SALT

in chemistry, substance produced by the reaction of an acid with a base. A salt consists of the positive ion of a base and the negative ion of an acid. The reaction between an acid and a base is called a neutralization reaction. The term salt is also used to refer specifically to common table salt, or sodium chloride. When in solution or the molten state, most salts are completely dissociated into negatively and positively charged ions and are good electrolytes (conductors of electricity). sodium chloride mineral substance of great importance. The mineral form halite, or rock salt, is sometimes called common salt to distinguish it from a class of chemical compounds called salts. Properties of common salt are shown in the table. Salt is essential to the health of both people and animals. Table salt, used universally as a seasoning, is fine-grained and of high purity. To ensure that this hygroscopic (i.e., water-attracting) substance will remain free-flowing when exposed to the atmosphere, small quantities of sodium aluminosilicate, tricalcium phosphate, or magnesium silicate are added. Iodized saltthat is, salt to which small quantities of potassium iodide have been addedis widely used in areas where iodine is lacking from the diet, a deficiency that can cause swelling of the thyroid gland, commonly called goitre. Livestock also require salt; it is often made available in solid blocks. The meat-packing, sausage-making, fish-curing, and food-processing industries use salt as a preservative or seasoning or both. It is employed for curing and preserving hides and as a brine for refrigeration. In the chemical industry, salt is required in the manufacture of sodium bicarbonate (baking soda), sodium hydroxide (caustic soda), hydrochloric acid, chlorine, and many other chemicals. Salt is also employed in soap, glaze, and porcelain enamel manufacture and enters into metallurgical processes as a flux (a substance promoting fusing of metals). When applied to snow or ice, salt lowers the melting point of the mixture. Thus, large amounts are used in northern climates to help rid thoroughfares of accumulated snow and ice. Salt is used in water-softening equipment that removes calcium and magnesium compounds from water. Additional reading Geology A factual survey of salt deposits of the world by country is given in Stanley J. Lefond, Handbook of World Salt Resources (1969). An authoritative work on the deposition and geologic history of salt deposits is Hermann Borchert and Richard Muir, Salt Deposits: The Origin, Metamorphism, and Deformation of Evaporites (1964; originally published in German, 1959). Additional geologic information may be found in I. Lerche and J.J. O'Brien (eds.), Dynamical Geology of Salt and Related Structures (1987). A good source for statistics on salt production and markets is the section on salt in the Minerals Yearbook, prepared by the U.S. Bureau of Mines. Technology D.W. Kaufmann (ed.), Sodium Chloride: The Production and Properties of Salt and Brine (1960, reissued 1978), is a technical reference. Salt geology, geochemistry, mining, rock mechanics, solution mining, underground storage, and other topics are treated in highly technical fashion in a series of proceedings papers: A.C. Bersticker (ed.), Symposium on Salt (1963); Jon L. Rau (ed.), Second Symposium on Salt, 2 vol. (1966); Jon L. Rau and Louis F. Dellwig (eds.), Third Symposium on Salt, 2 vol. (1970); Alan H. Coogan (ed.), Fourth Symposium on Salt, 2 vol. (1974); Alan H. Coogan and Lukas Hauber (eds.), Fifth Symposium on Salt, 2 vol. (1980); B. Charlotte Schreiber and H. Lincoln Harner (eds.), Sixth International Symposium on Salt, 2 vol. (1985); and Hidetake Kakihana et al. (eds.), Seventh Symposium on Salt, 2 vol. (1993). Dennis S. Kostick, Salt, in Donald D. Carr (ed.), Industrial Minerals and Rocks, 6th ed. (1994), pp. 851868, provides an overview of geology, technology, uses, economic factors, and environmental and health considerations. History The use and importance of salt is chronicled by Garnett Laidlow Eskew, Salt, the Fifth Element: The Story of a Basic American Industry (1948); Robert P. Multhauf, Neptune's Gift: A History of Common Salt (1978), discussing both naturally occurring and chemically produced salt; and S.A.M. Adshead, Salt and Civilization (1992), treating production and distribution from primitive to modern times as well as government involvement and taxation. Frank Osborne Wood John M. Hills Robert H. Ralston The Editors of the Encyclopdia Britannica