Dictionary of Chemistry [6th Ed.]

extra energy can come only from the

internal, or thermal, energy of the

substance.

adiabatic process Any process

that occurs without heat entering or

leaving a system. In general, an adia-

batic change involves a fall or rise in

temperature of the system. For exam-

ple, if a gas expands under adiabatic

conditions, its temperature falls

(work is done against the retreating

walls of the container). The adiabatic

equationdescribes the relationship

between the pressure (p) of an ideal

gas and its volume (V), i.e. pVγ= K,

where γis the ratio of the principal

speciÜc *heat capacities of the gas

and K is a constant.

adipic acidSee hexanedioic acid.

ADPSee atp.

adrenaline (epinephrine)A hor-

mone, produced by the medulla of

the adrenal glands, that increases

heart activity, improves the power

and prolongs the action of muscles,

and increases the rate and depth of

breathing to prepare the body for

‘fright,Ûight, orÜght’. At the same

time it inhibits digestion and excre-

tion.

adiabatic process 14

a

adsorbateA substance that is ad-

sorbed on a surface.

adsorbentA substance on the sur-

face of which a substance is ad-

sorbed.

adsorptionThe formation of a

layer of gas, liquid, or solid on the

surface of a solid or, less frequently,

of a liquid. There are two types de-

pending on the nature of the forces

involved. In chemisorptiona single

layer of molecules, atoms, or ions is

attached to the adsorbent surface by

chemical bonds. In physisorptionad-

sorbed molecules are held by the

weaker *van der Waals’ forces. Ad-

sorption is an important feature of

surface reactions, such as corrosion,

and heterogeneous catalysis. The

property is also utilized in adsorption

*chromatography.

adsorption indicator (absorption

indicator)A type of indicator used

in reactions that involve precipita-

tion. The yellow dyeÛuorescein is a

common example, used for the reac-

tion

NaCl(aq) + AgNO 3 (aq) →AgCl(s) +

NaNO 3 (aq)

As silver nitrate solution is added

to the sodium chloride, silver chlo-

ride precipitates. As long as Cl–ions

are in excess, they adsorb on the

precipitate particles. At the end

point, no Cl–ions are left in solution

and negativeÛuorescein ions are

then adsorbed, giving a pink colour

to the precipitate. The technique

is sometimes known as Fajan’s

method.

adsorption isotherm An equation

that describes how the amount of a

substance adsorbed onto a surface de-

pends on its pressure (if a gas) or its

concentration (if in a solution), at a

constant temperature. Several ad-

sorption isotherms are used in sur-

face chemistry including the *BET

isotherm and the *Langmuir ad-

sorption isotherm. The different

isotherms correspond to different as-

sumptions about the surface and the

adsorbed molecules.

OH

OH

N H

OH CH 3

Adrenaline