calcium carbonateA white solid,
CaCO 3 , which is only sparingly solu-
ble in water. Calcium carbonate
decomposes on heating to give cal-
cium oxide (quicklime) and carbon
dioxide. It occurs naturally as the
minerals calcite (rhombohedral; r.d.
2.71) and aragonite (rhombic; r.d.
2.93). Rocks containing calcium car-
bonate dissolve slowly in acidiÜed
rainwater (containing dissolved CO 2 )
to cause temporary hardness. In the
laboratory, calcium carbonate is pre-
cipitated 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 chlorideA white deli-
quescent compound, CaCl 2 , which is
soluble in water; r.d. 2.15; m.p.
782 °C; b.p. > 1600 °C. There are a
number of hydrated forms, including
the monohydrate, CaCl 2 .H 2 O, the di-
hydrate, CaCl 2 .2H 2 O (r.d. 0.84), and
the hexahydrate, CaCl 2 .6H 2 O (trigo-
nal; r.d. 1.71; the hexahydrate loses
4H 2 O at 30°C and the remaining
2H 2 O at 200°C). Large quantities of it
are formed as a byproduct of the
*Solvay process and it can be pre-
pared by dissolving calcium carbon-
ate 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 cal-
cium chloride is used in refrigeration
plants.
calcium cyanamideA colourless
solid, CaCN 2 , which sublimes at
1300 °C. It is prepared by heating cal-
cium dicarbide at 800°C in a stream
of nitrogen:
CaC 2 (s) + N 2 (g) →CaCN 2 (s) + C(s)The reaction has been used as a
method ofÜxing nitrogen in coun-
tries in which cheap electricity is
available to make the calcium dicar-
bide (the cyanamide process). Cal-
cium cyanamide can be used as a
fertilizer because it reacts with water
to give ammonia and calcium car-
bonate:
CaCN 2 (s) + 3H 2 O(l) →CaCO 3 (s) +
2NH 3 (g)
It is also used in the production of
melamine, urea, and certain cyanide
salts.calcium dicarbide (calcium
acetylide; calcium carbide; carbide)A
colourless solid compound, CaC 2 ;
tetragonal; r.d. 2.22; m.p. 450°C; b.p.
2300 °C. In countries in which elec-
tricity is cheap it is manufactured by
heating calcium oxide with either
coke or ethyne at temperatures
above 2000°C in an electric arc fur-
nace. The crystals consist of Ca2+and
C 2 – ions arranged in a similar way to
the ions in sodium chloride. When
water is added to calcium dicarbide,
the important organic raw material
ethyne (acetylene) is produced:
CaC 2 (s) + 2H 2 O(l) →Ca(OH) 2 (s) +
C 2 H 2 (g)calciumÛuorideA white crys-
talline solid, CaF 2 ; r.d. 3.2; m.p.
1360 °C; b.p. 2500°C. It occurs natu-
rally as the mineral *Ûuorite (or
Ûuorspar) and is the main source of
Ûuorine. See also fluorite structure.calcium hydrogencarbonate (cal-
cium bicarbonate)A compound,
Ca(HCO 3 ) 2 , that is stable only in solu-
tion and is formed when water con-
taining carbon dioxide dissolves
calcium carbonate:
CaCO 3 (s) + H 2 O(l) + CO 2 (g) →
Ca(HCO 3 ) 2 (aq)
It is the cause of temporary *hard-
ness in water, because the calcium93 calcium hydrogencarbonate
c