366 



lization from water. A certain amount is made, by evaporation, 

 from sea water. 



The salt crystallizes in white cubes. Its chief application is in 

 the manufacture of other compounds of sodium. 



Sodium Bicarbonate NaHCO 3 , Manufacture. This salt 

 is manufactured by the interaction of sodium chloride and am- 

 monium-hydrogen carbonate in the Solvay or ammonia-soda proc- 

 ess. Very concentrated solutions are used, and from them a 

 great part of the sodium-hydrogen carbonate, which is a much 

 less soluble salt, is precipitated : 



Nad + NH 4 HC0 3 + NaHC0 3 J, + NH 4 C1. (1) 



In practice salt is dissolved in water and. the solution is saturated 

 with ammonia gas. The mixture is placed in an iron tower 

 filled with perforated shelves. Carbon dioxide, made by heating 

 limestone in special kilns, is forced in at the bottom. The per- 

 forations split up the gas into small bubbles, and facilitate its 

 solution to form carbonic acid H 2 C0 3 . With the ammonium 

 hydroxide NH 4 OH in the liquid it gives ammonium-hydrogen 

 carbonate : 



NH 4 OH + H 2 C0 3 -* NH 4 HC0 3 + H 2 O. (2) 



This product interacts as in equation (1). The sodium bicar- 

 bonate is precipitated and is freed from the liquor in filter-presses. 

 The ammonia is recovered for use by treating the residual liquor, 

 containing ammonium chloride, with the quicklime CaO from the 

 kilns. The quicklime, with the water, gives slaked lime Ca(OH) 2 

 and the latter liberates the ammonia (p. 302) : 

 2NH 4 C1 + Ca(OH) 2 - CaCl 2 + 2NH 4 OH -> 2NH 3 f + 2H 2 O. 



Properties of Sodium Bicarbonate NaHCO 3 . This salt is 

 a fine, white, not obviously crystalline powder, which is only 

 slightly soluble in water. It is commonly known as baking soda. 



It decomposes slowly in an open vessel, even when cold. When 



