Lime is an important industrial chemical. The chemical name for lime is calcium oxide, and its chemical formula is CaO. The word lime is also used to refer to calcium hydroxide. Calcium hydroxide is formed by a reaction of calcium oxide with water. Calcium oxide is also known as quicklime, and calcium hydroxide is also called slaked lime and hydrated lime. Both substances are bases.
Slaked lime has a wide variety of uses. It serves as a flux in the production of steel (see Flux ). It also is used in the refining of aluminum, copper, and zinc. Lime “softens” water by removing minerals such as calcium, magnesium, and silica. It is also used in the treatment of sewage. Many farmers spread lime on their fields to neutralize acid in soil, and homeowners often use lime on their lawns to prevent the growth of moss. Lime also helps stabilize soil in the foundations of highways and airport runways. In the production of leather, tanneries use lime to remove hair from animal hides. The mortar that is layered between bricks or stones in the walls of buildings is composed of a mixture of lime, sand, and water. Lime is also a key ingredient in plaster and in a kind of cement called portland cement.
Most quicklime is made from small chunks of limestone. Limestone consists chiefly of calcium carbonate. To produce quicklime, manufacturers place crushed limestone in a special oven, called a kiln, and heat it to a temperature of about 2000 °F. (1204 °C). This process releases carbon dioxide from the limestone, leaving a lump of powdery, grayish-white quicklime.
The production of slaked lime involves a further process called slaking. Water is added to a lump of cooled quicklime, resulting in a chemical reaction that produces heat and steam. As the steam evaporates, the lump of quicklime disintegrates into a fine, dry, white powder. This powder is slaked lime. If water is added in excess of the amount needed for slaked lime, a slurry (soupy mixture) of partially dissolved lime is created. This slurry is commonly known as milk of lime. When slaked lime is dissolved completely in water, the resulting product is a clear liquid called limewater. Limewater is used to detect the presence of carbon dioxide gas in a substance. If carbon dioxide is present, it turns the limewater milky-white.
