h.c.p.Hexagonal close packing. See
close packing.headspaceThe space above a sam-
ple held in a sealed container. Head-
space analysis is used in forensic
science to investigate the volatile
constituents of a sample.heat capacity (thermal capacity)
The ratio of the heat supplied to an
object or specimen to its consequent
rise in temperature. The speciÜc heat
capacityis the ratio of the heat sup-
plied to unit mass of a substance to
its consequent rise in temperature.
The molar heat capacity is the ratio
of the heat supplied to unit amount
of a substance to its consequent rise
in temperature. In practice, heat ca-
pacity (C) is measured in joules per
kelvin, speciÜc heat capacity (c) in
JK–1kg–1, and molar heat capacity
(Cm) in J K–1mol–1. For a gas, the val-
ues of c and Cmare commonly given
either at constant volume, when only
its *internal energy is increased, or
at constant pressure, which requires
a greater input of heat as the gas is
allowed to expand and do work
against the surroundings. The sym-
bols for the speciÜc and molar heat
capacities at constant volume are cv
and Cv, respectively; those for the
speciÜc and molar heat capacities at
constant pressure are cpand Cp.
heat engineA device for convert-
ing heat into work. Engines usually
work on cycles of operation, the
most efÜcient of which would be the
*Carnot cycle.heat of atomizationThe energy
required to dissociate one mole of a
given substance into atoms.heat of combustionThe energy
liberated when one mole of a given
substance is completely oxidized.
heat of crystallizationThe en-
ergy liberated when one mole of agiven substance crystallizes from a
saturated solution of the same sub-
stance.heat of dissociationThe energy
absorbed when one mole of a given
substance is dissociated into its con-
stituent elements.
heat of formationThe energy lib-
erated or absorbed when one mole of
a compound is formed in their *stan-
dard states from its constituent el-
ements.
heat of neutralizationThe en-
ergy liberated in neutralizing one
mole of an acid or base.
heat of reaction The energy liber-
ated or absorbed as a result of the
complete chemical reaction of molar
amounts of the reactants.heat of solutionThe energy liber-
ated or absorbed when one mole of a
given substance is completely dis-
solved in a large volume of solvent
(strictly, to inÜnite dilution).heavy hydrogen See deuterium.
heavy metalA metal with a high
relative atomic mass. The term is
usually applied to common transition
metals, such as copper, lead, and
zinc. These metals are a cause of en-
vironmental *pollution (heavy-metal
pollution) from a number of sources,
including lead in petrol, industrial
efÛuents, and leaching of metal ions
from the soil into lakes and rivers by
acid rain.heavy sparA mineral form of
*barium sulphate, BaSO 4.heavy water (deuterium oxide)
Water in which hydrogen atoms,^1 H,
are replaced by the heavier isotope
deuterium,^2 H (symbol D). It is a
colourless liquid, which forms hexag-
onal crystals on freezing. Its physical
properties differ from those of ‘nor-
mal’ water; r.d. 1.105; m.p. 3.8°C; b.p.h.c.p. 266h