hydrocarbons. The liquid components are obtained by vacuum distillation and used in lubricating oils.
The solid components (parafÜn wax)
are obtained by solvent extraction.
The Ünal residue is a black tar containing free carbon (asphalt or bitumen).
petroleum ether A colourless
volatile Ûammable mixture of hydrocarbons (not an ether), mainly pentane and hexane. It boils in the range
30–70°C and is used as a solvent.
petroleum jelly (petrolatum) A
semi-solid mixture of hydrocarbons
extracted from petroleum. It is used
in skincare products and cosmetics,
and as a lubricant. Petroleum is
widely available under the tradename Vaseline.
pewter An alloy of lead and tin. It
usually contains 63% tin; pewter
tankards and food containers should
have less than 35% of lead so that the
lead remains in solid solution with
the tin in the presence of weak acids
in the food and drink. Copper is
sometimes added to increase ductility and antimony is added if a hard
alloy is required.
peyote See mescaline.
Pfund series See hydrogen spectrum.
PGA See phosphoglyceric acid.
pH See ph scale.
phane See cyclophane.
phase A homogeneous part of a
heterogeneous system that is separated from other parts by a distinguishable boundary. A mixture of ice
and water is a two-phase system. A
solution of salt in water is a singlephase system.
phase diagram A graph showing
the relationship between solid, liquid, and gaseous *phases over a
range of conditions (e.g. temperature
and pressure). See steel (illustration).
phase rule For any system at equilibrium, the relationship P + F = C + 2
holds, where P is the number of distinct phases, C the number of components, and F the number of degrees
of freedom of the system. The relationship derived by Josiah Willard
*Gibbs in 1876, is often called the
Gibbs phase rule.
phase space For a system with n
degrees of freedom, the 2n-dimensional space with coordinates (q 1 , q 2 ,
…, q n , p 1 , p 2 , …, p n ), where the qs describe the degrees of freedom of the
system and the ps are the corresponding momenta. Each point represents a state of the system. In a gas
of N point particles, each particle has
three positional coordinates and
three corresponding momentum coordinates, so that the phase space
has 6N-dimensions. If the particles
have internal degrees of freedom,
such as the vibrations and rotations
of molecules, then these must be included in the phase space, which is
consequently of higher dimension
than that for point particles. As the
system changes with time the representative points trace out a curve in
phase space known as a trajectory.
See also attractor; configuration
space; statistical mechanics.
phase transition A change in a
feature that characterizes a system.
Examples of phase transitions are
changes from solid to liquid, liquid to
gas, and the reverse changes. Other
examples of phase transitions include the transition from a paramagnet to a ferromagnet (see magnetism)
and the transition from a normally
conducting metal to a superconductor. Phase transitions can occur by al407
phase transition
p
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The solid components (parafÜn wax)
are obtained by solvent extraction.
The Ünal residue is a black tar containing free carbon (asphalt or bitumen).
petroleum ether A colourless
volatile Ûammable mixture of hydrocarbons (not an ether), mainly pentane and hexane. It boils in the range
30–70°C and is used as a solvent.
petroleum jelly (petrolatum) A
semi-solid mixture of hydrocarbons
extracted from petroleum. It is used
in skincare products and cosmetics,
and as a lubricant. Petroleum is
widely available under the tradename Vaseline.
pewter An alloy of lead and tin. It
usually contains 63% tin; pewter
tankards and food containers should
have less than 35% of lead so that the
lead remains in solid solution with
the tin in the presence of weak acids
in the food and drink. Copper is
sometimes added to increase ductility and antimony is added if a hard
alloy is required.
peyote See mescaline.
Pfund series See hydrogen spectrum.
PGA See phosphoglyceric acid.
pH See ph scale.
phane See cyclophane.
phase A homogeneous part of a
heterogeneous system that is separated from other parts by a distinguishable boundary. A mixture of ice
and water is a two-phase system. A
solution of salt in water is a singlephase system.
phase diagram A graph showing
the relationship between solid, liquid, and gaseous *phases over a
range of conditions (e.g. temperature
and pressure). See steel (illustration).
phase rule For any system at equilibrium, the relationship P + F = C + 2
holds, where P is the number of distinct phases, C the number of components, and F the number of degrees
of freedom of the system. The relationship derived by Josiah Willard
*Gibbs in 1876, is often called the
Gibbs phase rule.
phase space For a system with n
degrees of freedom, the 2n-dimensional space with coordinates (q 1 , q 2 ,
…, q n , p 1 , p 2 , …, p n ), where the qs describe the degrees of freedom of the
system and the ps are the corresponding momenta. Each point represents a state of the system. In a gas
of N point particles, each particle has
three positional coordinates and
three corresponding momentum coordinates, so that the phase space
has 6N-dimensions. If the particles
have internal degrees of freedom,
such as the vibrations and rotations
of molecules, then these must be included in the phase space, which is
consequently of higher dimension
than that for point particles. As the
system changes with time the representative points trace out a curve in
phase space known as a trajectory.
See also attractor; configuration
space; statistical mechanics.
phase transition A change in a
feature that characterizes a system.
Examples of phase transitions are
changes from solid to liquid, liquid to
gas, and the reverse changes. Other
examples of phase transitions include the transition from a paramagnet to a ferromagnet (see magnetism)
and the transition from a normally
conducting metal to a superconductor. Phase transitions can occur by al407
phase transition
p
www.AzShimi.ir www.AzShimi.com