What is Salt?

Sodium chloride, or common salt, is the chemical compound NaCl. Salt occurs naturally in many parts of the world as the mineral halite and as mixed evaporates in salt lakes. Seawater has lots of salt; it contains an average of 2.6% (by weight) NaCl, or 26 million metric tons per cubic kilometer (120 million short tons per cubic mile, an inexhaustible supply. Note: seawater also contains other Coarse Sonoma copy dissolved solids; salt represents about 77% of the Total Dissolved Solids). Underground salt deposits are found in both bedded sedimentary layers and domal deposits. Deposits have been found to have encapsulated ancient microorganisms including bacteria. Some salt is on the surface, the dried-up residue of ancient seas like the famed Bonneville Salt Flats in Utah. Salt even arrives on earth from outer space, and its presence on the planet Mars makes scientists think life may exist there. Conversely, surface salt deposits and man-made saltworks can be seen from space.

Sodium chloride crystals are cubic in form. Table salt consists of tiny cubes tightly bound together through ionic bonding of the sodium and chloride ions. The salt crystal is often used as an example of crystalline structure. It can be modified by temperature. Different types of crystal have different uses, as for food.

It varies in color from colorless, when pure, to white, gray or brownish, typical of rock salt (halite). Chemically, it is 60.663% elemental chlorine (Cl) and 39.337% sodium (Na). The atomic weight of elemental chlorine is 35.4527 and that of sodium is 22.989768. Properties of salt are collected in the U.S. Occupational Safety and Health Administration (OSHA) Chemical Sampling Information database.

Sodium chloride is sold in several different particle sizes (gradation) and forms, depending on the intended end use. Discrete crystals can be seen in rock salt used for deicing. Fine granules are typical of table salt and even finer popcorn salt. Kosher salt, pickling salt and ice cream salt are slightly coarser. Small compressed pellets are used in water softeners and large salt blocks are used as salt licks for livestock. When viewed under strong magnification, all sodium chloride is crystalline. Very large cubic crystals, of two, three or more inches in size, can be seen in some salt mines. They are transparent and cleave into perfect cubes when struck with a hard object.

Purity of rock salt produced in North America varies depending on the type of salt (evaporated, rock, solar) and on the source. Rock salt typically ranges between 95% and 99% NaCl, and mechanically evaporated salt and solar salt normally exceed 99% NaCl. Evaporated salt made with purified brine has the highest purity, in some cases 99.99% NaCl. Voluntary standards, such as those developed by the American Society for Testing and Materials (ASTM) and the American Water Works Association (AWWA) assure appropriate quality for the intended use. Mandatory specifications for food grade, drug/medical and analytical use include Food Chemicals Codex, U.S. Pharmacopoeia, and Reagent Grade Chemicals. Special devices, refractometers, are used to measure salinity.

Common salt or sodium chloride, is considered by the U.S. Food and Drug Administration as safe for its intended use as a food additive. This GRAS (generally recognized as safe) classification, and the universal use of sodium chloride since antiquity, affirms its safety. The Merck Index refers to sodium chloride as "not generally considered poisonous." Many substances in everyday use can be toxic in high concentrations, even water. Toxic levels of sodium chloride are reported as:

Oral toxicity (The Registry of Toxic Effects of Chemical Substances, 1986):
Human; TDLo: 12,357 mg/kg/23 D-C
Mouse; LD50: 4,000 mg/kg
Rat; LD50: 3,000 mg/kg
Rabbit; LDLo: 8,000 mg/kg

Acute aquatic toxicity (U.S. EPA, Ambient Water Quality Criteria for Chloride, 1988):
Rana Breviceps (frog); No observed effect concentration (NOEC): 400 mg/L.
Daphnia pulex 48-hour LC50 or EC50: 1,470 mg/L
Daphnia magna (water flea); 48 hour EC50: 3,310 mg/L
Myriophyllum spicatum (water milfoil); Phytotoxicity (EC50 for growth): 5,962 mg/L
Pimephales promealas (fathead minnow); 69-hour LC50: 7,650 mg/L
Lepomis macrochirus (Bluegill) LC50 or EC50: 7,846 mg/L
Anguilla rostrata (American eel) 48-hour LC50 or EC 50: 13,085 mg/L

EPA says that the chlorides of calcium, magnesium and potassium are generally more toxic to fresh water species than sodium chloride. Some Antarctic species depend on salt to protect them against the cold.