Soda. The substance known under the name of soda consists chemically of the carbonate of sodium, Na2Co3 in combination with water - The ordinary washing soda has 10 molecules of water - i.e. Na2Co310H2O - but loses some of it when exposed to the air. The carbonate of sodium is obtained, to a small extent, naturally, forming deposits upon the soil, and existing dissolved in soda lakes in Egypt and Hungary, and in the water of many geysers. Formerly also quantities of the compound were obtained from marine vegetation under the name of barilla (q.v.); the greater quantity, however, is obtained by artificial preparation from salt by one of two processes: (1) the Leblanc; (2) the ammoniacal process. In the first, the salt is heated with sulphuric acid in a furnace constructed for the purpose; sodium sulphate and hydrochloric acid result -
2NaCl + H2SO4 = Na2SO4 + 2HCl.
The sodium sulphate known as salt cake is then powdered, mixed with powdered limestone and coal, and heated strongly in another furnace. The fused mass, known as black ash, consists of soda and calcium sulphide, and from it the soda is dissolved out by warm water and recrystallised. In the ammoniacal process ammonia gas and carbonic acid are passed into a strong solution of brine, when bicarbonate of soda and ammonium chloride result, the former being converted into sodium carbonate by heating, and from the solution containing the ammonium chloride the ammonia is again evolved by the addition ot lime. Soda crystals, Na2CO3102HO, crystallise as large, prismatic crystals of the Monoclinic system, and dissolve in 2 parts of water at 38° C. The anhydrous sodium carbonate is a white powder which fuses to a porcelanous mass at about 800°. The bicarbonate of soda, NaHCO3, forms a white powder which is not as soluble in vater as the previous salt. Soda is very extensively used in a great number of technical and manufacturing processes as well as in pure chbmistry. Thus it is an important adjunct in many metallurgical processes, is employed in the manufacture of glass, soap, and of paper, and in the operations of bleaching and dyeing.