Sodium chloride, also known as common salt, table salt, or halite, is a chemical compound with formula NaCl.
Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms. It is commonly used as a flavour enhancer and preservative for food.
Sodium chloride forms crystals with cubic symmetry. In these, the larger chloride ions are arranged in a cubic close-packing, while the smaller sodium ions fill the octahedral gaps between them. Each ion is surrounded by six of the other kind. This same basic structure is found in many other minerals, and is known as the halite structure.
Sodium chloride is essential to life on Earth. Most biological tissues and body fluids contain a varying amount of salt. The concentration of sodium ions in the blood is directly related to the regulation of safe body-fluid levels. Propagation of nerve impulses by signal transduction is regulated by sodium ions.
0.9% sodium chloride in water is called a physiological solution
because it is isoosmotic with blood plasma. It is known medically as normal saline. Physiological solution is the mainstay of fluid replacement therapy that is widely used in medicine in prevention or treatment of dehydration, or as an intravenous therapy to prevent hypovolemic shock.
Humans are unusual among primates in secreting large amounts of salt by sweating.
Salt throughout history
Salt's preservative ability was a foundation of civilisation. It eliminated dependency on the seasonal availability of food and allowed travel over long distances. By the Middle Ages, caravans consisting of as many as forty thousand camels traversed four hundred miles of the Sahara bearing salt, sometimes trading it for slaves.
Until the 1900s, salt was one of the prime movers of national economies and wars. Salt has played a prominent role in determing the power and location of the world's great cities. Timbuktu was once a huge salt market. Liverpool rose from just a port in England to become the prime producer of the world's salt in the 1800s.
Salt created and destroyed empires. The salt mines of Poland led to a vast kingdom in the 1500s, only to be destroyed when Germans brought sea salt (often, to most of the world, considered 'superior' to rock salt). Venice fought and won a war with Genoa over salt. Genoa, however, had the last laugh. Genovites Christopher Columbus and Giovanni Caboto destroyed the Mediterranean trade by introducing the new world to the market.
Salt was once one of the most valuable commodities known to man. Salt was taxed, from as far back as the 20th century BC in China. In the Roman Empire, salt was sometimes even used as a currency, giving us the term salary. The Roman Republic and Empire controlled the price of salt, increasing it to raise money for wars, or lowering it to be sure that the poorest citizens could easily afford this important part of the diet. Throughout much of history, it influenced the conduct of wars, the fiscal policies of governments, and even the inception of revolutions.
In the empire of Mali, merchants in 12th-century Timbuktu—the gateway to the Sahara Desert and the seat of scholars—valued salt enough to buy it for its weight in gold; this trade led to the legends of the incredibly wealthy city of Timbuktu, and fueled inflation in Europe, which was exporting the salt.
In later times, for instance during the British colonial period, salt production and transport were controlled in India as a means of generating enormous tax revenues. This ultimately led to the Salt March to Dandi, led by Mahatma Gandhi in 1930 in which thousands of Indians went to the sea to illegally produce their own salt in protest of the British tax on salt.
The salt trade was based on one fact — it is more profitable to sell salted foodstuffs than to sell just salt. Thus sources of food to salt went hand in hand with salt making. Before the salt mines of Cheshire were discovered, a huge trade in British fish for French salt existed. This was not a happy accord, for each nation did not want to be dependent on each other. The search for fish and salt led to the Seven Years War between the two. With the British in control of saltworks in the Bahamas and North American cod, their sphere of influence quickly covered the world. The search for oil in the late 1800s and early 1900s used the technology and methods pioneered by salt miners, even to the degree that they looked where salt domes were located for oil.
There are thirty-two references to salt in the Bible, the most familiar probably being the story of Lot's wife, who was turned into a pillar of salt when she disobeyed the angels and looked back at the wicked city of Sodom (Genesis 19:26). Jesus also referred to his followers as the "salt of the earth" (Matthew 5:13), a reference to salt's great value in the ancient world.
Production and use
Nowadays, salt is produced by evaporation of seawater or brine from other sources, such as brine wells and salt lakes, and by mining rock salt, called halite.
While most people are familiar with the many uses of salt in cooking, they might be unaware that salt is used in a plethora of applications, from manufacturing pulp and paper to setting dyes in textiles and fabric, to producing soaps and detergents. In the northern USA, large quantities of rock salt are used to help clear highways of ice during winter
Salt is commonly used as a flavor enhancer for food and has been identified as one of the basic tastes.
Ironically, given its history, this has resulted in large sections of the developed world ingesting salt massively in excess of the required intake, particularly in colder climates where the required intake is much lower.
This causes elevated levels of blood pressure in some, which in turn is associated with increased risks of heart attack and stroke.
Many microorganisms cannot live in an overly salty environment: water is drawn out of their cells by osmosis. For this reason salt is used to preserve some foods, such as smoked bacon or fish. It has also been used to disinfect wounds.
While salt was a scarce commodity in history, industrialised production has now made salt plentiful. About 51% of world output is now used by cold countries to de-ice roads in winter. This works because salt and water form a eutectic mixture that has about a 10°C lower freezing point than pure water: the ions prevent regular ice crystals from forming (below -10°C salt will not prevent water from freezing). Concerns are arising that this use may be harmful to the environment though, and, in Canada, norms were developed to minimise the use of salt in de-icing.
The salt one buys for consumption today is not purely sodium chloride as most people assume. In 1911 Magnesium carbonate was first added to salt to make it flow more freely. In 1924 trace amounts of iodine in form of sodium iodide, potassium iodide or potassium iodate were first added, creating iodized salt to reduce the incidence of simple goiter.
Salt has also had influence on the English language. Many of its effects can still be seen today. Words and expressions related to salt mostly come from the Roman and Greek civilizations when salt was still a valuable commodity.
The Latin word for salt, sal, the French words solde (meaning "pay") and soldier, are all related. In the Italian language soldi means "money", soldato is "a soldier".
Roman soldiers were given a particular allowance to purchase salt (Latin: sal), salarium argentum, from which we take our English word salary. The Romans also preferred salting of their greens, which led to the Latin word for salt being integrated in the word salad (in Vulgar Latin salata literally meant "salted").
Also the expression "He is not worth his salt" can be traced back to ancient Greece where salt was traded for slaves.
It is worth noting that apparently, the English word doesn't come from the Latin word, but they both derive from a common ancestral PIE root-word *sal, which meant 'salt' in the Proto-Indo-European language, which is estimated to have been spoken over eight thousand years ago.
- Due to its high concentration of salt, the Dead Sea has such a high density that some objects which are not normally buoyant can float on its surface. Humans float easily, having a density slightly less than that of pure water. (But only 8% of the salt in the Dead Sea is sodium chloride, 53% is magnesium chloride, 37% potassium chloride).
- The Third Reich stored vast amounts of money, paintings and artworks in salt mines and many important documents and items continue to be stored in former salt mines to this day. They are also used to store nuclear waste.