associated with the *activation energy of the reaction.
potentiometric titration A titration in which the end point is found
by measuring the potential on an
electrode immersed in the reaction
mixture.
Pourbaix diagram A 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 powdered tungsten into incandescent
lamp Ülaments in the Ürst decade of
this century and is now widely used
for making self-lubricating bearings
and cemented tungsten carbide cutting tools.
The powders are produced by atomization of molten metals, chemical 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 × 10
6 Pa to 830 ×
10
6 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 radioisotopes have been produced. It is
used in mischmetal, a rare-earth
alloy containing 5% praseodymium,
for use in lighter Ûints. Another rareearth 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 direction of its axis of rotation. It arises
because the resultant of the angular
velocity of rotation and the increment of angular velocity produced by
the torque is an angular velocity
about a new direction; this commonly changes the axis of the applied torque and leads to sustained
rotation of the original axis of rotation.
A spinning top, the axis of which is
not exactly vertical, has a torque acting 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 magnetic Üelds.
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 deposited from the atmosphere; it includes 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 pressure is pressure measured on a gauge
that reads zero at zero pressure
rather than at atmospheric pressure.
Gauge pressure is measured on a
437
pressure
p
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potentiometric titration A titration in which the end point is found
by measuring the potential on an
electrode immersed in the reaction
mixture.
Pourbaix diagram A 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 powdered tungsten into incandescent
lamp Ülaments in the Ürst decade of
this century and is now widely used
for making self-lubricating bearings
and cemented tungsten carbide cutting tools.
The powders are produced by atomization of molten metals, chemical 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 × 10
6 Pa to 830 ×
10
6 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 radioisotopes have been produced. It is
used in mischmetal, a rare-earth
alloy containing 5% praseodymium,
for use in lighter Ûints. Another rareearth 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 direction of its axis of rotation. It arises
because the resultant of the angular
velocity of rotation and the increment of angular velocity produced by
the torque is an angular velocity
about a new direction; this commonly changes the axis of the applied torque and leads to sustained
rotation of the original axis of rotation.
A spinning top, the axis of which is
not exactly vertical, has a torque acting 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 magnetic Üelds.
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 deposited from the atmosphere; it includes 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 pressure is pressure measured on a gauge
that reads zero at zero pressure
rather than at atmospheric pressure.
Gauge pressure is measured on a
437
pressure
p
www.AzShimi.ir www.AzShimi.com