calcium carbonate A white solid,
CaCO 3 , 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 (rhombohedral; r.d.
2.71) and *aragonite (rhombic; r.d.
2.93). Rocks containing calcium carbonate dissolve slowly in acidiÜed
rainwater (containing dissolved CO 2 )
to cause temporary hardness. In the
laboratory, calcium 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 A white deliquescent 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 dihydrate, CaCl 2 .2H 2 O (r.d. 0.84), and
the hexahydrate, CaCl 2 .6H 2 O (trigonal; 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 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 cyanamide A colourless
solid, CaCN 2 , which sublimes at
1300°C. It is prepared by heating calcium 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 of Üxing nitrogen in countries in which cheap electricity is
available to make the calcium dicarbide (the cyanamide process). Calcium cyanamide can be used as a
fertilizer because it reacts with water
to give ammonia and calcium carbonate:
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 electricity is cheap it is manufactured by
heating calcium oxide with either
coke or ethyne at temperatures
above 2000°C in an electric arc furnace. The crystals consist of Ca
2+ 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)
calcium Ûuoride A white crystalline solid, CaF 2 ; r.d. 3.2; m.p.
1360°C; b.p. 2500°C. It occurs naturally as the mineral *Ûuorite (or
Ûuorspar) and is the main source of
Ûuorine. See also fluorite structure.
calcium hydrogencarbonate (calcium bicarbonate) A compound,
Ca(HCO 3 ) 2 , that is stable only in solution and is formed when water containing 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 *hardness in water, because the calcium
93
calcium hydrogencarbonate
c
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CaCO 3 , 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 (rhombohedral; r.d.
2.71) and *aragonite (rhombic; r.d.
2.93). Rocks containing calcium carbonate dissolve slowly in acidiÜed
rainwater (containing dissolved CO 2 )
to cause temporary hardness. In the
laboratory, calcium 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 A white deliquescent 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 dihydrate, CaCl 2 .2H 2 O (r.d. 0.84), and
the hexahydrate, CaCl 2 .6H 2 O (trigonal; 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 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 cyanamide A colourless
solid, CaCN 2 , which sublimes at
1300°C. It is prepared by heating calcium 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 of Üxing nitrogen in countries in which cheap electricity is
available to make the calcium dicarbide (the cyanamide process). Calcium cyanamide can be used as a
fertilizer because it reacts with water
to give ammonia and calcium carbonate:
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 electricity is cheap it is manufactured by
heating calcium oxide with either
coke or ethyne at temperatures
above 2000°C in an electric arc furnace. The crystals consist of Ca
2+ 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)
calcium Ûuoride A white crystalline solid, CaF 2 ; r.d. 3.2; m.p.
1360°C; b.p. 2500°C. It occurs naturally as the mineral *Ûuorite (or
Ûuorspar) and is the main source of
Ûuorine. See also fluorite structure.
calcium hydrogencarbonate (calcium bicarbonate) A compound,
Ca(HCO 3 ) 2 , that is stable only in solution and is formed when water containing 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 *hardness in water, because the calcium
93
calcium hydrogencarbonate
c
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