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Together with water, salt has been the most important substance ever in the history of mankind. Empires have been built thanks to salt. Ways of obtaining common salt. Why can't we drink sea-water?

- Salt is, perhaps, the most important substance in the history of mankind
- It had enormous economic implications: wars, revolutions; it was present in the origin and decline of empires. It influenced the development of civilisation.
- The Phoenitians: trade; they were the first to salt fish.
- The Arabs bartered salt for gold and other goods.
- The Roman Empire: soldiers received part of their wages in salt ( hence the word “salary” from “salarium”).
- Africa: the slave trade was financed with salt.
- Until gas and petroleum made their appearances, salt was considered to be the most solid international currency in this part of the world.

-Common salt is one of the most abundant substances found in Nature. Localisation.
-Its composition; its properties.
-The sea is an inmense deposit of salt in solution.
-The salinity of sea-water.
-Fresh-water coming from the mouths of rivers or melted ice reduce salinity.
-In areas where rainfall is scarce and evaporation is high, salinity is higher.
-Lakes of salt water also exist inland, and they usually have a higher salinity level than open seas.

-Quite apart from needing water, living-beings need to maintain and internal determined balance of salt to survive.
-Plants need determined quantities of common salt.
-Animals need to ingest greater quantities of salt than plants do, so their tissues are saltier.
-Salt intervenes in many different biological processes: it maintains the balance of body fluids within our body vessels, aids our digestion and helps destroy bacteria which may have entered our bodies simultaneously with our food.
-Sodium iones are present in a mother’s milk and participate in the transmission of nervous impulses.
-Its main task is to maintain the osmotic pressure.
-An excess of salt can produce illnesses.

-Two main sources: salt is extracted in its solid state from deposits and some comes directly from sea-water.
-In China brine was extracted from wells. Deep-drilling techniques.
-Salt flats along the coast.
-Ways of eliminating salt from sea-water. Freezing.
-Common salt has more than 14,000 uses.

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