Dictionary of Chemistry [6th Ed.]

adhesives, food packaging, and

moulded articles. It is derived from

the fungus Aureobasidium pullulans.

pumiceA porous volcanic rock that

is light and full of cavities due to ex-

panding gases that were liberated

from solution in the lava while it so-

lidiÜed. Pumice is often light enough

toÛoat on water. It is usually acid

(siliceous) in composition, and is used

as an abrasive and for polishing.

pumpA device that imparts energy

to aÛuid in order to move it from

one place or level to another or to

raise its pressure (compare vacuum

pump). Centrifugal pumps and tur-

bines have rotating impellers, which

increase the velocity of theÛuid, part

of the energy so acquired by theÛuid

then being converted to pressure en-

ergy. Displacement pumps act di-

rectly on theÛuid, forcing it toÛow

against a pressure. They include pis-

ton, plunger, gear, screw, and cam

pumps.

purineAn organic nitrogenous base

(see formula), sparingly soluble in

water, that gives rise to a group of

biologically important derivatives,

notably *adenine and *guanine,

which occur in nucleotides and nu-

cleic acids (DNA and RNA).

pumice 444

p

N

N

N

N

Purine

O

2 CH

2 CH

C

H 2

CH 2

CH 2

Pyran

N

N

Pyrazine

N

N

H

Pyrazole

N

Pyridine

PVA See polyvinylacetate.

PVCSee polychloroethene.

pyranA heterocyclic compound

having an oxygen atom and two dou-

ble bonds in a six-membered ring,

C 5 H 6 O. There are two isomers de-

pending on the position of the CH 2

group.

pyridineA colourless liquid with a

strong unpleasant smell, C 5 H 5 N (see

formula); r.d. 0.98; m.p. –42°C; b.p.

115 °C. Pyridine is an aromatic hete-

rocyclic compound present in coal

pyrazoleAn unsaturated hetero-

cyclic compound having two nitro-

gen atoms in aÜve-membered ring,

C 3 H 4 N 2 ; m.p. 66–70°C; b.p. 168–

188 °C. Pyrazine is a symmetric

diazine.

pyranoseA *sugar having a six-

membered ring containingÜve car-

bon atoms and one oxygen atom.

pyrazineAn unsaturated hetero-

cyclic compound having two nitro-

gen atoms in a six-membered ring,

C 4 H 4 N 2 ; r.d. 1.03; m.p. 52°C; b.p.

115 °C. Pyrazine is a symmetric di-

azine.