"Soda ash" refers to anhydrous Na2CO3, obtained by dehydration of hydrous forms, or by the Solvay process (which results in a product
containing about 1% impurities, these consisting of sulfate and chloride and silicate). It is used in soaps, petroleum refining, paper manufacture,
ceramics, glass-making, and making other Na salts.
Sodium carbonate can be also crystallized from saturated solutions as either the monohydrate, heptahydrate, and decahydrate, depending on the
temperature. Sal soda, or washing soda, is usually the decahydrate, Na2CO3.10H20, although the other hydrates are also used for the same purpose; the
monohydrate is also used in photography.
That used in baking is usually baking soda, sodium bicarbonate, NaHCO3. A similar mixed carbonate is trona, Na3H(CO3)2.2H2O, which occurs as a mineral
in northeast Africa. The ancient Egyptians used it for mummifying bodies.
John W.
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