Chemistry, Third edition

454 GLOSSARY

carboxylic acid One of a series of organic compounds

which contain a –COOH group, e.g. CH 3 COOH.

catalyst A substance which, although it is not consumed,

speeds up a reaction

cathode In a battery or electrochemical cell, the electrode

at which reduction takes place.

chain reaction A sequence of self-sustaining reactions.

chemiluminescence The emission of light from a chemi-

cal reaction.

chromatography A technique used to separate mixtures.

The separation takes place on a specially selected

material, e.g. alumina.

colloid Particles of substances in solution which are

smaller than those in suspensions but larger than

individual molecules or ions.

common ion effect A reduction in the concentration of a

species ‘AB’ in solution, due to the swamping of the mix-

ture with A or B ions from another source.

complex (inorganic chemistry) Species consisting of lig-

ands bonded to a central metal ion, e.g. [Ag(NH 3 ) 2 ].

compound A substance consisting of atoms of different

elements joined together in a definite ratio.

conjugate acid Species made when a base gains a proton.

conjugate base Species made when an acid loses a proton.

coordination number The number of nearest neighbours.

These are ions of opposite charge in an ionic crystal, or

the number of atoms that are attached to the central

metal ion in a complex.

critical pressure The pressure required to liquefy a sub-

stance at its critical temperature.

critical temperature The temperature above which a

substance cannot be liquefied, whatever the applied

pressure.

decay constant (k) The rate constant (first order) for the

decay of a radionuclide.

deionized water Pure water obtained by passing tap water

through an ion-exchange resin.

delocalization The ‘spreading’ out of electron density

over a molecule or ion.

density Mass per unit volume of substance; common unit

gcm^3.

diatomic molecule A molecule that consists of two

atoms, e.g. HCl and O 2.

dipole A partial positive and equal partial negative charge

separated by a distance.

dissociation Another word for ‘break up’.

distillation The separation of a mixture, making use of the

fact that its components have different boiling points.

electrochemical cell Two electrodes in electrical contact

with each other and with a solution (the electrolyte).

electrode An experimental arrangement (normally

involving a metal conductor) at which oxidation or

reduction occurs, e.g. in an electrochemical cell.

electron A negatively charged sub-atomic particle.

electron-gain enthalpy (H—eg^ ) The standard enthalpy

change when one mol of anions is formed, all species

being in the gas phase.

electronegativity The measure of the ability of an atom

to attract bonding electrons to itself when it is joined to

another atom by a covalent bond.

element A substance in which all the atoms have the same

atomic number. Matter that cannot be broken down into

anything simpler by chemical reactions.

elementary reaction An individual reaction step; part of

an overall reaction mechanism.

emission (of light) The giving out of a photon of light by

an atom, molecule or ion.

empirical formula A chemical formula that uses the

smallest whole number ratio of the atoms present in a

compound.

emulsifying agent A substance that stabilizes an emulsion

so allowing emulsification.

emulsion A colloidal solution consisting of one liquid dis-

persed in another.

end point In a titration, the point at which enough solu-

tion has been added so that the indicator begins to

change colour. Ideally, the end point should equal the

equivalence point.

endothermic reaction A reaction in which heat is taken in.

enthalpy The energy (or ‘heat content’) of a substance at

constant pressure.

enthalpy of fusion (H^ —fus) The standard enthalpy change

when 1 g (or 1 mol) of a liquid is formed from 1 g (or

1 mol) of solid without a change in temperature; units

are J g^1 or J mol^1. See the website.

enthalpy of vaporization (H—vap^ ) The standard enthalpy

change when 1 g (or 1 mol) of a vapour is formed from

1 g (or 1 mol) of liquid without a change in temperature.

Units are J g^1 or J mol^1. See the website.

equilibrium A dynamic equilibrium is a state of balance

that is produced by two opposing processes (e.g. forward

and back reactions) which are occurring at the same rate.

equilibrium composition A particular set of concentra-

tions of reactants and products at equilibrium.

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