Sodium chloride occurs very extensively in nature as rock salt or halite deposits, which consist of crystalline sodium chloride often discolored by impurities. It is also present in sea-water with an average concentration of 3 percent, but in the Dead Sea up to 7.2 percent. The rock salt deposits are usually mined, or occasionally, water is pumped down the brine, containing about 25 percent sodium chloride, brought to the surface. The brine is evaporated by vacuum evaporators; impurities separate first and can be removed. In warm climates, salt is obtained by evaporation of shallow sea water by the sun, to give ‘bay salt’.

Pure sodium chloride is prepared by passing hydrogen chloride gas into a saturated solution of impure salt; the pure compound is precipitated.

Properties: sodium chloride crystallizes in colourless transparent cubes; the crystals have a simple cubic lattice. Sodium chloride melts at 804˚C and can be vaporized at high temperatures; the vapour is mainly composed of ‘ion-pairs’, Na⁺Cl^-, but contains a few discrete ions, Na⁺ and Cl^-. Its solubility changes very little with temperature, 35.5g at 0˚C, and 39 g at 100˚C. A solution containing 23.6 percent of sodium chloride forms a eutectic, freezing at -23˚C; this is therefore the lowest temperature obtainable with an ice and salt freezing mixture.

Ordinary common salt is usually somewhat deliquescent owing to small amounts of magnesium chloride present as impurity. Sodium phosphate may be added to convert this into magnesium phosphate, which is not deliquescent; the salt then ‘runs’ better.

Uses: sodium chloride is a constituent of many foods, being added for flavouring and curing. Electrolysis of sodium chloride solution is used to produce caustic soda and chlorine, sodium hypochlorite and sodium chlorate. If salt is thrown into a furnace where clay is being fired to make earthenware, it volatilizes and reacts with the silicate:

2NaCl + SiO₂ + H₂O Na₂SiO₃ + 2HCl

These two salts can be prepared in the laboratory by mixing solutions of sodium hydroxide and dilute sulphuric acid, and evaporating to crystallizing point. If equivalent amount of base and acid are used, the normal sulphate is formed:



With two equivalents of acid to one of base, the bisulphate is obtained:

NaOH + H₂SO₄ NaHSO₄ + H₂O

The bisulphate dissolves in water dissociating to give the ions Na⁺, H⁺ and SP₄^2-; it is therefore acidic in solution, and in fact, behaves very much like a solution of dilute sulphuric acid. Normal sodium sulphate is neutral and is not hydrolysed in solution.

Sodium bisulphate is a by-product in the manufacture of nitric acid from sodium nitrate, and of hydrochloric acid from sodium chloride:

NaNO₂ + H₂SO₄ NaHSO₄ + HNO₃

NaCl + H₂SO₄ NaHSO₄ + HCl   

The normal sulphate can be obtained by heating the bisulphate with sodium chloride in a furnace:

NaCl + NaHSO₄ Na₂SO₄ + HCl

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