Common salt this is also called as sodium chloride. This is one of the important substance used in our daily life. It gives taste to food. Sodium chloride is available in sea water to an extent of 2.8%. It is also available as rock salt in earth crust. Deposits of rock salt are found in Punjab, Himachal Pradesh and Rajasthan in our country.
Production of common salt from sea-water:
Common salt is manufactured from sea water by evaporation of water by solar heat and wind. The evaporation is carried out in wide flat ponds called salt meadows. These meadows are constructed at the sea shores on special grounds. The ground should be free from sand and contain clay which prevent the percolation of sea water during evaporation. If the meadows are below the sea water level, tides push the water into meadows. Other wise the sea water is pumped in to the meadows. The sea water is pumped first in to the deposit meadows and allowed to evaporate for some days. The concentrated salt solution is pumped into less deeper meadows. Again this solution is evaporated. This process is repeated till sodium chloride starts separating as crystals.
Purification of common salt:
Common salt obtained from sea-water contains insoluble impurities such as clay and sand and soluble impurities like sodium sulphate, calcium chloride and Magnesium chloride. This salt is purified by two methods.
Method-1
The salt is dissolved in sufficient quantity of water to get a high concentrated solution. Calculated quantities of sodium
carbonate and calcium hydroxide are added to this solution. Impurities such as magnesium and calcium salts are present in salt react with these substances and form in soluble calcium and magnesium carbonates. The insoluble substances are filtered off and the filtrate is evaporated to get pure crystals of sodium chloride. This may contain traces of sodium sulphate.
Method-2
Common salt of highest purity can be obtained by the following method. Impure common salt is dissolved in water and hydrogen chloride gas is bubbled into the solution. This decreases the solubility of sodium chloride and so it separates out as crystals. The impurities remain in solution. The crystals are separated by filtration and seived for grading.
Uses of Common salt:
* To give taste to food.
* For storage of meat and fish and to make pickles.
* To prepare freezing mixture with ice to get temperatures below 0 c.
* In the manufacture of chlorine, sodium metal and hydrochloric acid, Caustic soda, washing soda and sodium sulphate.
* In soap industry for salting out soap.
* In tanning industry as preservative of animal leather.