I

ice See water.

ice pointThe temperature at
which there is equilibrium between
ice and water at standard atmos-
pheric pressure (i.e. the freezing or
melting point under standard condi-
tions). It was used as aÜxed point (0°)
on the Celsius scale, but the kelvin
and the International Practical Tem-
perature Scale are based on the
*triple point of water.

icosahedron A polyhedron having
20 triangular faces withÜve edges
meeting at each vertex. The symme-
try of an icosahedron (known as
icosahedral symmetry) hasÜvefold ro-
tation axes. It is impossible in crys-
tallography to have a periodic crystal
with the point group symmetry of an
icosahedron (icosahedral packing).
However, it is possible for short-
range order to occur with icosahedral
symmetry in certain liquids and
glasses because of the dense packing
of icosahedra. Icosahedral symmetry
also occurs in certain quasicrystals,
such as alloys of aluminium and
manganese.

ideal crystalA single crystal with a
perfectly regular lattice that contains
no impurities, imperfections, or
other defects.

ideal gas (perfect gas)A hypotheti-
cal gas that obeys the *gas laws ex-
actly. An ideal gas would consist of
molecules that occupy negligible
space and have negligible forces be-
tween them. All collisions made be-
tween molecules and the walls of the
container or between molecules and
other molecules would be perfectly
elastic, because the molecules would

have no means of storing energy ex-

cept as translational kinetic energy.

ideal solutionSee raoult’s law.

IEIonization energy. See ionization

potential.

ignition temperature The tem-

perature to which a substance must

be heated before it will burn in air.

Ilkovic equationA relation used

in polarography relating the diffu-

sion current iaand the concentration

c. The Ilkovic equation has the form

ia= kc, where k is a constant.

imidesOrganic compounds con-

taining the group –CO.NH.CO.– (the

imido group).

A

• Information about IUPAC nomenclature

imido group See imides.

iminesCompounds containing the

group –NH– in which the nitrogen

atom is part of a ring structure, or

the group =NH, in which the nitro-

gen atom is linked to a carbon atom

by a double bond. In either case, the

group is referred to as an imino

group.

A

• Information about IUPAC nomenclature
  imino group See imines.

Imperial unitsThe British system

of units based on the pound and the

yard. The former f.p.s. system was

used in engineering and was loosely

based on Imperial units; for all

scientiÜc purposes *SI units are now

used. Imperial units are also being re-

placed for general purposes by met-

ric units.