Salt

I

Introduction

Salt, also sodium chloride, chemical compound that has the formula NaCl. The term salt is also applied to substances produced by the reaction of an acid with a base, known as a neutralization reaction. Salts are characterized by ionic bonds, relatively high melting points, electrical conductivity when melted or when in solution, and a crystalline structure when in the solid state. See Acids and Bases; Chemical Reaction. This article focuses on salt, formula NaCl.

Salt is a white solid, soluble in hot or cold water, slightly soluble in alcohol, but insoluble in concentrated hydrochloric acid. In the crystalline form the compound is transparent and colourless, shining with an ice-like lustre. The compound usually includes traces of magnesium chloride (MgCl₂), magnesium sulphate (MgSO ₄), calcium sulphate (CaSO₄), potassium chloride (KCl), and magnesium bromide (MgBr₂).

Salt is widely distributed in nature. It is found in solution in ocean water in concentrations of about 30 g/litre (about 4.08 oz/gal) of water, meaning that salt makes up about 3 per cent of ocean water by weight. The compound is also distributed throughout many rivers and inland lakes and seas, the concentration varying from 0.002 per cent in the Mississippi River to 30 per cent in the Dead Sea. Salt can occur as a surface crust or layer in swamps and dry lake bottoms, especially in extremely arid regions. The mineral halite, more commonly known as rock salt or massive salt, occurs in beds deposited by the dehydration of ancient bodies of salt water. The compound is constantly being formed by the action of rivers and streams on rocks containing chlorides and compounds of sodium. Salt melts at 804° C (1479° F) and begins to vaporize at temperatures just slightly above this; it has a relative density of 2.17.

II

History

Important since prehistoric times as a seasoning agent and to preserve foods, salt was also commonly used in the religious rites of the Greeks, Romans, Hebrews, and Christians. It was an important medium of exchange in the commercial ventures across the Mediterranean, Aegean, and Adriatic seas, and has been subject to severe taxation in Asian countries from ancient to modern times. In the form of salt cakes, it served as money in ancient Ethiopia and Tibet. The English term salary, which formerly represented a soldier's money allowance for salt, was derived from salarium, the Latin term referring to the salt allotment that was issued to soldiers serving in the Roman army.

The simplest method of obtaining salt from areas near oceans or seas is by the evaporation of salt water; this process is expensive, however, and is used only when cheaper methods are unavailable. In most regions, rock salt is obtained from subterranean sources by mining or by wells sunk to the deposits. In the latter method, the salt is first dissolved in water forced down one of the well tubes, and the resulting brine is raised to the surface through other tubes. After being freed of clay impurities, the salt solution is evaporated. Several methods of evaporation are used, the most important of which are solar evaporation, in which the heat is derived from the rays of the Sun; steam evaporation in vacuum pans and covered kettles; and direct-heat evaporation in open kettles and pans. Most commercial salt is produced by steam or direct-heat evaporation of rock-salt brine.

III

Uses

The most familiar use of salt is as a seasoning. Salt is an essential constituent in the diet of human beings and other warm-blooded animals. Certain peoples restrict the consumption of salt, but they obtain necessary quantities of it by eating salt-containing raw or cooked meat and fish. Common table salt marketed for consumption in inland areas often has small quantities of iodides added to prevent the occurrence of goitre. Wild animals often congregate at salt streams or surface encrustations of salt, called salt licks, where they lick the salt deposits.

Industrially, salt is the source of chlorine and its principal compounds and the source of sodium and its compounds. Chlorine compounds of commercial importance include hydrochloric acid, chloroform, carbon tetrachloride, and bleaching powder. Important sodium compounds include sodium carbonate (see Soda), sodium sulphate, baking soda, sodium phosphate, and sodium hydroxide. Salt is widely used as a preservative for meats and is employed in some refrigeration processes, in dyeing, and in the manufacture of soap and glass. Because they are transparent to infrared radiation, salt crystals are used for making the prisms and lenses of instruments used in the study of infrared radiation.