sodium chloride, NaCl, common salt.

Properties

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).

Natural Occurrence and Commercial Preparation

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.

Biological Importance and Uses

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.

silver chloride, chemical compound, AgCl, a white cubic crystalline solid. It is nearly insoluble in water but is soluble in a water solution of ammonia, potassium cyanide, or sodium thiosulfate ("hypo"). On exposure to light it becomes a deep grayish blue due to its decomposition into metallic silver and atomic chlorine. This light-sensitive behavior is the basis of photographic processes (see photography, still). Since silver bromide, AgBr, and silver iodide, AgI, react similarly, all three of these silver halide salts are used in making photographic films and plates. Both the bromide and iodide are less soluble in water and more sensitive to light than the chloride. The bromide forms light yellow cubic crystals; the iodide forms yellow hexagonal or yellow-orange cubic crystals, depending on the temperature. Besides use in photography, silver chloride is used in silver plating, and silver iodide is used for seeding clouds. The chloride, bromide, and iodide occur naturally as the minerals cerargyrite, bromyrite, and iodyrite, respectively. Silver fluoride, AgF, forms colorless cubic crystals; it is much more soluble in water than the other silver halides.

potassium chloride, chemical compound, KCl, a colorless or white, cubic, crystalline compound that closely resembles common salt (sodium chloride). It is soluble in water, alcohol, and alkalies. Potassium chloride occurs pure in nature as the mineral sylvite and is found combined in many minerals and in brines and ocean water. It is recovered (with other compounds) from the brine of Searles Lake in California. It is produced from sylvinite, a sodium chloride-potassium chloride mineral that is mined extensively near Carlsbad, N.Mex., and it is refined by fractional crystallization and by a flotation process. It is also recovered from lake brines in Utah and from ores in Saskatchewan, Canada. The chief use of potassium chloride is in the production of fertilizers; it is also used in chemical manufacture. For agricultural use it is often called muriate of potash; the concentration of potassium chloride in muriate of potash is expressed as a corresponding concentration of potassium oxide (K₂O), i.e., the concentration of potassium oxide that there would be if the potassium were present as its oxide instead of as its chloride. Thus, muriate of potash that contains (typically) 80% or 97% KCl by weight is said to contain 50% or 60% K₂O, respectively. Manure salts contain some potassium chloride.

polyvinyl chloride (PVC), thermoplastic that is a polymer of vinyl chloride. Resins of polyvinyl chloride are hard, but with the addition of plasticizers a flexible, elastic plastic can be made. This plastic has found extensive use as an electrical insulator for wires and cables. Cloth and paper can be coated with it to produce fabrics that may be used for upholstery materials and raincoats.

mercurous chloride, mercury (I) chloride, or calomel, chemical compound, Hg₂Cl₂, a white crystalline powder, very slightly soluble in water. It was once used medicinally as a purgative, cathartic, liver stimulant, and to eliminate parasitic worms, but is rarely so used today because it is readily decomposed into metallic mercury and the very poisonous mercuric chloride on exposure to sunlight or if heated in the presence of moisture. Mercurous chloride is a less dangerous poison than mercuric chloride chiefly because it is much less soluble; it is highly toxic if retained in the body. Mercurous chloride is prepared by sublimation from a mixture of mercury and mercuric chloride or by precipitation from a mercurous chloride solution on adding chloride ion. It is also found in nature as horn quicksilver. The calomel electrode, often used as a reference in determining electric potentials and for measuring the pH of solutions, contains mercurous chloride, mercury metal, and potassium chloride solution.

mercuric chloride or mercury (II) chloride, chemical compound, HgCl₂, a white powder of colorless rhombohedral crystals, somewhat soluble in water. It is also called bichloride of mercury or corrosive sublimate. It is extremely poisonous. Raw egg white may be given as an antidote, since mercuric chloride reacts with egg albumin to form a nearly insoluble precipitate; medical treatment should be sought immediately. Mercuric chloride is sometimes used in dilute solution as an antiseptic for inanimate objects and as a fungicide. It is also used in preparing other mercury compounds; it reacts with mercury metal to form mercurous chloride. Mercuric chloride is prepared by reacting mercury with chlorine gas or by subliming a mixture of mercuric sulfate and sodium chloride (common salt).

hydrogen chloride, chemical compound, HCl, a colorless, poisonous gas with an unpleasant, acrid odor. It is very soluble in water and readily soluble in alcohol and ether. It fumes in moist air. It is not flammable, and the liquid is a poor conductor of electricity. Hydrogen chloride is prepared commercially by the reaction of sulfuric acid with sodium chloride (common salt); niter cake, a mixture of sodium bisulfite and sulfuric acid that is a byproduct of nitric acid manufacture, is sometimes used in place of sulfuric acid. Hydrogen chloride is also produced as a byproduct of the manufacture of chlorinated organic chemicals. It can be prepared directly by reaction of hydrogen and chlorine gases; the reaction is very exothermic and takes place readily in sunlight or at elevated temperatures. Although anhydrous (water-free) hydrogen chloride is commercially available as a high-pressure compressed gas in steel cylinders, most of the gas produced is dissolved in water to form hydrochloric acid (see acids and bases), a commercially important chemical. Pure grades of hydrochloric acid are colorless, but technical grades, commonly called muriatic acid, are often yellow-colored because of impurities such as dissolved metals. Most hydrochloric acid produced has a concentration of 30% to 35% hydrogen chloride by weight. The major use of hydrochloric acid is in the manufacture of other chemicals. It is also used in large amounts in pickling (cleaning) metal surfaces, e.g., iron before galvanizing. It reacts with most common metals, releasing hydrogen and forming the metal chloride; with most metal oxides and hydroxides it reacts to form water and the metal chloride. Hydrochloric acid is also used in small amounts in processing glucose and other foods and for various other uses. Concentrated solutions are strong acids and highly corrosive. Hydrochloric acid is not an oxidizing agent but can be oxidized by very strong oxidizing agents, liberating chlorine gas. In dilute solutions of the acid the hydrogen chloride is almost completely dissociated into hydrogen and chloride ions. A solution containing 20.24% hydrogen chloride by weight is azeotropic, boiling at a constant temperature of 110°C; at atmospheric pressure. Hydrogen chloride also forms monohydrates, dihydrates, and trihydrates that are liquids at room temperature.

chloride of lime: see bleaching powder.

chloride, chemical compound containing chlorine. Most chlorides are salts that are formed either by direct union of chlorine with a metal or by reaction of hydrochloric acid (a water solution of hydrogen chloride) with a metal, a metal oxide, or an inorganic base. Chloride salts include sodium chloride (common salt), potassium chloride, calcium chloride, and ammonium chloride. Most chloride salts are readily soluble in water, but mercurous chloride (calomel) and silver chloride are insoluble, and lead chloride is only slightly soluble. Some chlorides, e.g., antimony chloride and bismuth chloride, decompose in water, forming oxychlorides. Many metal chlorides can be melted without decomposition; two exceptions are the chlorides of gold and platinum. Most metal chlorides conduct electricity when fused or dissolved in water and can be decomposed by electrolysis to chlorine gas and the metal. Chlorine forms compounds with the other halogens and with oxygen; when chlorine is the more electronegative element in the compound, the compound is called a chloride. Thus, compounds with bromine and iodine are bromine chloride, BrCl, and iodine chloride, ICI, but compounds with oxygen or fluorine (which are more electronegative than chlorine) are oxides (e.g., chlorine dioxide, ClO₂) or fluorides (e.g., chlorine fluoride, ClF) respectively. Many organic compounds contain chlorine, as is indicated by common names such as carbon tetrachloride, methylene chloride, and methyl chloride. However, in the nomenclature system for organic chemistry adopted by the International Union of Pure and Applied Chemistry (IUPAC), the presence in a compound of chlorine bonded to a carbon atom is indicated by the prefix or infix chloro; thus, carbon tetrachloride is tetrachloromethane, methylene chloride is dichloromethane, and methyl chloride is chloromethane.

calcium chloride, CaCl₂, chemical compound that is crystalline, lumpy, or flaky, is usually white, and is very soluble in water. The anhydrous compound is hygroscopic; it rapidly absorbs water and is used to dry gases by passing them through it. Calcium chloride is commercially available usually as the dihydrate, CaCl₂·2H₂O; it is used to melt ice on roads, to control dust, in brines for refrigeration, and as a preservative in foods. It is also used in the monohydrate and hexahydrate forms. Calcium chloride is a byproduct of the Solvay process (a major source of the compound) and is present in natural brines.

ammonium chloride, chemical compound, NH₄Cl, a white or colorless, odorless, water-soluble, cubic crystalline salt with a biting taste, commonly known as sal ammoniac. It is prepared commercially by reacting ammonia, NH₃, with hydrogen chloride, HCl, and is used chiefly in the manufacture of electric dry-cell batteries, in soldering fluxes, in textile printing, and in making other compounds. It is also used in certain medical treatments. It occurs in nature in volcanic regions.

or chloroethylene

Colourless, flammable, toxic gas (H₂CdoublehorzbondCHCl), belonging to the family of organic compounds of halogens. It is produced in very large quantities and used principally to make PVC, as well as in other syntheses and in adhesives. Prolonged exposure to vinyl chloride is linked to several kinds of cancer.

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in full polyvinyl chloride

Synthetic resin, an organic polymer made by treating vinyl chloride monomers with a peroxide. It may be blended with more rubbery polymers or copolymerized with other vinyls to obtain products with desired properties. PVC resin mixed with plasticizers (see Waldo Semon), stabilizers, and pigments is made into flexible articles (e.g., raincoats, toys, containers). Nonplasticized resin has been used for rigid products (e.g., water pipes, plumbing fittings, phonograph records). Concern over leaching of vinyl chloride into foods has resulted in restrictions on its use in food containers; its decomposition into hydrogen chloride when burned has also raised concerns. Today it is produced in larger quantities than any other plastic except polyethylene.

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or carbonyl chloride

Colourless, highly toxic gas used in chemical warfare as well as in industrial processes including the making of dyestuffs and polyurethane resins. Either alone or in combination with chlorine, it was used against troops in World War I. It smells like musty hay. Inhalation causes severe lung injury several hours after exposure. First prepared in 1811, it is manufactured by the reaction of carbon monoxide and chlorine in the presence of a catalyst. Gaseous phosgene is usually stored and transported as a liquid under pressure in steel cylinders or as a solution in toluene. Mixed with water, it forms carbon dioxide and hydrochloric acid.

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