used as a rough *indicator of acidity
or alkalinity, both in solution and as
litmus paper (absorbent paper soaked
in litmus solution).
litre Symbol l. A unit of volume in
the metric system regarded as a spe-
cial name for the cubic decimetre. It
was formerly deÜned as the volume
of 1 kilogram of pure water at 4°C at
standard pressure, which is equiva-
lent to 1.000 028 dm^3.
lixiviationThe separation of mix-
tures by dissolving soluble con-
stituents in water.
LNGSee liquefied petroleum gas.
localizationThe conÜnement of
electrons to a particular atom in a
molecule or to a particular chemical
bond.
localized bondA *chemical bond
in which the electrons forming the
bond remain between (or close to)
the linked atoms. Compare delocal-
ization.
lock-and-key mechanism A
mechanism proposed in 1890 by
Emil Fischer (1852–1919) to explain
binding between the active site of an
enzyme and a substrate molecule.
The active site was thought to have a
Üxed structure (the lock), which ex-
actly matched the structure of a
speciÜc substrate (the key). Thus the
enzyme and substrate interact to
form an *enzyme–substrate complex.
The substrate is converted to prod-
ucts that no longerÜt the active site
and are therefore released, liberating
the enzyme. Recent observations
made by X-ray diffraction studies
have shown that the active site of an
enzyme is moreÛexible than the
lock-and-key theory would suggest.
lodestoneSee magnetite.
logarithmic scale 1.A scale of
measurement in which an increase
or decrease of one unit represents a
tenfold increase or decrease in the
quantity measured. Decibels and pH
measurements are common exam-
ples of logarithmic scales of measure-
ment. 2.A scale on the axis of a
graph in which an increase of one
unit represents a tenfold increase in
the variable quantity. If a curve y = xn
is plotted on graph paper with loga-
rithmic scales on both axes, the re-
sult is a straight line of slope n, i.e.
logy = nlogx, which enables n to be
determined.London formulaA formula giving
an expression for the induced-
dipole–induced-dipole interaction
between molecules (called the dis-
persion interaction or London inter-
action). The London formula for the
interaction energy V is given by V=- C/r^6 , where C = ⅔α′ 1 α′ 2 I 1 I 2 /(I 1 + I 2 ).
Here α′ 1 and α′ 2 are the polarizability
volumes of molecule 1 and 2 respec-
tively, I 1 and I 2 are the ionization
energies of molecules 1 and 2 respec-
tively, and r is the distance between
the molecules. The London formula
is named after Fritz London (1900–
54), who derived it. The interaction
described by the London formula is
usually the dominant term in inter-
molecular forces (unless hydrogen
bonds are present).
lone pairA pair of electrons having
opposite spin in an orbital of an
atom. For instance, in ammonia the
nitrogen atom hasÜve electrons,
three of which are used in forming
single bonds with hydrogen atoms.
The other two occupy aÜlled atomic
orbital and constitute a lone pair (see
illustration). The orbital containing
these electrons is equivalent to a sin-
gle bond (sigma orbital) in spatial ori-
entation, accounting for the
pyramidal shape of the molecule. In
the water molecule, there are two
lone pairs on the oxygen atom. In331 lone pair
l