one neutron. The abundance of deu-
terium 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 deu-
terium compounds tend to react
rather more slowly than the corre-
sponding hydrogen compounds. Its
physical properties are slightly differ-
ent 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 amor-
phous 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 pro-
fessor at Cambridge University, while
carrying out much of his experimen-
tal work at the Royal Institution in
London. He began studying gases at
low temperatures and in 1872 in-
vented the DewarÛask. In 1891, to-
gether with Frederick Abel (1827–
1902), he developed the smokeless
propellant explosive cordite, and in
1898 was theÜrst to liquefy hydro-
gen.
Dewar benzene An isomer of ben-
zene, C 6 H 6. Dewar benzene has the
systematic name bicyclo [2,2,0] hexa-
2,5-diene, and is a nonplanar bicyclic
molecule. It has a high bond strain
and will spontaneously slowly con-
vert to benzene. The unsubstituted
compound wasÜrst synthesized in
1963.DewarÛaskA vessel for storing
hot or cold liquids so that they main-
tain their temperature independently
of the surroundings. Heat transfer to
the surroundings is reduced to a min-
imum: 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 radi-
ation; and the vessel is stoppered to
prevent evaporation. It was devised
around 1872 by Sir James *Dewar
and is also known by itsÜrst trade
name ThermosÛask. See also cryo-
stat.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 con-
tribute to the resonance hybrid of
benzene.dextranA glutinous glucose poly-
mer produced by certain bacteria. It
can be made by fermenting sucrose
(cane sugar) and is used as a thicken-
ing agent, as a stabilizer in ice cream,
and as a substitute for plasma in
blood transfusions. Esters with sul-
phuric acid yield sodium salts that
are employed as anticoagulant drugs.
dextrinAn intermediate polysac-
charide compound resulting from
the hydrolysis of starch to maltose by
amylase enzymes.dextro- form See optical activity.dextrorotatory Denoting a chemi-171 dextrorotatory
d