ions react with soap to give scum.
Calcium hydrogencarbonate is unsta-
ble when heated and decomposes to
give solid calcium carbonate. This ex-
plains why temporary hardness is re-
moved 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 manufac-
tured 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 nitrateA white deliques-
cent 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 trihy-
drate, Ca(NO 3 ) 2 .3H 2 O. The anhydrous
salt can be obtained from the hy-
drate by heating but it decomposes
on strong heating to give the oxide,
nitrogen dioxide, and oxygen. Cal-
cium 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 con-
taining 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 oflimestone is heated in a tall tower
(lime kiln) to a temperature above
550 °C:
CaCO 3 (s) ˆCaO(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 cal-
cium 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 mix-
ing solutions containing calcium ions
and hydrogenphosphate ions in the
presence of an alkali:
HPO 4 2–+ OH–→PO 4 3–+ H 2 O3Ca2++ 2PO 4 3–→Ca 3 (PO 4 ) 2
It is used extensively as a fertilizer.
The compound was formerly called
calcium orthophosphate (see phos-
phates).calcium stearate See calcium oc-
tadecanoate.calcium sulphateA 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 mono-
clinic form at 200°C. More com-
monly, 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 *hard-
ness of water. It is used in the manu-calcium hydroxide 94c