one neutron. The abundance of deuterium in natural hydrogen is about
0.015%. It is present in water as the
oxide HDO (see also heavy water),
from which it is usually obtained by
electrolysis or fractional distillation.
Its chemical behaviour is almost
identical to hydrogen although deuterium compounds tend to react
rather more slowly than the corresponding hydrogen compounds. Its
physical properties are slightly different from those of hydrogen, e.g. b.p.
23.6 K (hydrogen 20.4 K).
deuterium oxide See heavy
water.
deuteron A nucleus of a deuterium
atom, consisting of a proton and a
neutron bound together; the ion D
+
formed by ionization of a deuterium
atom. See also hydron.
Devarda’s alloy An alloy of copper
(50%), aluminium (45%) and zinc (5%),
used in chemical tests for the nitrate
ion (in alkaline solutions it reduces a
nitrate to ammonia).
devitriÜcation Loss of the amorphous nature of glass as a result of
crystallization.
Dewar, Sir James (1842–1923)
British chemist and physicist, born in
Scotland. In 1875 he became a professor at Cambridge University, while
carrying out much of his experimental work at the Royal Institution in
London. He began studying gases at
low temperatures and in 1872 invented the *Dewar Ûask. In 1891, together with Frederick Abel (1827–
1902), he developed the smokeless
propellant explosive *cordite, and in
1898 was the Ürst to liquefy hydrogen.
Dewar benzene An isomer of benzene, C 6 H 6 . Dewar benzene has the
systematic name bicyclo [2,2,0] hexa2,5-diene, and is a nonplanar bicyclic
molecule. It has a high bond strain
and will spontaneously slowly convert to benzene. The unsubstituted
compound was Ürst synthesized in
1963.
Dewar Ûask A vessel for storing
hot or cold liquids so that they maintain their temperature independently
of the surroundings. Heat transfer to
the surroundings is reduced to a minimum: the walls of the vessel consist
of two thin layers of glass (or, in
large vessels, steel) separated by a
vacuum to reduce conduction and
convection; the inner surface of a
glass vessel is silvered to reduce radiation; and the vessel is stoppered to
prevent evaporation. It was devised
around 1872 by Sir James *Dewar
and is also known by its Ürst trade
name Thermos Ûask. See also cryostat.
Dewar structure A structure of
*benzene proposed by Sir James
*Dewar, having a hexagonal ring of
six carbon atoms with two opposite
atoms joined by a long single bond
across the ring and with two double
C–C bonds, one on each side of the
hexagon. Dewar structures contribute to the resonance hybrid of
benzene.
dextran A glutinous glucose polymer produced by certain bacteria. It
can be made by fermenting sucrose
(cane sugar) and is used as a thickening agent, as a stabilizer in ice cream,
and as a substitute for plasma in
blood transfusions. Esters with sulphuric acid yield sodium salts that
are employed as anticoagulant drugs.
dextrin An intermediate polysaccharide compound resulting from
the hydrolysis of starch to maltose by
amylase enzymes.
dextro- form See optical activity.
dextrorotatory Denoting a chemi171
dextrorotatory
d
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0.015%. It is present in water as the
oxide HDO (see also heavy water),
from which it is usually obtained by
electrolysis or fractional distillation.
Its chemical behaviour is almost
identical to hydrogen although deuterium compounds tend to react
rather more slowly than the corresponding hydrogen compounds. Its
physical properties are slightly different from those of hydrogen, e.g. b.p.
23.6 K (hydrogen 20.4 K).
deuterium oxide See heavy
water.
deuteron A nucleus of a deuterium
atom, consisting of a proton and a
neutron bound together; the ion D
+
formed by ionization of a deuterium
atom. See also hydron.
Devarda’s alloy An alloy of copper
(50%), aluminium (45%) and zinc (5%),
used in chemical tests for the nitrate
ion (in alkaline solutions it reduces a
nitrate to ammonia).
devitriÜcation Loss of the amorphous nature of glass as a result of
crystallization.
Dewar, Sir James (1842–1923)
British chemist and physicist, born in
Scotland. In 1875 he became a professor at Cambridge University, while
carrying out much of his experimental work at the Royal Institution in
London. He began studying gases at
low temperatures and in 1872 invented the *Dewar Ûask. In 1891, together with Frederick Abel (1827–
1902), he developed the smokeless
propellant explosive *cordite, and in
1898 was the Ürst to liquefy hydrogen.
Dewar benzene An isomer of benzene, C 6 H 6 . Dewar benzene has the
systematic name bicyclo [2,2,0] hexa2,5-diene, and is a nonplanar bicyclic
molecule. It has a high bond strain
and will spontaneously slowly convert to benzene. The unsubstituted
compound was Ürst synthesized in
1963.
Dewar Ûask A vessel for storing
hot or cold liquids so that they maintain their temperature independently
of the surroundings. Heat transfer to
the surroundings is reduced to a minimum: the walls of the vessel consist
of two thin layers of glass (or, in
large vessels, steel) separated by a
vacuum to reduce conduction and
convection; the inner surface of a
glass vessel is silvered to reduce radiation; and the vessel is stoppered to
prevent evaporation. It was devised
around 1872 by Sir James *Dewar
and is also known by its Ürst trade
name Thermos Ûask. See also cryostat.
Dewar structure A structure of
*benzene proposed by Sir James
*Dewar, having a hexagonal ring of
six carbon atoms with two opposite
atoms joined by a long single bond
across the ring and with two double
C–C bonds, one on each side of the
hexagon. Dewar structures contribute to the resonance hybrid of
benzene.
dextran A glutinous glucose polymer produced by certain bacteria. It
can be made by fermenting sucrose
(cane sugar) and is used as a thickening agent, as a stabilizer in ice cream,
and as a substitute for plasma in
blood transfusions. Esters with sulphuric acid yield sodium salts that
are employed as anticoagulant drugs.
dextrin An intermediate polysaccharide compound resulting from
the hydrolysis of starch to maltose by
amylase enzymes.
dextro- form See optical activity.
dextrorotatory Denoting a chemi171
dextrorotatory
d
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