due to the presence of impurities. It
forms well-shaped orthorhombic
crystals. It occurs in the USA, Spain,
and SW Africa.
cetane See hexadecane.
cetane number A number that
provides a measure of the ignition
characteristics of a Diesel fuel when
it is burnt in a standard Diesel engine. It is the percentage of cetane
(hexadecane) in a mixture of cetane
and 1-methylnaphthalene that has
the same ignition characteristics as
the fuel being tested. Compare octane number.
CFC See chlorofluorocarbon.
CGE Capillary gel electrophoresis.
See capillary electrophoresis.
c.g.s. units A system of *units
based on the centimetre, gram, and
second. Derived from the metric system, it was badly adapted to use with
thermal quantities (based on the inconsistently deÜned *calorie) and
with electrical quantities (in which
two systems, based respectively on
unit permittivity and unit permeability of free space, were used). For scientiÜc purposes c.g.s. units have now
been replaced by *SI units.
chain A line of atoms of the same
type in a molecule. In a straight chain
the atoms are attached only to single
atoms, not to groups. Propane, for instance, is a straight-chain alkane,
CH 3 CH 2 CH 3 , with a chain of three
carbon atoms. A branched chain is
one in which there are side groups
attached to the chain. Thus, 3-ethyloctane, CH 3 CH 2 CH(C 2 H 5 )C 5 H 11 , is a
branched-chain alkane in which
there is a side chain (C 2 H 5 ) attached
to the third carbon atom. A closed
chain is a *ring of atoms in a molecule; otherwise the molecule has an
open chain.
Chain, Sir Ernst Boris (1906–79)
German-born British biochemist,
who began his research career at
Cambridge University in 1933. Two
years later he joined *Florey at Oxford, where they isolated and
puriÜed *penicillin. They also developed a method of producing the drug
in large quantities and carried out its
Ürst clinical trials. The two men
shared the 1945 Nobel Prize for physiology or medicine with penicillin’s
discoverer, Alexander *Fleming.
chain reaction A reaction that is
self-sustaining as a result of the products of one step initiating a subsequent step. Chemical chain reactions
usually involve free radicals as intermediates. An example is the reaction
of chlorine with hydrogen initiated
by ultraviolet radiation. A chlorine
molecule is Ürst split into atoms:
Cl 2 → Cl• + Cl•
These react with hydrogen as follows
Cl• + H 2 → HCl + H•
H• + Cl 2 → HCl + Cl• etc.
Combustion and explosion reactions
involve similar free-radical chain reactions.
chair See ring conformations.
chair conformation See conformation.
chalcedony A mineral consisting
of a microcrystalline variety of
*quartz. It occurs in several forms,
including a large number of semiprecious gemstones; for example,
sard, carnelian, jasper, onyx, chrysoprase, agate, and tiger’s-eye.
chalcogens See group 16 elements.
chalconides Binary compounds
formed between metals and group 16
elements; i.e. oxides, sulphides, selenides, and tellurides.
chalcopyrite (copper pyrites) A
brassy yellow mineral consisting of a
cetane
112
c
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forms well-shaped orthorhombic
crystals. It occurs in the USA, Spain,
and SW Africa.
cetane See hexadecane.
cetane number A number that
provides a measure of the ignition
characteristics of a Diesel fuel when
it is burnt in a standard Diesel engine. It is the percentage of cetane
(hexadecane) in a mixture of cetane
and 1-methylnaphthalene that has
the same ignition characteristics as
the fuel being tested. Compare octane number.
CFC See chlorofluorocarbon.
CGE Capillary gel electrophoresis.
See capillary electrophoresis.
c.g.s. units A system of *units
based on the centimetre, gram, and
second. Derived from the metric system, it was badly adapted to use with
thermal quantities (based on the inconsistently deÜned *calorie) and
with electrical quantities (in which
two systems, based respectively on
unit permittivity and unit permeability of free space, were used). For scientiÜc purposes c.g.s. units have now
been replaced by *SI units.
chain A line of atoms of the same
type in a molecule. In a straight chain
the atoms are attached only to single
atoms, not to groups. Propane, for instance, is a straight-chain alkane,
CH 3 CH 2 CH 3 , with a chain of three
carbon atoms. A branched chain is
one in which there are side groups
attached to the chain. Thus, 3-ethyloctane, CH 3 CH 2 CH(C 2 H 5 )C 5 H 11 , is a
branched-chain alkane in which
there is a side chain (C 2 H 5 ) attached
to the third carbon atom. A closed
chain is a *ring of atoms in a molecule; otherwise the molecule has an
open chain.
Chain, Sir Ernst Boris (1906–79)
German-born British biochemist,
who began his research career at
Cambridge University in 1933. Two
years later he joined *Florey at Oxford, where they isolated and
puriÜed *penicillin. They also developed a method of producing the drug
in large quantities and carried out its
Ürst clinical trials. The two men
shared the 1945 Nobel Prize for physiology or medicine with penicillin’s
discoverer, Alexander *Fleming.
chain reaction A reaction that is
self-sustaining as a result of the products of one step initiating a subsequent step. Chemical chain reactions
usually involve free radicals as intermediates. An example is the reaction
of chlorine with hydrogen initiated
by ultraviolet radiation. A chlorine
molecule is Ürst split into atoms:
Cl 2 → Cl• + Cl•
These react with hydrogen as follows
Cl• + H 2 → HCl + H•
H• + Cl 2 → HCl + Cl• etc.
Combustion and explosion reactions
involve similar free-radical chain reactions.
chair See ring conformations.
chair conformation See conformation.
chalcedony A mineral consisting
of a microcrystalline variety of
*quartz. It occurs in several forms,
including a large number of semiprecious gemstones; for example,
sard, carnelian, jasper, onyx, chrysoprase, agate, and tiger’s-eye.
chalcogens See group 16 elements.
chalconides Binary compounds
formed between metals and group 16
elements; i.e. oxides, sulphides, selenides, and tellurides.
chalcopyrite (copper pyrites) A
brassy yellow mineral consisting of a
cetane
112
c
www.AzShimi.ir www.AzShimi.com