Salt (Sodium Chloride)

Common salt is the chemical compound sodium chloride, formula NaCl. Each tiny crystal of salt is built from a regular pattern of sodium ions (Na+) and chloride ions (Cl-) locked together by powerful electrical attraction. Salt is so woven into human life that the words "salary" and "soldier" both come from Latin words for paying people in salt. Today salt is cheap and used by the tonne in cooking, food preservation, road de-icing, water softening and the chemical industry. It is also one of the few rocks you can eat.

  • FormulaNaClSodium + chloride
  • Crystal shapeCubeVisible under a magnifying glass
  • Made fromSodium (Na) + chlorine (Cl)Two dangerous elements, safe compound
  • Melts at801 CHotter than a campfire
  • In sea water35 g per litre3.5 per cent by weight
  • Daily intakeAround 5 g recommendedMost people eat more

How salt is made

Salt is formed when the metal sodium reacts with the gas chlorine. Both pure elements are extremely dangerous:

  • Sodium is a soft, silvery metal that bursts into flame in water.
  • Chlorine is a yellow-green poisonous gas used as a weapon in World War One.

Yet when these two react, sodium gives one electron to chlorine. Each sodium atom becomes a Na+ ion. Each chlorine atom becomes a Cl- ion. The two ions stick together in a 1:1 ratio, building rigid cubes of solid salt. This is a perfect example of ionic bonding.

The result is a safe, edible white solid that we sprinkle on chips. Chemistry can be astonishing.

Where salt comes from

Most salt today comes from three sources:

  • Sea salt: produced by evaporating sea water in shallow pans, usually in hot dry places like the Mediterranean, India and parts of Africa.
  • Rock salt: mined from underground deposits left behind when ancient seas dried up. The largest mines are in Ukraine, Russia, the USA and Poland. The salt mines of Wieliczka in Poland have been worked for nearly 1,000 years and are now a UNESCO World Heritage site.
  • Brine wells: water is pumped down to dissolve underground salt deposits, then the salty water is pumped back up and evaporated.
Fact The "salt of the earth" is no joke. The total amount of salt in the worlds oceans is about 50 million billion tonnes. If you spread it all over Earths land, it would form a layer about 150 metres thick. That is taller than a 40-storey skyscraper.

Salt in cooking

Salt has been the most important kitchen ingredient for thousands of years.

  • Flavour: salt enhances the natural flavour of almost any food. A pinch can transform a dull dish.
  • Preservation: salt sucks water out of meat, fish and vegetables, stopping bacteria from growing. Before refrigerators, this is how people stored food through winter.
  • Texture: salt strengthens the gluten in bread dough and helps hold meat juices in during cooking.
  • Brining: soaking meat in salty water for hours before cooking makes it juicier and more tender.

Salt in your body

Your body needs sodium and chloride to work properly. Sodium helps:

  • Carry nerve signals along your nerves
  • Keep your muscles (including your heart) contracting properly
  • Balance the amount of water in your body

You lose salt through sweat, tears and urine, so you need to replace it. A typical adult only needs about 5 grams of salt a day (about a teaspoon). Most people in rich countries eat 9 or 10 grams a day, mostly hidden in processed food. Too much salt is linked to high blood pressure and heart disease. That is why doctors and the NHS advise checking food labels.

Did you know? Roman soldiers were sometimes paid in salt, or with a small allowance to buy it. The Latin word for salt is sal, which is where the words "salary" and "soldier" come from. Salt was also so valuable in some places that it was traded gram-for-gram with gold along ancient trade routes across the Sahara.

Other uses of salt

  • De-icing roads: rock salt is spread on roads in winter. Salt lowers the freezing point of water, so ice melts even at slightly below 0 degrees Celsius.
  • Water softening: hard water is run through resin beads that swap calcium ions for sodium ions, leaving softer water behind. Salt is used to "regenerate" the beads when they run out of sodium.
  • Making chlorine: industry passes electricity through salt water (a process called electrolysis) to make pure chlorine gas, sodium hydroxide and hydrogen. These three chemicals are the starting point for thousands of others.
  • Salt-cured leather: traditional tanneries pack animal hides in salt to dry them out before tanning.
  • Salt baths: dissolved salt in a warm bath relaxes muscles and may help skin conditions.
  • Aquariums: salt-water tanks need very precisely measured amounts of mixed salts to keep marine fish healthy.

Why a salt crystal is cubic

Look at a few grains of table salt through a magnifying glass and you will see they are almost perfect tiny cubes. This shape comes directly from the way the ions are arranged. Each sodium ion is surrounded by 6 chloride ions, and each chloride ion is surrounded by 6 sodium ions. They form a perfectly repeating 3D grid called a cubic lattice. Salt crystals naturally grow into cubes because this is the most efficient way to stack the lattice.

Try this Grow a salt crystal at home. Dissolve as much salt as you can into a glass of hot water (keep adding and stirring until no more will dissolve). Tie a piece of cotton thread around a pencil and let the thread hang down into the salt water without touching the bottom. Place the glass somewhere warm and undisturbed. Over the next few days, as the water slowly evaporates, beautiful tiny cubic crystals will grow on the thread. Look at them through a magnifying glass and you will see the cubic structure of NaCl up close.
Deeper dive: the salt mines that built Europe

Several great medieval cities of Europe owe their wealth to salt mining. Without it, much of European history would have unfolded differently.

In Austria, the town of Salzburg (literally "salt castle") was named after the giant rock-salt deposits in the nearby mountains. The salt was so important that the prince-archbishops of Salzburg grew enormously wealthy from controlling its trade. Money from salt paid for the famous baroque palaces, churches and gardens. It also made Salzburg rich enough to support a remarkable family of musicians: the young Mozart was born there in 1756.

In Poland, the Wieliczka salt mine just outside Krakow operated continuously from the 1200s until 1996. Generations of miners carved out vast underground chambers and even decorated them: chapels, statues, chandeliers and life-sized salt sculptures of saints, all carved from rock salt. The mine is now a museum visited by over a million people a year.

In northern Germany, the Hanseatic city of Lubeck built much of its medieval wealth on the salt-and-fish trade. Salt from the inland mines of Luneburg was shipped to the Baltic coast, used to preserve herring, and traded across the North Sea. The "salt road" between Lubeck and Luneburg was one of the most travelled trade routes in medieval Europe.

Salt also shaped wars. Napoleons French armies struggled in Russia partly because they ran out of salt to preserve food. The American Revolution and the American Civil War both featured battles for control of salt supplies. Cheap chemical-industry salt has made these wars unimaginable today, but for most of human history, controlling salt meant controlling food, money and power.

For more, see ions and chemical bonds.