models of biological systems (e.g.
predator–prey relationships).
low-energy electron diffraction
See leed.
lowering of vapour pressure A
reduction in the saturated vapour
pressure of a pure liquid when a
solute is introduced. If the solute is
a solid of low vapour pressure, the
decrease in vapour pressure of
the liquid is proportional to the
concentration of particles of solute;
i.e. to the number of dissolved mol-
ecules or ions per unit volume. To a
Ürst approximation, it does not de-
pend on the nature of the particles.
See colligative properties; raoult’s
law.
lowest unoccupied molecular
orbital(LUMO)The orbital in a
molecule that has the lowest unoccu-
pied energy level at the absolute zero
of temperature. The lowest unoccu-
pied molecular orbital and the high-
est occupied molecular orbital
(HOMO) are the two *frontier or-
bitals of the molecule.
Lowry–Brønsted theory See acid.
LSDSee lysergic acid diethy-
lamide.
L-seriesSee absolute configura-
tion.
lubrication The use of a substance
to prevent contact between solid sur-
faces in relative motion in order to
reduce friction, wear, overheating,
and rusting. Liquid hydrocarbons
(oils), either derived from petroleum
or made synthetically, are the most
widely used lubricants as they are
relatively inexpensive, are good
coolants, provide the appropriate
range of viscosities, and are ther-
mally stable. Additives include poly-
meric substances that maintain the
desired viscosity as the temperature
increases, antioxidants that prevent
the formation of a sludge, and alka-
line-earth phenates that neutralize
acids and reduce wear.
At high temperatures, solid lubri-
cants, such as graphite or molybde-
num disulphide, are often used.
SemiÛuid lubricants (greases) are
used to provide a seal against mois-
ture and dirt and to remain attached
to vertical surfaces. They are made
by adding gelling agents, such as
metallic soaps, to liquid lubricants.
luciferaseSee bioluminescence.luciferinSee bioluminescence.
lumenSymbol lm. The SI unit of lu-
minousÛux equal to theÛux emitted
by a uniform point source of 1 can-
dela in a solid angle of 1 steradian.luminescenceThe emission of
light by a substance for any reason
other than a rise in its temperature.
In general, atoms of substances emit
*photons of electromagnetic energy
when they return to the *ground
state after having been in an excited
state (see excitation). The causes of
the excitation are various. If the ex-
citing cause is a photon, the process
is called photoluminescence; if it is
an electron it is called electrolumi-
nescence. Chemiluminescenceis lumi-
nescence resulting from a chemical
reaction (such as the slow oxidation
of phosphorus); *bioluminescence is
the luminescence produced by a liv-
ing organism (such as aÜreÛy). If the
luminescence persists signiÜcantly
after the exciting cause is removed it
is called phosphorescence; if it does
not it is called Ûuorescence. This dis-
tinction is arbitrary since there must
always be some delay; in some deÜni-
tions a persistence of more than 10
nanoseconds (10–8s) is treated as
phosphorescence.
luminol testA *presumptive test
for blood. The reagent is a mixture of
3-aminophthalhydrazide, sodium car-333 luminol test
l