at constant temperature (e.g. from
solid to liquid at the melting point or
from liquid to gas at the boiling
point). For example, the latent heat
of vaporization is the energy a sub-
stance absorbs from its surroundings
in order to overcome the attractive
forces between its molecules as it
changes from a liquid to a gas and in
order to do work against the external
atmosphere as it expands. In thermo-
dynamic terms the latent heat is the
*enthalpy of evaporation (∆H), i.e. L =
∆H = ∆U + p∆V, where ∆U is the
change in the internal energy, p is
the pressure, and ∆V is the change in
volume.
The speciÜc latent heat (symbol l) is
the heat absorbed or released per
unit mass of a substance in the
course of its isothermal change of
phase. The molar latent heat is the
heat absorbed or released per unit
amount of substance during an
isothermal change of state.latexNatural rubber as it is ob-
tained from a rubber tree or any sta-
ble suspension in water of a similar
synthetic polymer. Synthetic latexes
are used to make articles from rub-
ber or plastics by such techniques as
dipping (rubber gloves), spreading
(waterproof cloth), and electrodeposi-
tion (plastic-coated metal). They are
also employed in paints and adhe-
sives.Latimer diagramA simple dia-
gram summarizing the standard po-
tentials for an element in different
oxidation states. The different
species are written in a horizontal
line in order of decreasing oxidation
state, with the most highly oxidized
species on the left. Arrows are writ-
ten between adjacent species with
the standard potential in volts indi-
cated above the arrow. Often the oxi-
dation number is included in the
diagram. The standard electrode po-tential for nonadjacent species can
be calculated, but values for common
couples are often indicated by addi-
tional arrows, Latimer diagrams de-
pend on pH and are commonly given
for both acidic and alkaline condi-
tions. See also frost diagram.latticeThe regular arrangement of
atoms, ions, or molecules in a crys-
talline solid. See crystal lattice.lattice energyA measure of the
stability of a *crystal lattice, given by
the energy that would be released
per mole if atoms, ions, or molecules
of the crystal were brought together
from inÜnite distances apart to form
the lattice. See born–haber cycle.lattice vibrationsThe periodic vi-
brations of the atoms, ions, or mol-
ecules in a *crystal lattice about their
mean positions. On heating, the am-
plitude of the vibrations increases
until they are so energetic that the
lattice breaks down. The temperature
at which this happens is the melting
point of the solid and the substance
becomes a liquid. On cooling, the am-
plitude of the vibrations diminishes.
At *absolute zero a residual vibration
persists, associated with the *zero-
point energy of the substance. The
increase in the electrical resistance
of a conductor is due to increased
scattering of the free conduction
electrons by the vibrating lattice
particles.
laughing gasSee dinitrogen
oxide.
lauric acidSee dodecanoic acid.Lavoisier, Antoine Laurent
(1743–1794) French chemist, who col-
lected taxes for the government in
Paris. In the 1770s he discovered oxy-
gen and nitrogen in air and demol-
ished the *phlogiston theory of
combustion by demonstrating the
role of oxygen in the process. In 1783latex 318l