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Calcium and its compounds

Basalt is a fine-grained basic igneous rock. It is composed chiefly of calcium-rich plagioclase feldspar and pyroxene; other minerals present may be olivine, magnetite, and apatite. Basalt is the commonest type of lava.

Calcium is a soft grey metallic element belonging to group 2 (formerly 11 A) of the periodic table: a.n. is 20; r.a.m. is 40.08; r.d. is 1.54; m.p. is 839°C; b.p. is 1484°C. Calcium compounds are common in the Earth's crust, e.g. limestone and marble (CaCO₃), gypsum (CaSO₄.2H₂O) and fluorite (CaF₂). The element is extracted by electrolysis of fused calcium chloride and is used as a getter in vacuum systems and a deoxidizer in producing nonferrous alloys. It is also used as a reducing agent in the extraction of such metals as thorium, zirconium, and uranium.

Calcium is an essential element for living organisms, being required for normal growth and development. In animals it is an important constituent of bones and teeth and is present in the blood, being required for muscle contraction and other metabolic processes. In plants it is a constituent (in the form of calcium pectate) of the middle lamella.

Calcium carbonate is a white solid, CaCO₃, which is only sparingly soluble in water. Calcium carbonate decomposes on heating to give calcium oxide (quicklime) and carbon dioxide. It occurs naturally as the minerals calcite ( its shape is rhombohedral; r.d. is 2.71) and aragonite ( its shape is rhombic; r.d. is 2.93). Rocks containing calcium carbonate dissolve slowly in acidified rainwater (containing dissolved CO₂) to cause temporary hardness. In the laboratorycalcium carbonate is precipitated from limewater by carbon dioxide. Calcium carbonate is used in making lime (calcium oxide) and is the main raw material for the Solvay process.

Calcium chloride is a white deliquescent compound, CaCl₂, which is soluble in water; r.d. is 2.15; m.p. is 782°C; b.p. is >1600°C. There are a number of hydrated forms, including the monohydrate, CaCl₂.H₂O, the dihydrate, CaCl₂.2H₂O (r.d. is 0.84) and the hexahydrate, CaCl₂.6H₂O (trigonal; r.d. is 1.71). The hexahydrate loses 4H₂O at 30°C and the remaining 2H₂O at 200°C. Large quantities of it are formed as a byproduct of the Solvay process and it can be prepared by dissolving calcium carbonate or calcium oxide in hydrochloric acid. Crystals of the anhydrous salt can only be obtained if the hydrated salt is heated in a stream of hydrogen chloride. Solid calcium chloride is used in mines and on roads to reduce dust problems, whilst the molten salt is the electrolyte in the extraction of calcium. An aqueous solution of calcium chloride is used in refrigeration plants.

Calcium fluoride is a white crystalline solid, CaF₂; r.d. is 3.2; m.p. is 1360°C; b.p. is 2500°C. It occurs naturally as the mineral fluorite (or fluorspar) and is the main source of fluorine. The calcium fluoride structure (fluorite structure)is a crystal structure in which the calcium ions are each surrounded by eight fluoride ions arranged at the corners of a cube. Each fluoride ion is surrounded by four calcium ions at the corners of a tetrahedron.

Calcium hydrogencarbonate (calcium bicarbonate) is a compound Ca(HCO₃)₂, that is stable only in solution and is formed when water containing carbon dioxide dissolves calcium carbonate.

It is the cause of temporary hardness in water because calcium ions react with soap to give scum. Calcium hydrogencarbonate is unstable when heated and decomposes to give solid calcium carbonate. This explains why temporary hardness is removed by boiling and the formation of scale in kettles and boilers.

Calcium hydroxide is a white solid, Ca(OH)₂, which dissolves sparingly in water; its shape is hexagonal; r.d. is 2.24. It is manufactured by adding water to calcium oxide, a process that evolves much heat and is known as slaking. It is used as a cheap alkali to neutralize the acidity in certain soils and in the manufacture of mortar, whitewash, bleaching powder and glass.

Calcium oxide is a white solid compound, CaO, formed by heating calcium in oxygen or by the thermal decomposition of calcium carbonate. Its shape is cubic; r.d. is 3.35; m.p. is 2580°C; b.p. is 2850^o C. On a large scale, calcium carbonate in the form of limestone is heated in a tall tower (lime kiln) to a temperature above 550°C:

CaCO₃(s) = CaO (s) + CO₂(g).

Although the reaction is reversible, the carbon dioxide is carried away by the upward current through the kiln and all the limestone decomposes. Calcium oxide is used to make calcium hydroxide, as a cheap alkali for treating acid soil and in extractive metallurgy to produce a slag with the impurities (especially sand) present in metal ores.

Calcium phosphate (V) is a white insoluble powder, Ca₃(PO₄)₂; r.d. is 3.14. It is found naturally in the mineral apatite, Ca₅(PO₄)₃(OH, F, Cl) and as rock phosphate. It is also the main constituent of animal bones. Calcium phosphate can be prepared by mixing solutions containing calcium ions and hydrogenphosphate ions in the presence of an alkali. It is used extensively as a fertilizer. The compound was formerly called calcium orthophosphate.

Calcium sulphate is a white solid compound, CaSO₄: r.d. is 2.96; 1450°C It occurs naturally as the mineral anhydrite, which has a rhombic structure, transforming to a monoclinic form at 200°C. More commonly, it is found as the dihydrate, gypsum, CaSO₄.2H₂O (monoclinic; r.d. is 2.32). When heated, gypsum loses water at 128°C to give the hemihydrate, 2CaSO₄.H₂O, better known as plaster of Paris. Calcium sulphate is sparingly-soluble in water and is a cause of permanent hardness of water. It is used in the manufacture of certain paints, ceramics and paper. The naturally occurring forms are used in the manufacture of sulphuric (V1) acid.

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