456 GLOSSARYmean bond enthalpy HA–B An average bond diss-
ociation energy for a bond A–B, for example averaged
over several different types of molecules.melting point The temperature at which a solid melts at a
particular pressure (see normal melting point).metalloid Elements with properties that lie between those
typical of metals and non-metals. Also known as semi-
metals.miscible Liquids which dissolve in each other (i.e. mix).molar absorption coefficient Symbolized ; A measure
of the intensity of absorption of a substance at a particu-
lar wavelength.molar solubility The concentration of a substance (in
units of moles per cubic decimetre, i.e. mol dm^3 ) in a
saturated solution of that substance at that temperature.mole (mol) The SI unit of amount of substancemole fraction The number of moles of a substance in a
mixture divided by the total number of moles of all the
components of the mixture.molecule A neutral particle consisting of two or more
atoms combined in a definite ratio.neutral solution A solution with equal numbers of hydro-
nium and hydroxide ions, with a pH equal to 7 at 25 °C.neutralization reaction The chemical reaction in which a
base ‘cancels out’ an acid, producing a salt and water only.neutron A neutral particle found in atoms of all elements
(except for protium 11 H).NMR Nuclear magnetic resonance – a type of spectroscopy.noble gas A Group 18 element.normal boiling point Also known as normal boiling tem-
perature – the temperature at which the vapour pressure
of a liquid equals 1 atm.normal freezing point Also called normal freezing tem-
perature– the temperature at which a liquid freezes
under a pressure of 1 atm.normal melting point Also called melting temperature- the temperature at which a solid melts under a
pressure of 1 atm. Equals the normal freezing point.
nucleon A name for the particles in the nucleus (i.e. for
neutrons and protons).nuclide Anatomof a particular isotope.orbital The volume in which an electron is to be found
with a certain probability (e.g. 90%). Each orbital holds
up to two electrons, which may be thought of as spinning
in opposite directions.osmosis The flow of solvent (usually water) through a
semipermeable membrane.osmotic pressure The pressure which must be applied
across a semipermeable membrane in order to stop
osmosis.oxidizing agent A substance which causes another sub-
stance to be oxidized in a chemical reaction and is itself
reduced.pascal (Pa) SI unit of pressure, 101 325 Pa = 1 atm.percent transmittance The percentage of light at a par-
ticular wavelength which is allowed through a sample in
a spectrometer; defined as (I/Io)100.pH The negative logarithm of the hydronium ion concen-
tration in an aqueous solution. At 25 °C, pure water is
neutral with a pH of 7.0.phenol An organic compound in which an –OH group is
directly attached to a benzene ring, e.g. C 6 H 5 OH.photolysis The decomposition of a substance using light
as the energy source.photon A ‘particle’ of light; it has no mass and consists of
pure energy.pKa The negative logarithm of an acidity constant.pKb The negative logarithm of a basicity constant.polar molecule A molecule having one end that is posi-
tively charged while the other is equally negatively
charged.polyatomic molecule A molecule that contains more
than two atoms, e.g. O 3 , H 2 O.precipitation The formation of an insoluble substance in
solution.precision The closeness of repeat measurements.proton A positively charged particle in the nucleus of all
atoms; also an isolated hydrogen ion.pyrolysis The decomposition of a substance using heat as
the energy source.radioactivity The spontaneous disintegration of nuclei.radioisotope An isotope that is radioactive.radionuclide An atom of a radioisotope.random errors Errors which cause repeat measurements
to be scattered (up and down) around the true value.rate constant (k) The rate of a reaction (at a particular
temperature) when the reacting substances are at a con-
centration of 1 mol dm^3 ;kvalues measure the relative
speeds of different reactions.rate expression An expression, found by experiment,
which relates the reaction rate to the rate constant and to
the concentrations of the reactants.0230_000118_30_Gloss.qxd 3/2/06 2:24 pm Page 456