Chemistry and physics
In its broader sense, "salt" is also the name for a substance that is created in a reaction between an acid and a base. Characteristic features of salts are ionic bonds, high melting points, electrical conductivity in melts and solutions and a crystalline structure.
Common salt, sodium chloride
Salt, consisting of the metallic element sodium and the gaseous element chlorine, is a white solid which dissolves in hot and cold water. Common salt does not dissolve easily in alcohol and it is insoluble in concentrated hydrochloric acid. Common salt is widespread in nature. Vast quantities of it are present in the sea, which contains an average of about 35 grammes per litre of water. Many rivers, lakes and inland seas also contain common salt.
The rock salt deposits formed throughout the earth’s history, which have been known for about 600 million years, are inexhaustible.
Sodium
Symbol Na; a highly reactive, silver-white soft metallic element with atomic number 11. It belongs to the first main group of the periodic system and is therefore one of the alkaline metals. The metal was discovered in 1807 by the British chemist Sir Humphry Davy. Elemental sodium is so soft that it can be cut with a knife. When exposed to air, it turns matt grey within a few seconds (due to a layer of sodium hydroxide). Sodium enters into a violent chemical reaction with water, releasing sodium hydroxide and hydrogen. There is a risk of explosion when it reacts with halogens or chlorinated hydrocarbons.
In the order of frequency of elements occurring in the earth’s crust, sodium is in seventh position. About 2.5% of the uppermost layer of the earth’s crust consists of chemically bonded sodium. Another characteristic of this element is that it is an essential component of living organisms. For instance, sodium plays a key part in transmitting stimuli along the neural pathways.
Chlorine
Symbol Cl; a yellowish-green gas. Together with fluorine, bromine, iodine and astatine, it belongs to the halogen group. Elemental chlorine was isolated for the first time in 1774 by the Swedish chemist Carl Wilhelm Scheele. Chlorine is gaseous at room temperature, but it can easily be liquefied by increasing the pressure.
The gas has a pungent odour and is very poisonous in higher concentrations. Chlorine does not occur in nature it its elemental form. Its compounds are mineral substances (mostly chlorides) which are frequently encountered. It is in 20th position in the frequency ranking for elements found in the earth’s crust. Chlorine reacts easily with many substances such as water, organic compounds and numerous metals.
The main technical methods of obtaining chlorine involve the electrolysis of aqueous salt solutions (sodium chloride and also potassium chloride). Alkaline lye is obtained as a by-product of this process (chlorine-alkaline electrolysis).
