Dictionary of Chemistry [6th Ed.]

decalin(decahydronaphthalene) A
liquid bicyclic hydrocarbon, C 10 H 18 ,
used as a solvent. There are two
stereoisomers, cis (b.p. 198°C) and
trans (b.p. 185°C), made by the cat-
alytic hydrogenation of naphthalene
at high temperatures and pressures.

165 degassing

d

H

H

H

H

H

H

H

H

H

H

Decalin

decanedioic acid (sebacic acid)A
white crystalline dicarboxylic acid,
HOOC(CH 2 ) 8 COOH; r.d. 1.12; m.p.
131–134.5°C; b.p. 294.4°C (100
mmHG). Obtained from castor oil, it
is used in plasticizers, lubricants, and
cosmetics and in the production of
other organic chemicals.

decanoic acid (capric acid)A white
crystalline straight-chain saturated
*carboxylic acid, CH 3 (CH 2 ) 8 COOH;
m.p. 31.5°C. Its esters are used in per-
fumes andÛavourings.

decantationThe process of sepa-
rating a liquid from a settled solid
suspension or from a heavier immis-
cible liquid by carefully pouring it
into a different container.

decarboxylationThe removal of
carbon dioxide from a molecule. De-
carboxylation is an important reac-
tion in many biochemical processes,
such as the *Krebs cycle and the syn-
thesis of fatty acids.

decay1.The spontaneous transfor-
mation of one radioactive nuclide
into a daughter nuclide, which may
be radioactive or may not, with the
emission of one or more particles or
photons. The decay of N 0 nuclides to
give N nuclides after time t is given
by N = N 0 exp(–γt), where γis called

the decay constant or the disintegra-

tion constant. The reciprocal of the

decay constant is the mean life. The

time required for half the original

nuclides to decay (i.e. N = ½N 0 ) is

called the half-lifeof the nuclide. The

same terms are applied to elemen-

tary particles that spontaneously

transform into other particles. For

example, a free neutron decays into a

proton and an electron. 2.The rever-

sion of excited states of atoms or

molecules to the ground state.

deci-Symbol d. A preÜx used in the

metric system to denote one tenth.

For example, 0.1 coulomb = 1 deci-

coulomb (dC); 0.1 metre = 1 decime-

tre (dm).

decoctionA solution made by boil-

ing material (e.g. plant substances) in

water, followed byÜltration.

decomposition1.The chemical

breakdown of organic matter into its

constituents by the action of bacteria

and other organisms. 2.A chemical

reaction in which a compound

breaks down into simpler com-

pounds or into elements.

decrepitation A crackling noise

produced when certain crystals are

heated, caused by changes in struc-

ture resulting from loss of water of

crystallization.

defectSee crystal defect.

defect stateA quantum mechani-

cal state that exists due to the pres-

ence of a *crystal defect.

deÜnite proportions See chemi-

cal combination.

deÛagrationA type of explosion

in which the shock wave arrives be-

fore the reaction is complete (be-

cause the reaction front moves more

slowly than the speed of sound in the

medium).

degassingThe removal of dis-