A vapor may co-exist with a liquid (or solid). When
this is true, the two phases will be in equilibrium, and the gas pressure will
equal the equilibrium
vapor pressure of the liquid (or solid).[2]
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A vapor (American spelling) or vapour (see
spelling differences) is a substance in the
gasphase at a
temperature lower than its
critical temperature.[1]
This means that the vapor can be
condensed to a
liquid or to a
solid by increasing its
pressure, without reducing the temperature.
For example,
water has a critical temperature of 374°C (or 647 K) which is the highest
temperature at which liquid water can exist. In the
atmosphere at ordinary temperatures, therefore, gaseous water is known as
water vapor and will condense to liquid if itspartial
pressure is increased sufficiently.
A vapor may co-exist with a liquid (or solid). When
this is true, the two phases will be in equilibrium, and the gas pressure will
equal the equilibrium
vapor pressure of the liquid (or solid).[2]
Properties
Vapor refers to a gas phase at a temperature where the same substance can
also exist in the
liquid or
solid state, below the
critical temperature of the substance. If the vapor is in contact with a
liquid or solid phase, the two phases will be in a state of
equilibrium. The term gas refers to a compressible fluid phase. Fixed gases
are gases for which no liquid or solid can form at the temperature of the gas
(such as air at typical ambient temperatures). A liquid or solid does not have
to boil to release a vapor.
Vapor is responsible for the familiar processes of
cloud formation and
condensation. It is commonly employed to carry out the physical processes of
distillation and
headspace extraction from a liquid sample prior to
gas chromatography.
The constituent
molecules of a vapor possess vibrational, rotational, and translational
motion. These motions are considered in the
kinetic theory of gases.
Vapor pressure
Main article:
Vapor pressure
Liquid-Vapor Equilibrium
The
vapor pressure is the equilibrium pressure from a liquid or a solid at a
specific temperature. The equilibrium vapor pressure of a liquid or solid is not
affected by the amount of contact with the liquid or solid interface.
The
normal boiling point of a liquid is the
temperature at which the
vapor pressure is equal to one
atmosphere (unit).[3]
For two-phase systems (e.g., two liquid phases), the vapor pressure of the
system is the sum of the vapor pressures of the two liquids. In the absence of
stronger inter-species attractions between like-like or like-unlike molecules,
the vapor pressure follows
Raoult's Law, which states that the
partial pressure of each component is the product of the vapor pressure of
the pure component and its mole fraction in the mixture. The total vapor
pressure is the sum of the component partial pressures.[4]
The
physical chemistry behind
distillation is based on manipulating the
equilibriumoccurring between the liquid and vapor phases of a molecule in
solution.
Examples
Water vapor is responsible for humidity
Measuring vapor
Since it is in the gas phase, the amount of vapor present is quantified by
the
partial pressure of the gas. Also, vapors obey the
barometric formula in a gravitational field just as conventional atmospheric
gases do.
Vapors of flammable liquids
Flammable liquids do not burn when ignited. It is the vapor cloud above the
liquid that will burn if the vapor's concentration is between the
Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL) of the flammable
liquid.
See also
References
-
^ R.H.Petrucci, W.S.Harwood and F.G.Herring, "General Chemistry", 8th
edition (Prentice-Hall 2002), p.486
-
^ Petrucci et al. p.483
-
^ Petrucci et al. p.484
-
^ Thomas Engel and Philip Reid, "Physical Chemistry" (Pearson
Benjamin-Cummings 2006) p.194
Retrieved from "http://en.wikipedia.org/wiki/Vapor"
Categories:
Gases |
Pressure |
Chemical properties
http://en.wikipedia.org/wiki/Vapor
