h.c.p. Hexagonal close packing. See
close packing.
headspace The space above a sample held in a sealed container. Headspace 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
capacity is the ratio of the heat supplied 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 capacity (C) is measured in joules per
kelvin, speciÜc heat capacity (c) in
J K
–1 kg
–1
, and molar heat capacity
(C m ) in J K
–1 mol
–1
. For a gas, the values of c and C m are 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 symbols for the speciÜc and molar heat
capacities at constant volume are c v
and C v , respectively; those for the
speciÜc and molar heat capacities at
constant pressure are c p and C p .
heat engine A device for converting heat into work. Engines usually
work on cycles of operation, the
most efÜcient of which would be the
*Carnot cycle.
heat of atomization The energy
required to dissociate one mole of a
given substance into atoms.
heat of combustion The energy
liberated when one mole of a given
substance is completely oxidized.
heat of crystallization The energy liberated when one mole of a
given substance crystallizes from a
saturated solution of the same substance.
heat of dissociation The energy
absorbed when one mole of a given
substance is dissociated into its constituent elements.
heat of formation The energy liberated or absorbed when one mole of
a compound is formed in their *standard states from its constituent elements.
heat of neutralization The energy liberated in neutralizing one
mole of an acid or base.
heat of reaction The energy liberated or absorbed as a result of the
complete chemical reaction of molar
amounts of the reactants.
heat of solution The energy liberated or absorbed when one mole of a
given substance is completely dissolved in a large volume of solvent
(strictly, to inÜnite dilution).
heavy hydrogen See deuterium.
heavy metal A 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 environmental *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 spar A 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 hexagonal crystals on freezing. Its physical
properties differ from those of ‘normal’ water; r.d. 1.105; m.p. 3.8°C; b.p.
h.c.p.
266
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close packing.
headspace The space above a sample held in a sealed container. Headspace 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
capacity is the ratio of the heat supplied 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 capacity (C) is measured in joules per
kelvin, speciÜc heat capacity (c) in
J K
–1 kg
–1
, and molar heat capacity
(C m ) in J K
–1 mol
–1
. For a gas, the values of c and C m are 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 symbols for the speciÜc and molar heat
capacities at constant volume are c v
and C v , respectively; those for the
speciÜc and molar heat capacities at
constant pressure are c p and C p .
heat engine A device for converting heat into work. Engines usually
work on cycles of operation, the
most efÜcient of which would be the
*Carnot cycle.
heat of atomization The energy
required to dissociate one mole of a
given substance into atoms.
heat of combustion The energy
liberated when one mole of a given
substance is completely oxidized.
heat of crystallization The energy liberated when one mole of a
given substance crystallizes from a
saturated solution of the same substance.
heat of dissociation The energy
absorbed when one mole of a given
substance is dissociated into its constituent elements.
heat of formation The energy liberated or absorbed when one mole of
a compound is formed in their *standard states from its constituent elements.
heat of neutralization The energy liberated in neutralizing one
mole of an acid or base.
heat of reaction The energy liberated or absorbed as a result of the
complete chemical reaction of molar
amounts of the reactants.
heat of solution The energy liberated or absorbed when one mole of a
given substance is completely dissolved in a large volume of solvent
(strictly, to inÜnite dilution).
heavy hydrogen See deuterium.
heavy metal A 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 environmental *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 spar A 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 hexagonal crystals on freezing. Its physical
properties differ from those of ‘normal’ water; r.d. 1.105; m.p. 3.8°C; b.p.
h.c.p.
266
h
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