salt
To most people, the word 'salt'
means the substance we use in
cooking and on food. But to a
scientist, it is used to describe a
large number of chemical sub-
stances including common salt.
Salts are formed when the hydrogen in an acid is replaced by a metal. All salts are made up of crystals. In the case of common salt (table salt),these crystals are white and cube-shaped.Common saltSodium chloride (common salt) is the most useful mineral in the world.It has been estimated that there are 16,000 uses for it. Apart from being used in cooking and to make other
chemicals, common salt is used in the food industry to preservemeat, fish and vegetables, as well as formaking such foods as butter and margarine. Salt is used in the manufacture of dyes, paper, pottery,leather and many medicines. Salt is also essential for keeping roads and pavements free of ice and snow in winter. Pure water freezes at 0°C (32°F), but brine (salty water) freezes at a much lower temperature.The salt melts the ice and snow and prevents it freezing again.In addition, salt is essential, in small quantities, for the proper working of the body. We have to eat salt, because the body loses it all the time. For example, sweat contains
salt, which is why it tastes salty.

Rock salt and sea salt

There is plenty of salt in the world, but it is not always easy to get at.'Rock salt' comes from thick layers deep underground. These were formed long ago when prehistoric seas dried up.There are famous salt mines in Canada, Poland, Siberia in the USSR, and Cheshire, England.
Sometimes rock salt is mined by
digging for it underground. Usually, however, water or steam is pumped down the mine, forming brine. This is pumped to the surface, where the
water is evaporated, leaving the salt. In some countries with hot sunshine,salt is obtained by trapping sea water in shallow pans or lagoons. 

When the sea water evaporates, ‘sea salt’ is left behind. Pure salt melts at 800°C.Brine (a concentrated solution of salt in water) freezes at temperatures as low as -20°C.water. Sea water contains about 100 gm of salt in 5 litres of water.  

The Dead Sea is a very salty lake 470 m below sea level. It contains about 800 gm of salt in 5 litres of water.

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