Chem. 1. Also called soda ash. an anhydrous, grayish-white, odorless, water-soluble powder, Na2CO3, usually obtained by the Solvay process and containing about 1 percent of impurities consisting of sulfates, chlorides, and bicarbonates of sodium: used in the manufacture of glass, ceramics, soaps, paper, petroleum products, sodium salts, as a cleanser, for bleaching, and in water treatment.
2. Also called sal soda, soda, washing soda. the decahydrated form of this salt, Na2CO3·10H2O, used similarly.
3. the monohydrated form of this salt, Na2CO3·H2O, used similarly, esp. in photography.