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(also quicklime), CaO, a compound of calcium and oxygen; colorless crystals with a density of 3.4 g/cm3 and a melting point of 2585°C. The industrial product is a white porous substance. Calcium oxide reacts vigorously with water, liberating a considerable amount of heat and forming calcium hydroxide: CaO + H2O = Ca(OH)2 (slaking process). Calcium oxide is obtained by the calcination of limestone or chalk. It is widely used in construction, the chemical industry, metallurgy, agriculture, and water purification.