adhesives, food packaging, and
moulded articles. It is derived from
the fungus Aureobasidium pullulans.
pumice A porous volcanic rock that
is light and full of cavities due to expanding gases that were liberated
from solution in the lava while it solidiÜed. Pumice is often light enough
to Ûoat on water. It is usually acid
(siliceous) in composition, and is used
as an abrasive and for polishing.
pump A device that imparts energy
to a Ûuid in order to move it from
one place or level to another or to
raise its pressure (compare vacuum
pump). Centrifugal pumps and turbines have rotating impellers, which
increase the velocity of the Ûuid, part
of the energy so acquired by the Ûuid
then being converted to pressure energy. Displacement pumps act directly on the Ûuid, forcing it to Ûow
against a pressure. They include piston, plunger, gear, screw, and cam
pumps.
purine An 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 nucleic acids (DNA and RNA).
pumice
444
p
N
N
N
N
Purine
O
C
H 2
C
H 2
C
H 2
CH 2
CH 2
Pyran
N
N
Pyrazine
N
N
H
Pyrazole
N
Pyridine
PVA See polyvinylacetate.
PVC See polychloroethene.
pyran A heterocyclic compound
having an oxygen atom and two double bonds in a six-membered ring,
C 5 H 6 O. There are two isomers depending on the position of the CH 2
group.
pyridine A 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 heterocyclic compound present in coal
pyrazole An unsaturated heterocyclic compound having two nitrogen atoms in a Üve-membered ring,
C 3 H 4 N 2 ; m.p. 66–70°C; b.p. 168–
188°C. Pyrazine is a symmetric
diazine.
pyranose A *sugar having a sixmembered ring containing Üve carbon atoms and one oxygen atom.
pyrazine An unsaturated heterocyclic compound having two nitrogen 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 diazine.
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moulded articles. It is derived from
the fungus Aureobasidium pullulans.
pumice A porous volcanic rock that
is light and full of cavities due to expanding gases that were liberated
from solution in the lava while it solidiÜed. Pumice is often light enough
to Ûoat on water. It is usually acid
(siliceous) in composition, and is used
as an abrasive and for polishing.
pump A device that imparts energy
to a Ûuid in order to move it from
one place or level to another or to
raise its pressure (compare vacuum
pump). Centrifugal pumps and turbines have rotating impellers, which
increase the velocity of the Ûuid, part
of the energy so acquired by the Ûuid
then being converted to pressure energy. Displacement pumps act directly on the Ûuid, forcing it to Ûow
against a pressure. They include piston, plunger, gear, screw, and cam
pumps.
purine An 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 nucleic acids (DNA and RNA).
pumice
444
p
N
N
N
N
Purine
O
C
H 2
C
H 2
C
H 2
CH 2
CH 2
Pyran
N
N
Pyrazine
N
N
H
Pyrazole
N
Pyridine
PVA See polyvinylacetate.
PVC See polychloroethene.
pyran A heterocyclic compound
having an oxygen atom and two double bonds in a six-membered ring,
C 5 H 6 O. There are two isomers depending on the position of the CH 2
group.
pyridine A 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 heterocyclic compound present in coal
pyrazole An unsaturated heterocyclic compound having two nitrogen atoms in a Üve-membered ring,
C 3 H 4 N 2 ; m.p. 66–70°C; b.p. 168–
188°C. Pyrazine is a symmetric
diazine.
pyranose A *sugar having a sixmembered ring containing Üve carbon atoms and one oxygen atom.
pyrazine An unsaturated heterocyclic compound having two nitrogen 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 diazine.
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