associated with the *activation en-
ergy of the reaction.
potentiometric titrationA titra-
tion in which the end point is found
by measuring the potential on an
electrode immersed in the reaction
mixture.
Pourbaix diagramA diagram
showing how oxidation–reduction
behaviour for compounds of a given
element depends on pH.
powder metallurgy A process in
which powdered metals or alloys are
pressed into a variety of shapes at
high temperatures. The process
started with the pressing of pow-
dered tungsten into incandescent
lampÜlaments in theÜrst decade of
this century and is now widely used
for making self-lubricating bearings
and cemented tungsten carbide cut-
ting tools.
The powders are produced by at-
omization of molten metals, chemi-
cal decomposition of a compound of
the metal, or crushing and grinding
of the metal or alloy. The parts are
pressed into moulds at pressures
ranging from 140 × 106 Pa to 830 ×
106 Pa after which they are heated in
a controlled atmosphere to bond the
particles together (see sintering).
praseodymium Symbol Pr. A soft
silvery metallic element belonging to
the *lanthanoids; a.n. 59; r.a.m.
140.91; r.d. 6.773; m.p. 931°C; b.p.
3512 °C. It occurs in bastnasite and
monazite, from which it is recovered
by an ion-exchange process. The only
naturally occurring isotope is
praseodymium–141, which is not
radioactive; however, fourteen ra-
dioisotopes have been produced. It is
used in mischmetal, a rare-earth
alloy containing 5% praseodymium,
for use in lighterÛints. Another rare-
earth mixture containing 30%
praseodymium is used as a catalyst in
cracking crude oil. The element was
discovered by C. A. von Welsbach in
1885.
A- Information from the WebElements site
precessional motion A form of
motion that occurs when a torque is
applied to a rotating body in such a
way that it tends to change the direc-
tion of its axis of rotation. It arises
because the resultant of the angular
velocity of rotation and the incre-
ment of angular velocity produced by
the torque is an angular velocity
about a new direction; this com-
monly changes the axis of the ap-
plied torque and leads to sustained
rotation of the original axis of rota-
tion.
A spinning top, the axis of which is
not exactly vertical, has a torque act-
ing on it as a result of gravity. Instead
of falling over, the top precesses
about a vertical line through the
pivot. Precessional effects occur in
atoms in magneticÜelds.precipitate A suspension of small
solid particles produced in a liquid by
chemical reaction.precipitation 1.All liquid and
solid forms of water that are de-
posited from the atmosphere; it in-
cludes rain, drizzle, snow, hail, dew,
and hoar frost. 2.The formation of a
precipitate.precursor A compound that leads
to another compound in a series of
chemical reactions.pressure The force acting normally
on unit area of a surface or the ratio
of force to area. It is measured in
*pascals in SI units. Absolute pres-
sure is pressure measured on a gauge
that reads zero at zero pressure
rather than at atmospheric pressure.
Gauge pressure is measured on a437 pressure
p