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A white deliquescent compound, CaCl2, which is soluble in water; r.d. 2.15; m.p. 782°C; b.p. >1600°C. There are a number of hydrated forms, including the monohydrate, CaCl2.H2O, the dihydrate, CaCl2.2H2O (r.d. 0.84), and the hexahydrate, CaCl2.6H2O (trigonal; r.d. 1.71; the hexahydrate loses 4H2O at 30°C and the remaining 2H2O at 200°C). Large quantities of it are formed as a byproduct of the Solvay process and it can be prepared by dissolving calcium carbonate or calcium oxide in hydrochloric acid. Crystals of the anhydrous salt can only be obtained if the hydrated salt is heated in a stream of hydrogen chloride. Solid calcium chloride is used in mines and on roads to reduce dust problems, whilst the molten salt is the electrolyte in the extraction of calcium. An aqueous solution of calcium chloride is used in refrigeration plants.