Sodium chloride is readily soluble in water and insoluble or only slightly soluble in most other liquids. It forms small, transparent, colorless to white cubic crystals. Sodium chloride is odorless but has a characteristic taste. It is an ionic compound, being made up of equal numbers of positively charged sodium and negatively charged chloride ions. When it is melted or dissolved in water the ions can move about freely, so that dissolved or molten sodium chloride is a conductor of electricity; it can be decomposed into sodium and chlorine by passing an electrical current through it (see electrolysis).
Nearly all chemical compounds that contain either sodium or chlorine are ultimately derived from salt. Salt is widely and abundantly distributed in nature. It makes up nearly 80% of the dissolved material in seawater, and is the greater part of dissolved matter in the Dead Sea, the Great Salt Lake, and in salt wells in various parts of the world. It is also widely distributed in solid form. The mineral halite is pure salt. Rock, or mineral, salt is usually less pure; it is found in large deposits in the United States, notably in New York, Michigan, Ohio, Kansas, Texas, and Louisiana, and also in Great Britain, France, Germany, Russia, China, and India.
The manufacture and use of salt is one of the oldest chemical industries. Salt is mined from deposits or is obtained as a brine by introducing water into the deposits to dissolve the salt and then pumping the solution to the surface. Salt is also obtained by evaporation of seawater, usually in shallow basins warmed by sunlight; salt so obtained was formerly called bay salt, and is now often called sea salt or solar salt. Most salt for table use is obtained from seawater. It is usually not pure sodium chloride—it may contain natural impurities that provide dietary minerals, or small amounts of other substances (e.g., magnesium carbonate, hydrated calcium silicate, or tricalcium phosphate) may be added to prevent lumping.
Salt is important in many ways. It is an essential part of the diet of both humans and animals and is a part of most animal fluids, such as blood, sweat, and tears. It aids digestion by providing chlorine for hydrochloric acid, a small but essential part of human digestive fluid. Persons with hypertensive heart disease often must restrict the amount of salt in their diet.
Salt is widely used as a seasoning for foods and is used in curing meats and preserving fish and other foods. Iodized table salt usually contains small amounts of potassium iodide, sodium carbonate, and sodium thiosulfate. As a chemical salt is used in making glass, pottery, textile dyes, and soap. It is used in large amounts to melt ice and snow on streets and highways. The major use of salt is as a raw material for the production of chlorine, sodium metal, and sodium hydroxide; it is also used in large amounts in the Solvay process for making sodium carbonate. Historically, salt has been used as money; a high tax on salt was a contributing cause of the French Revolution.
See G. L. Eskew, Salt, the Fifth Element (1948); D. W. Kaufmann, ed., Sodium Chloride (1968); G. Mamantov and R. Marassi, ed., Molten Salt Chemistry (1987).