Industrially important compounds of sodium include sodium carbonate, sodium chloride and sodium bicarbonate. Important compounds of calcium are calcium oxide, calcium hydroxide, calcium sulphate, calcium carbonate and cement. These are industrially important compounds.
SODIUM CARBONATE (WASHING SODA), Na2CO3.10H2O
Sodium carbonate is generally prepared by Solvay Process. In this process, advantage is taken of the low solubility of sodium hydrogencarbonate whereby it gets precipitated in the reaction of sodium chloride with ammonium hydrogen carbonate. The latter is prepared by passing CO3 to a concentrated solution of sodium chloride saturated with ammonia, where ammonium carbonate followed by ammonium hydrogen carbonate are formed.
Sodium hydrogen carbonate crystal separates. These are heated to give sodium carbonate.
In this process NH3 is recovered when the solution containing NH4Cl is treated with Ca(OH)3. Calcium chloride is obtained as a by-product.
It may be mentioned here that Solvay process cannot be extended to the manufacture of potassium carbonate because potassium hydrogen carbonate is too soluble to be precipitated by the addition of ammonium hydrogen carbonate to a saturated solution of potassium chloride.
PROPERTIES
Sodium carbonate is a white crystalline solid which exists as a decahydrate, Na3CO3.10H3O. This is also called washing soda. It is readily soluble in water. On heating, the decahydrate loses its water of crystallisation to form monohydrate. Above 373K, the monohydrate becomes completely anhydrous and changes to a white powder called soda ash.
Carbonate part of sodium carbonate gets hydrolyzed by water to form an alkaline solution.
USES:
(i) It is used in water softening, laundering and cleaning.
(ii) It is used in the manufacture of glass, soap, borax and caustic soda.
(iii) It is used in paper, paints and textile industries.
(iv) It is an important laboratory reagent both in qualitative and quantitative analysis.
SODIUM CHLORIDE, NaCl
The most abundant source of sodium chloride is sea water which contains 2.7 to 2.9% by mass of the salt. In tropical countries like India, common salt is generally obtained by evaporation of seawater. Approximately 50 lakh tons of salt are produced annually in India by solar evaporation. Crude sodium chloride, generally obtained by crystallisation of brine solution, contains sodium sulphate, calcium sulphate, calcium chloride and magnesium chloride as impurities. Calcium chloride, CaCl3, and magnesium chloride, MgCl3 are impurities because they are deliquescent (absorb moisture easily from the atmosphere). To obtain pure sodium chloride, the crude salt is dissolved in a minimum amount of water and filtered to remove insoluble impurities. The solution is then saturated with hydrogen chloride gas. Crystals of pure sodium chloride separate out. Calcium and magnesium chloride, being more soluble than sodium chloride, remain in solution. Sodium chloride melts at 1081K. It has a solubility of 36.0 g in 100 g of water at 273 K. The solubility does not increase appreciably with increase in temperature.
USES : (i) It is used as a common salt or table salt for domestic purposes.
(ii) It is used for the preparation of Na3O3, NaOH and Na3CO3.
CALCIUM OXIDE OR QUICK LIME, CaO
It is prepared on a commercial scale by heating limestone (CaCO3) in a rotary kiln at 1070-1270 K.
The carbon dioxide is removed as soon as it is produced to enable the reaction to proceed to completion. Calcium oxide is a white amorphous solid. It has a melting point of 2870 K. On exposure to atmosphere, it absorbs moisture and carbon dioxide
The addition of a limited amount of water breaks the lump of lime. This process is called slaking of lime. Quick lime slaked with soda gives solid sodalime. Being a basic oxide, it combines with acidic oxides at high temperature.
USES: (i) It is an important primary material for manufacturing cement and is the cheapest form of alkali.
(ii) It is used in the manufacture of sodium carbonate from caustic soda.
(iii) It is employed in the purification of sugar and in the manufacture of dye stuff.
CALCIUM HYDROXIDE (SLAKED LIME), Ca(OH)2
Calcium hydroxide is prepared by adding water to quick lime, CaO. It is a white amorphous powder. It is sparingly soluble in water. The aqueous solution is known as lime water and a suspension of slaked lime in water is known as milk of lime. When carbon dioxide is passed through lime water it turns milky due to the formation of calcium carbonate.
On passing excess of carbon dioxide, the precipitate dissolves to form calcium hydrogencarbonate
Milk of lime reacts with chlorine to form hypochlorite, a constituent of bleaching powder.
USES: (i) It is used in the preparation of mortar, a building material.
(ii) It is used in white wash due to its disinfectant nature.
(iii) It is used in glass making, in the tanning industry, for the preparation of bleaching powder and for purification of sugar.
CALCIUM CARBONATE, CaCO3
Calcium carbonate occurs in nature in several forms like limestone, chalk, marble etc. It can be prepared by passing carbon dioxide through slaked lime or by the addition of sodium carbonate to calcium chloride.
Excess of carbon dioxide should be avoided since this leads to the formation of water-soluble calcium hydrogencarbonate. Calcium carbonate is a white fluffy powder. It is almost insoluble in water. When heated to 1200 K, it decomposes to evolve carbon dioxide
It reacts with dilute acid to liberate carbon dioxide.
USES: It is used as a building material in the form of marble and in the manufacture of quicklime. Calcium carbonate along with magnesium carbonate is used as a flux in the extraction of metals such as iron. Specially precipitated CaCO3 is extensively used in the manufacture of high-quality paper. It is also used as an antacid, mild abrasive in toothpaste, a constituent of chewing gum, and a filler in cosmetics.
CONCLUSION
Sodium carbonate is generally prepared by Solvay Process.
The most abundant source of sodium chloride is sea water which contains 2.7 to 2.9% by mass of the salt. In tropical countries like India, common salt is generally obtained by evaporation of seawater.
It is prepared on a commercial scale by heating limestone (CaCO3) in a rotary kiln at 1070-1270 K.
Calcium hydroxide is prepared by adding water to quick lime, CaO.
Calcium carbonate occurs in nature in several forms like limestone, chalk, marble etc. It can be prepared by passing carbon dioxide through slaked lime or by the addition of sodium carbonate to calcium chloride.
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