between them, and collisions be-
tween molecules would be perfectly
elastic. In practice, however, the be-
haviour of real gases deviates from
the gas laws because their molecules
occupy aÜnite volume, there are
small forces between molecules, and
in polyatomic gases collisions are to a
certain extent inelastic (see equation
of state).
gas chromatography A technique
for separating or analysing mixtures
of gases by *chromatography. The
apparatus consists of a very long tube
containing the stationary phase. This
may be a solid, such as kieselguhr
(gas–solid chromatography, or GSC),
or a nonvolatile liquid, such as a hy-
drocarbon oil coated on a solid sup-
port (gas–liquid chromatography, or
GLC). The sample is often a volatile
liquid mixture, which is vaporized
and swept through the column by a
carrier gas (e.g. hydrogen). The com-
ponents of the mixture pass through
the column at different rates because
they adsorb to different extents on
the stationary phase. They are de-
tected as they leave, either by meas-
uring the thermal conductivity of the
gas or by aÛame detector.
Gas chromatography is usually
used for analysis; components can be
identiÜed by the time they take to
pass through the column. It is some-
times also used for separating mix-
tures.
Gas chromatography is often used
to separate a mixture into its compo-
nents, which are then directly in-
jected into a mass spectrometer. This
technique is known as gas chro-
matography–mass spectroscopy or
GCMS.
gas chromatography infrared
(GC-IR)A form of *Fourier-transform
infrared (FT-IR) used to identify small
amounts of gas obtained by gas chro-
matography. Since functional groupsin molecules are characteristic in in-
frared spectra this produces informa-
tion to supplement that obtained by
mass spectrometry. It is easier to per-
form FT-IR for gas chromatography
than for liquid chromatography be-
cause carrier solvents in liquid
chromatography absorb infrared
radiation.
gas constant (universal molar gas
constant)Symbol R. The constant
that appears in the universal gas
equation(see gas laws). It has the
value 8.314 510(70) J K–1mol–1.
gas equationSee gas laws.gasiÜcation The conversion of
solid or liquid hydrocarbons to fuel
gas. Solid fuels such as coal or coke
are converted into producer gas (car-
bon monoxide) or water gas (carbon
monoxide and hydrogen) by the ac-
tion of air (or oxygen) and steam.
Solid fuels may also be hydrogenated
to produce methane. Liquid fuels,
from petroleum, are gasiÜed to pro-
duce synthesis gas (carbon monoxide
and hydrogen) or town gas (mostly
hydrogen and methane), usually by
*cracking or *hydrogenation.
gas lawsLaws relating the temper-
ature, pressure, and volume of an
*ideal gas. *Boyle’s law states that
the pressure (p) of a specimen is in-
versely proportional to the volume
(V) at constant temperature (pV = con-
stant). The modern equivalent of
*Charles’ law states that the volume
is directly proportional to the ther-
modynamic temperature (T) at con-
stant pressure (V/T = constant);
originally this law stated the con-
stant expansivity of a gas kept at con-
stant pressure. The pressure law
states that the pressure is directly
proportional to the thermodynamic
temperature for a specimen kept at
constant volume. The three laws can
be combined in the universal gasgas chromatography 242g