Common Salt (Sodium Chloride) Content from the guide to life, the universe and everything

Common Salt (Sodium Chloride)

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A salt cellar.

Table salt, the white stuff in your salt cellar, is the most commonly known form of salt. In chemistry it is called sodium chloride and has the symbol NaCl. The use of this as a seasoning for food is known to most, but many don't see the great importance and multiple uses of this simple substance. 'Sea salt', also known as 'bay salt', is produced by evaporation of sea water, and contains other salts beside sodium chloride, for example magnesium chloride, magnesium sulphate and sodium sulphate. These components impart a somewhat different taste compared to table salt, which is sometimes preferred by gourmet chefs.

Sodium chloride, which is formed by the reaction between hydrochloric acid and sodium hydroxide, consists of a sodium ion  — the cation, chemical symbol Na+, which has a positive charge — and a negatively-charged chloride ion, the anion, chemical symbol Cl-. Sodium is located in group 1 — the alkaline metals — in the periodic table and has a relative atomic mass of 22.99. Due to its positive nature Na+ is readily attracted to the negative chloride ion. The chemical element chlorine is located in group 17 — the halogens — in the periodic table, with a relative atomic mass of 35.45.

The attraction between the sodium and chlorine ions causes them to come together in such a manner as to produce an ionic bond. Large numbers of these ions come together to form a giant ionic lattice, for which the symbol is NaCl. This compound consists of 60.663% by weight of Cl and 39.337% Na. Since the compound results from the chemical reaction of a metal with a non-metal the ending 'ide' is given to the chlorine, to give the name sodium chloride. Alone, both sodium and chlorine are poisonous and highly reactive with oxygen, but the combination of the two forms a neutral, safe compound. When dissolved in water the NaCl completely dissociates, or breaks up into its separate Na+ and Cl- ions, and forms a solution with a neutral pH. NaCl forms colourless cubic crystals, melts at 801°C and boils at about 1,450°C. It has a density of 2.170 kg/l. The solubility of NaCl in water is 35.9 g/100 ml at 25°C.

Salt can be a corrosive element: copper is frequently used for roofing, and the metal is sufficiently reactive to be attacked by chemicals present in the atmosphere, such as salt from sea-spray. The distinctive green patina of copper can be seen in the panoramic view of some European cities. This oxidation also happens to bronze statues, giving them a fine greenish crust.

History of Salt

Salt was the first garnish or additive used with food. Primitive man obtained his daily dietary requirement of salt by consuming animal blood. As humans moved away from being hunter-gatherers and towards agriculture and the domestication of animals, he became aware of the preservative properties of salt. Salt creates a hostile environment for micro-organisms by dehydrating their cells. This observation freed early farmers from dependency on seasonal availability of certain foodstuffs, and hence the demand for salt increased. It is therefore no accident that Man's earliest civilisations, for example, the Sumerians, emerged in arid (desert) regions, close to known salt deposits, despite the obvious hardships of living in such places. This is because, as salt is water-soluble, it dissolved in rain water and was flushed out of the soil to give rise to salty seas and lakes. Early Man didn't have the technology to extract this. However, salt could be obtained from exposed rock outcrops, or from dried out former lagoons or from the shores of seas and salt lakes, present in desert regions where the rainfall is low.

Documentation of the use of salt can be dated back to China in 2700 BC. Pong-Tzao-Kan-Au noted the relative importance and use of salt, as well as distinguishing between 40 different kinds. The Chinese obviously felt the same about salt as the Inuit did about snow!

Salt was used in ancient Greece for trading slaves. Europeans brought salt on their expedition to the Americas, where it was found in abundance, and called 'white gold'. Used by early native Americans in Canada and Mexico, salt was one of the greatest economically important substances for many years, often used in the form of money and held as a high commodity. There are 30 references made to salt in the Bible  — possibly the most memorable is when Lot's wife Edith looked back upon the destruction of Sodom and was turned into a pillar of salt. What people probably/possibly saw was a salt floe which had been sculpted by the waves of the extremely salty Dead Sea, formed into a human shape.

There is an old saying that a man was 'worth his weight in salt' because when an essential commodity is scarce it increases its value. Romans valued salt so highly that their soldiers were given a salt allowance, or actually paid in it; the modern word 'salary' (Salarium argentum, meaning 'salt money') is a derivative of this. The modern abbreviation for pound weight is lb. This comes from the word 'libra' (Latin for 'balance') as scales were used to measure out a Roman soldier's pay — a pound1 of salt. The British currency symbol £ is an elaborate 'L' for the same reason: it is the initial letter of Libra.

Nomads were said to have spread the use of salt westward to the European countries such as France, The Netherlands, Spain, Italy, then on to West Africa and South Africa, through trade. In Japan they scrub their bodies with salt before partaking of a sauna. Their national sport of Sumo wrestling involves throwing salt into the ring for purification purposes before the combatants enter it.

Where is Salt Found?

Salt is found naturally all over the world. Salt can be found in oceans and sea water as well as in underground salt deposits2. Salt can be found in great beds left behind when salt-water lakes and inland seas have completely evaporated; the Dead Sea is a present-day example of this process in action. Biologically it is on the skin of animals, in sweat glands, in tears, and excreted by the body as a major component of urine. In fact humans are salty creatures; blood, sweat, tears, nasal excretions and semen all taste salty. Salt is found in meteors from space, and has been found on Mars. It is believed that, because of its presence, one of the conditions for life, namely large quantities of surface water, may once have existed on Mars, although it is no longer present.

The Cheshire Salt Mine and Museum

They were mining salt in the county of Cheshire in England long before the Romans invaded, and it is the only large-scale producer of salt in Britain today. At one time you could go on a tour of the working mine but that is no longer possible now. There has been a salt museum at London Road Northwich for over a century. The museum - now called the Weaver Hall Museum and Workhouse - is housed in the old Brunner Library3 building, which used to be the old Northwich Workhouse. If you visit the museum, don't forget to pop round the back and see the marine engine.

Ingesting Salt

First and foremost it must be understood that humans as well as animals cannot live without salt, more specifically, without the sodium in salt. Sodium maintains the extracellular and intracellular pressure in a cell as well as maintaining ion concentrations, and the sodium ion maintains blood pressure.

The chloride ion functions to maintain acid base balance and concentrations in the body so as to keep the pH at an ideal level so that the enzymes in the body are able to function at their optimum rate. The chloride ion also enhances the blood's ability to carry carbon dioxide through the bloodstream and out of the body.

The body cannot manufacture salt and must get it in some other way, always by consumption. Salt is a great additive to food for flavour such as in the recipe for a uniform bread dough; however, it shouldn't be added while cooking and baking where the recipe doesn't call for it. Let the person eating the meal you have cooked add their own salt to taste.

Salt is considered an essential accompaniment to raw tomatoes and some people can't eat an omelette, boiled or scrambled eggs without a pinch of salt. And edging the cocktail glass with salt is an extremely important ingredient in the preparation of a perfect Margarita.

The average person should be responsible for their own health and this means being careful what they eat; processed ready-made meals probably contain or even exceed the total salt required for daily needs. Many adults in the developed world eat as much as 10g of salt per day due to processed foods with high salt content. Such ready meals are required by law to display their nutritional value and contents, so do check the salt content before purchase.

We are being advised by health representatives to reduce our daily salt intake, and dieticians recommend we avoid sea salt in particular, to minimise raised blood pressure, which can lead to heart disease and/or stroke. Guideline daily amounts (GDA) for a typical adult rate salt as between 4g (USA) and 6g (UK) per day. It's possible to get the full GDA by eating many foods which don't taste in the least salty. Adding salt is almost invariably unnecessary except for taste. Always remember — too much salt would be poisonous.

Some people now react to salt in the same way that the body reacts to hard drugs, which suggests that salt need and cravings may be linked to the same brain pathways as addiction and drug abuse. There definitely is a feel-good factor with some processed foods, which can have a similar effect of an antidepressant! Researchers have tried to promote the unhealthiness of eating regularly in fast food restaurants, but as always, moderation is key.

It's quite shocking to recall that once upon a time a trip to McDonald's meant witnessing the french fries being routinely doused with liberal doses of salt prior to delivery to the customer. No doubt the policy was to ensure the diner became thirsty enough to order another drink before they left the premises, but the practice is now banned. No-one wants to be a killjoy and the occasional treat after a cinema trip is enjoyable for all the family.

The Salt-eating Elephants of Kenya

An adult elephant requires 100g of salt daily just to stay alive. A BBC Natural History Unit documentary recorded the path of an elephant herd which followed an ancient well-worn trail to a cave on Mount Elgon, an extinct volcano on the Ugandan border of Kenya. Linking trunk to tail, the huge beasts head underground through the precariously steep descent, the leader and all the followers guiding their fellow behind with utmost care into the deeper pitch black. When they arrive at the salt-drenched walls they lick their prize and also use their tusks to mine for more. The fascinating story is one of the episodes in Sir David Attenborough's  Life of Mammals.

Other Uses of Salt

  • Salts are used in highway safety: it is cheap and abundant and melts snow and ice readily by lowering the freezing point of water. Before predicted snow storms rock salt is usually spread on roads and paths in order to prevent dangerous slippery conditions for motorists and pedestrians.

  • Heat storage: (molten salt) in solar collection systems.

  • Farmers fill their rear tractor tyres with salt water (because it's heavier than pure water) in order improve traction.

  • Add salt to a pan of water to raise the boiling point when you're making hard-boiled eggs.

  • Most people know that salt is important in the art of stain removal, eg spilled red wine and blood. But not many know that you can vanish tarnish with it!

  • Some exasperated gardeners are tempted to use salt to kill pests such as slugs and snails but this is not advisable as the build-up of salt in the soil will eventually kill the plants you are trying to protect. It would be better to read the tips in the Entry on natural slug control.

Salt Wisdom

'Salt of the Earth' is a description of someone who can be depended upon. It is mentioned in the Bible, in Matthew 5.13 among others.

'Adding salt to a wound' is a metaphor for making things worse, which can be used in almost any situation. An 'old salt' is slang for a retired seaman who is usually a reliable source for stories, however, many such tales you hear can be taken with 'a pinch of salt'. That saying itself probably derived from non-superstitious people who don't think ancient folklore has any basis in fact. For example superstitious types believe that if you spill salt you should gather some and throw it over your left shoulder in order to ward off the devil sat there, and he wouldn't like an eyeful of salt (who would?) so would fall off. Better remember to sweep up the spilt salt after your good-luck routine though, or the housework  fairy will be on your trail!

There is a fairy tale about a magic salt mill at the bottom of the sea, which is why sea water is always salty...

Salty EG Entries

1A Roman pound was about three quarters of a modern pound.2Halite is the geological term for NaCl, therefore it's the techy term for rock salt.3Salt proprietors John Brunner and Thomas Ward donated the Northwich Salt Museum and Library to Northwich in 1889.

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