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 molecules or ions per unit volume. To a
Ürst approximation, it does not depend 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 unoccupied energy level at the absolute zero
of temperature. The lowest unoccupied molecular orbital and the highest occupied molecular orbital
(HOMO) are the two *frontier orbitals of the molecule.
Lowry–Brønsted theory See acid.
LSD See lysergic acid diethylamide.
L-series See absolute configuration.
lubrication The use of a substance
to prevent contact between solid surfaces 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 thermally stable. Additives include polymeric substances that maintain the
desired viscosity as the temperature
increases, antioxidants that prevent
the formation of a sludge, and alkaline-earth phenates that neutralize
acids and reduce wear.
At high temperatures, solid lubricants, such as graphite or molybdenum disulphide, are often used.
SemiÛuid lubricants (greases) are
used to provide a seal against moisture and dirt and to remain attached
to vertical surfaces. They are made
by adding gelling agents, such as
metallic soaps, to liquid lubricants.
luciferase See bioluminescence.
luciferin See bioluminescence.
lumen Symbol lm. The SI unit of luminous Ûux equal to the Ûux emitted
by a uniform point source of 1 candela in a solid angle of 1 steradian.
luminescence The 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 exciting cause is a photon, the process
is called photoluminescence; if it is
an electron it is called electroluminescence. Chemiluminescence is luminescence resulting from a chemical
reaction (such as the slow oxidation
of phosphorus); *bioluminescence is
the luminescence produced by a living organism (such as a ÜreÛy). If the
luminescence persists signiÜcantly
after the exciting cause is removed it
is called phosphorescence; if it does
not it is called Ûuorescence. This distinction is arbitrary since there must
always be some delay; in some deÜnitions a persistence of more than 10
nanoseconds (10
–8 s) is treated as
phosphorescence.
luminol test A *presumptive test
for blood. The reagent is a mixture of
3-aminophthalhydrazide, sodium car333
luminol test
l
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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 molecules or ions per unit volume. To a
Ürst approximation, it does not depend 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 unoccupied energy level at the absolute zero
of temperature. The lowest unoccupied molecular orbital and the highest occupied molecular orbital
(HOMO) are the two *frontier orbitals of the molecule.
Lowry–Brønsted theory See acid.
LSD See lysergic acid diethylamide.
L-series See absolute configuration.
lubrication The use of a substance
to prevent contact between solid surfaces 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 thermally stable. Additives include polymeric substances that maintain the
desired viscosity as the temperature
increases, antioxidants that prevent
the formation of a sludge, and alkaline-earth phenates that neutralize
acids and reduce wear.
At high temperatures, solid lubricants, such as graphite or molybdenum disulphide, are often used.
SemiÛuid lubricants (greases) are
used to provide a seal against moisture and dirt and to remain attached
to vertical surfaces. They are made
by adding gelling agents, such as
metallic soaps, to liquid lubricants.
luciferase See bioluminescence.
luciferin See bioluminescence.
lumen Symbol lm. The SI unit of luminous Ûux equal to the Ûux emitted
by a uniform point source of 1 candela in a solid angle of 1 steradian.
luminescence The 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 exciting cause is a photon, the process
is called photoluminescence; if it is
an electron it is called electroluminescence. Chemiluminescence is luminescence resulting from a chemical
reaction (such as the slow oxidation
of phosphorus); *bioluminescence is
the luminescence produced by a living organism (such as a ÜreÛy). If the
luminescence persists signiÜcantly
after the exciting cause is removed it
is called phosphorescence; if it does
not it is called Ûuorescence. This distinction is arbitrary since there must
always be some delay; in some deÜnitions a persistence of more than 10
nanoseconds (10
–8 s) is treated as
phosphorescence.
luminol test A *presumptive test
for blood. The reagent is a mixture of
3-aminophthalhydrazide, sodium car333
luminol test
l
www.AzShimi.ir www.AzShimi.com