two linked *monosaccharide mol-
ecules. For example, sucrose com-
prises one glucose molecule and one
fructose molecule bonded together.
discharge1.The conversion of the
chemical energy stored in a *sec-
ondary cell into electrical energy.
2.The release of electric charge from
a capacitor in an external circuit.
3.The passage of charge carriers
through a gas at low pressure in a
discharge tube. A potential difference
applied between cathode and anode
creates an electricÜeld that acceler-
ates any free electrons and ions to
their appropriate electrodes. Colli-
sions between electrons and gas mol-
ecules create more ions. Collisions
also produce excited ions and mol-
ecules (see excitation), which decay
with emission of light in certain
parts of the tube.
disconnection See retrosynthetic
analysis.
disilaneSee silane.
dislocationSee crystal defect.
disodium hydrogenphos-
phate(V)(disodium orthophos-
phate)A colourless crystalline solid,
Na 2 HPO 4 , soluble in water and insol-
uble in ethanol. It is known as the di-
hydrate (r.d. 2.066), heptahydrate
(r.d. 1.68), and dodecahydrate (r.d.
1.52). It may be prepared by titrating
phosphoric acid with sodium hydrox-
ide to an alkaline end point (phe-
nolphthalein) and is used in treating
boiler feed water and in the textile
industry.
disodium orthophosphateSee
disodium hydrogenphosphate(v).
disodium tetraborate-10-water
See borax.
d-isomerSee optical activity.
d-isomer See absolute configura-
tion.
disordered solid A material that
neither has the structure of a perfect
*crystal lattice nor of a crystal lattice
with isolated *crystal defects. In a
random alloy, one type of disordered
solid, the order of the different types
of atom occurs at random. Another
type of disordered solid is formed by
introducing a high concentration of
defects, with the defects distributed
randomly throughout the solid. In an
*amorphous solid, such as glass,
there is a random network of atoms
with no lattice.disperse dyeSee dyes.
disperse phaseSee colloids.dispersion forces See van der
waals’ force.displacement reaction See substi-
tution reaction.disproportionation A type of
chemical reaction in which the same
compound is simultaneously reduced
and oxidized. For example, copper(I)
chloride disproportionates thus:
2CuCl →Cu + CuCl 2
The reaction involves oxidation of
one molecule
CuI→CuII+ e
and reduction of the other
CuI+ e →Cu
The reaction of halogens with hy-
droxide ions is another example of a
disproportionation reaction, for ex-
ample
Cl 2 (g) + 2OH–(aq) ˆCl–(aq) +
ClO–(aq) + H 2 O(l)
The reverse process is *compropor-
tionation.dissipative systemA system that
involves *irreversible processes. All
real systems are dissipative (in con-
trast to such idealized systems as the
frictionless pendulum, which is in-
variant under time reversal). In a dis-181 dissipative system
d