Salt is as old as civilization. Its chemical formula is sodium chloride. Salt is necessary for life, and in some cultures the regular diet does not provide enough. Settlements and commercial operations throughout history have located at sources of salt.
Roman legionaries were famously paid in salt, from which we get the word salary (Latin salarium). Salt was also a currency in sub-Saharan Africa and elsewhere. Trade routes have developed to carry salt, such as the Via Salaria between Rome and its seaport.
As a major commodity, salt has been taxed. As a tax on a necessity, this has been hated. The British ruling India taxed salt, which Mahatma Gandhi defied in 1930 by marching to the sea and taking his own salt.
Seawater contains 3.5% salt, more or less. If the water is evaporated, most of what remains is salt, which can be used as is or purified. Salt is also present in the lakebeds of ancient lakes, one example being the Bonneville Salt Flats of Utah. A salt dome is an underground or exposed upthrusting of salt rock formed by geologic processes. All of these are and have been sources of salt.
Properties and Uses
Salt forms cubic clear crystals. As a mineral, it is called halite or rock salt. Salt is obviously edible and is one of the five basic tastes. Table salt is ubiquitous in the human diet, and as a result is used to deliver various nutritional additives. Potassium iodide (for iodine) is the most common additive to consumer salt, but folic acid (vitamin B9), fluoride, and iron salts can all be found.
Salt dissolves readily in water, and saltwater differs from freshwater in several ways. Saltwater freezes below the freezing point of fresh water (0 °C); Daniel Fahrenheit set the zero of his temperature scale at that point. This is the reason that rock salt added to roads in winter can melt the ice. Saltwater is also more conductive than fresh water, which is why radio waves do not penetrate.
As a chemical, salt is the main source of its two components, in the form of sodium hydroxide (lye) and chlorine gas. Salt is the feed stock for a great deal of industrial chemistry.
The sodium in salt works with potassium in the body to regulate communication between nerve cells. Sodium also regulates water retention, and too much sodium can cause high blood pressure. Excessive sodium is a real possibility in Western diets.