Chemistry, Third edition

Glossary

absolute temperature (0 K) The lowest possible temper-

ature (273.15 °C).

absorbance A measure of the extent to which a sample in

a cell absorbs light at a particular wavelength. Defined as

log (Io/I).

absorption (of light) The taking in of a photon of light by

an atom, molecule or ion.

abundance The percentage (in terms of numbers of

atoms) of an isotope in a sample of an element.

accuracy The closeness of measurement(s) to the true

value.

acid A substance that produces hydronium ions (H 3 Oor

H(aq)) in solution.

acidic solution A solution with a pH below 7 at 25 °C.

acidity constant (Ka) The equilibrium constant for the

ionisation of an acid in water.

activation energy The minimum energy required by col-

liding reactant molecules so that chemical change occurs.

activity The number of disintegrations of a radioactive

sample per second (see the website).

addition reaction A reaction in which a carbon–carbon

multiple bond is broken and parts of another molecule

are added to each carbon atom of the multiple bond.

alcohol A series of organic compounds in which an alkyl

group is bonded to –OH. e.g. CH 3 OH

aldehyde One of a series of organic compounds which

contain a –CHO group, e.g. CH 3 CHO.

alkali A water-soluble base.

alkali metal A Group 1 element.

alkaline earth metal A Group 2 element.

alkaline solution A solution of a base – a solution with a

pH above 7 at 25 °C.

alkane A hydrocarbon of general formula CnH 2 n 2 , e.g.

CH 4.

alkene A hydrocarbon of general formula CnH 2 n, e.g.

C 2 H 4.

alkyne A hydrocarbon of general formula CnH2n 2 , e.g.

C 2 H 2.

allotropes Alternative forms of an element in which

the atoms are joined together in different ways, e.g.

diamond, graphite and buckminsterfullerene.

alpha particle A helium ion, produced in some types of

radioactive decay.

amine Compounds in which organic groups replace the

hydrogen atoms in ammonia, e.g. CH 3 NH 2 ,(CH 3 ) 2 NH

and (CH 3 ) 3 N

amphoteric substance A substance is amphoteric when

it shows reactions that are characteristic of both acids

and bases.

anode In an electrochemical cell (or battery), the electrode

at which oxidation takes place.

aromatic compound An organic compound that contains

a benzene ring, e.g. C 6 H 5 Cl.

aryl group An aromatic group, e.g. –C 6 H 5.

atmosphere (1) The gaseous envelope around a planet.

(2) A unit of pressure, equal to 101 kPa.

atomic mass (of an element) The average mass of one

atom of that element in atomic mass units; symbolized

m(element).

atomic mass unit (u) A very small unit of mass (1 u

1.66 10 ^24 g).

atomic number (Z) The number of protons in an atom.

Avogadro’s constant (NA) The number of objects per

mol of objects (NA6.022 1023 mol^1 ).

Avogadro’s law (or principle) Equal volumes of all gases

at the same pressure and temperature contain the same

number of molecules (or moles).

base A substance that accepts a proton from an acid,

forming a salt and water only.

basicity constant (Kb) The equilibrium constant for the

ionisation of a base in water.

Beer–Lambert equation (law) An equation relating the

absorbanceAof a substance to the cell path length b

and the molar absorption coefficient of the substance.

Formally expressed as Acb.

blood alcohol concentration (BAC) The concentration

of ethanol in blood, usually axpressed in units of mg of

alcohol per 100ml of blood.

bond dissociation enthalpy (HA–B—^ ) A measure of the

strength of a bond. The standard enthalpy change when

1 mol of bonds are broken, all species being in the gas phase.

buffer (solution) A solution which resists changes in pH

upon the addition of acid, base or upon dilution.

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