-40), the other 

 product, chlorine, being of great commercial value also. The 

 Nelson cell (Fig. 58) is now most extensively used. A porous 

 diaphragm of asbestos separates the perforated steel cathode 

 from the carbon anode, which is immersed in a current of brine 

 flowing through the cell between the electrodes. Chlorine is 

 liberated at the anode and rises in bubbles to the surface of the 

 solution. It is drawn off, dried, and compressed to liquid form 

 in iron cylinders, or is made directly into bleaching compounds 

 (p. 224). Sodium, as we have seen in the discussion of electroly- 



