Chemistry

Salt is a chemical compound with a number of interesting properties:

  • Crystals or white crystalline powder.
  • Transparent and colourless in crystalline form – rather like ice.
  • Crystallises in the isometric system, usually in the form of cubes.
  • Soluble in water (35.6g/100g at 0°C and 39.2g/100g at 100°).
  • Slightly soluble in alcohol, but insoluble in concentrated hydrochloric acid.
  • Melts at 801°C and begins to vaporize at temperatures just slightly above this boiling point 1,413°C.
  • Hardness of 2.5 on the MOH scale of hardness.
  • Specific gravity of 2.165.
  • Non-combustible – low toxicity.
  • Hygroscopic – absorbs moisture from damp atmospheres above 75 per cent relative humidity – below this, it will dry out.

In its natural form, salt often includes traces of magnesium chloride, magnesium sulphate, magnesium bromide, and others. These impurities can tint the otherwise transparent crystals, yellow, red, blue or purple.

Chemicals from Salt

When an electric current is passed through a strong solution of salt in water, electrolysis occurs and three products are formed:

  • Chlorine (Cl2)
  • Sodium hydroxide (NaOH)
  • Hydrogen (H2).

Because hydrogen and chlorine gases form an explosive mixture, it is important to keep them separated. All three products are useful individually and they can also be combined together to make further products. Sodium hydroxide and chlorine combine to form sodium hypochlorite solution which is widely used in the home as domestic bleach. A stronger solution of sodium hypochlorite is used as a dairy and industrial disinfectant.

Under different reaction conditions, sodium hydroxide and chlorine will react to form sodium chlorate. This is produced as white crystals which can be highly explosive or inflammable if mixed with organic matter. Solutions of sodium chlorate are widely used as a herbicide.

When chlorine gas is burned in hydrogen, the two gases react to form hydrogen chloride. The hydrogen chloride dissolves in water to form hydrochloric acid. Hydrochloric acid made in this way is very pure, and can be used safely in the food and pharmaceutical industries.

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