Dictionary of Chemistry [6th Ed.]

tering such variables as temperature

and pressure.

Phase transitions can be classiÜed

by their order. If there is non-zero

*latent heat at the transition it is

said to be a Ürst-order transition. If

the latent heat is zero it is said to be

a second-order transition.

Some models describing phase

transitions, particularly in low-

dimensional systems, are amenable

to exact mathematical solutions.

Techniques for investigating transi-

tions may include the feature of uni-

versality, in which very different

physical systems behave in the same

way near a phase transition.

phencyclidine(PCP)A hallucino-

genic drug, originally used as a veti-

nary anaesthetic. It is usually used as

a powder (known as ‘angel dust’).

phencyclidine 408

p

N

Phencyclidine

phenol(carbolic acid)A white crys-

talline solid, C 6 H 5 OH; r.d. 1.1; m.p.

43 °C; b.p. 182°C. It is made by the

*cumene process or by the *Raschig

process. Phenol reacts readily to form

substituted derivatives. It is an im-

portant industrial chemical used in

making Nylon, phenolic and epoxy

resins, dyestuffs, explosives, and

pharmaceuticals. See also phenols.

phenol–formaldehyde resin A

class of resins produced by polymer-

izing phenols with formaldehyde

(methanol) using acid or basic cata-

lysts. They are generally cross-linked

thermosetting materials. *Bakelite is

the original example.

phenolic resins Synthetic resins

made by copolymerizing phenols

with aldehydes. *Phenol–formalde-

hyde resins are the commonest type.

phenolphthaleinA dye used as an

acid-base *indicator. It is colourless

below pH 8 and red above pH 9.6. It

is used in titrations involving weak

acids and strong bases. It is also used

as a laxative.

phenolphthalein testSee kastle

meyer test.

phenolsOrganic compounds that

contain a hydroxyl group (–OH)

bound directly to a carbon atom in a

benzene ring. Unlike normal alco-

hols, phenols are acidic because of

the inÛuence of the aromatic ring.

Thus, phenol itself (C 6 H 5 OH) ionizes

in water:

C 6 H 5 OH →C 6 H 5 O–+ H+

Phenols are made by fusing a sul-

phonic acid salt with sodium hydrox-

ide to form the sodium salt of the

phenol. The free phenol is liberated

by adding sulphuric acid.

phenomenological relations See

onsager relations.

phenoxy resins Thermoplastic

materials made by condensation of

phenols, used mainly for packaging.

phenylalanineSee amino acid.

phenylamine(aniline; aminoben-

zene)A colourless oily liquid aro-

matic *amine, C 6 H 5 NH 2 , with an

‘earthy’ smell; r.d. 1.0217; m.p.

–6.3°C; b.p. 184.1°C. The compound

turns brown on exposure to sunlight.

It is basic, forming the phenylammo-

nium(or anilinium) ion, C 6 H 5 NH 3 +,

with strong acids. It is manufactured

by the reduction of nitrobenzene or

by the addition of ammonia to

chlorobenzene using a copper(II) salt