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 (slaked lime) A
white solid, Ca(OH) 2 , which dissolves
sparingly in water (see limewater);
hexagonal; r.d. 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 nitrate A white deliquescent compound, Ca(NO 3 ) 2 , that is
very soluble in water; cubic; r.d. 2.50;
m.p. 561°C. It can be prepared by
neutralizing nitric acid with calcium
carbonate and crystallizing it from
solution as the tetrahydrate
Ca(NO 3 ) 2 .4H 2 O, which exists in two
monoclinic crystalline forms (α, r.d.
1.9; β, r.d. 1.82). There is also a trihydrate, Ca(NO 3 ) 2 .3H 2 O. The anhydrous
salt can be obtained from the hydrate by heating but it decomposes
on strong heating to give the oxide,
nitrogen dioxide, and oxygen. Calcium nitrate is sometimes used as a
nitrogenous fertilizer.
calcium octadecanoate (calcium
stearate) An insoluble white salt,
Ca(CH 3 (CH 2 ) 16 COO) 2 , which is formed
when soap is mixed with water containing calcium ions and is the scum
produced in hard-water regions.
calcium oxide (quicklime) A white
solid compound, CaO, formed by
heating calcium in oxygen or by the
thermal decomposition of calcium
carbonate; cubic; r.d. 3.35; m.p.
2580°C; b.p. 2850°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:
CaCO 3 (s) ˆ CaO(s) + CO 2 (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) A white
insoluble powder, Ca 3 (PO 4 ) 2 ; r.d. 3.14.
It is found naturally in the mineral
*apatite, Ca 5 (PO 4 ) 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:
HPO 4
2– + OH
–
→ PO 4
3– + H 2 O
3Ca
2+ + 2PO 4
3–
→ Ca 3 (PO 4 ) 2
It is used extensively as a fertilizer.
The compound was formerly called
calcium orthophosphate (see phosphates).
calcium stearate See calcium octadecanoate.
calcium sulphate A white solid
compound, CaSO 4 ; r.d. 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, CaSO 4 .2H 2 O (monoclinic;
r.d. 2.32). When heated, gypsum
loses water at 128°C to give the
hemihydrate, 2CaSO 4 .H 2 O, 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 manucalcium hydroxide
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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 (slaked lime) A
white solid, Ca(OH) 2 , which dissolves
sparingly in water (see limewater);
hexagonal; r.d. 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 nitrate A white deliquescent compound, Ca(NO 3 ) 2 , that is
very soluble in water; cubic; r.d. 2.50;
m.p. 561°C. It can be prepared by
neutralizing nitric acid with calcium
carbonate and crystallizing it from
solution as the tetrahydrate
Ca(NO 3 ) 2 .4H 2 O, which exists in two
monoclinic crystalline forms (α, r.d.
1.9; β, r.d. 1.82). There is also a trihydrate, Ca(NO 3 ) 2 .3H 2 O. The anhydrous
salt can be obtained from the hydrate by heating but it decomposes
on strong heating to give the oxide,
nitrogen dioxide, and oxygen. Calcium nitrate is sometimes used as a
nitrogenous fertilizer.
calcium octadecanoate (calcium
stearate) An insoluble white salt,
Ca(CH 3 (CH 2 ) 16 COO) 2 , which is formed
when soap is mixed with water containing calcium ions and is the scum
produced in hard-water regions.
calcium oxide (quicklime) A white
solid compound, CaO, formed by
heating calcium in oxygen or by the
thermal decomposition of calcium
carbonate; cubic; r.d. 3.35; m.p.
2580°C; b.p. 2850°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:
CaCO 3 (s) ˆ CaO(s) + CO 2 (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) A white
insoluble powder, Ca 3 (PO 4 ) 2 ; r.d. 3.14.
It is found naturally in the mineral
*apatite, Ca 5 (PO 4 ) 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:
HPO 4
2– + OH
–
→ PO 4
3– + H 2 O
3Ca
2+ + 2PO 4
3–
→ Ca 3 (PO 4 ) 2
It is used extensively as a fertilizer.
The compound was formerly called
calcium orthophosphate (see phosphates).
calcium stearate See calcium octadecanoate.
calcium sulphate A white solid
compound, CaSO 4 ; r.d. 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, CaSO 4 .2H 2 O (monoclinic;
r.d. 2.32). When heated, gypsum
loses water at 128°C to give the
hemihydrate, 2CaSO 4 .H 2 O, 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 manucalcium hydroxide
94
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