Vapor pressure or equilibrium vapor pressure is defined as the
pressure exerted by a vapor in thermodynamic equilibrium with its
condensed phases (solid or liquid) at a given temperature in a closed
system. The equilibrium vapor pressure is an indication of a liquid's
evaporation rate. It relates to the tendency of particles to escape
from the liquid (or a solid). A substance with a high vapor pressure
at normal temperatures is often referred to as volatile. The pressure
exhibited by vapor present above a liquid surface is known as vapor
pressure. As the temperature of a liquid increases, the kinetic energy
of its molecules also increases. As the kinetic energy of the
molecules increases, the number of molecules transitioning into a
vapor also increases, thereby increasing the vapor pressure.
The vapor pressure of any substance increases non-linearly with
temperature according to the Clausius–Clapeyron relation. The
atmospheric pressure boiling point of a liquid (also known as the
normal boiling point) is the temperature at which the vapor pressure
equals the ambient atmospheric pressure. With any incremental increase
in that temperature, the vapor pressure becomes sufficient to overcome
atmospheric pressure and lift the liquid to form vapor bubbles inside
the bulk of the substance. Bubble formation deeper in the liquid
requires a higher pressure, and therefore higher temperature, because
the fluid pressure increases above the atmospheric pressure as the
depth increases. More important at shallow depths is the higher
temperature required to start bubble formation. The surface tension of
the bubble wall leads to an overpressure in the very small, initial
bubbles. Thus, thermometer calibration should not rely on the
temperature in boiling water.
The vapor pressure that a single component in a mixture contributes to
the total pressure in the system is called partial pressure. For
example, air at sea level, and saturated with water vapor at
20 °C, has partial pressures of about 2.3 kPa of water, 78 kPa
of nitrogen, 21 kPa of oxygen and 0.9 kPa of argon, totaling 102.2
kPa, making the basis for standard atmospheric pressure.
1 Measurement and units
2 Estimating vapor pressures with Antoine equation
3 Relation to boiling point of liquids
4 Liquid mixtures
Boiling point of water
7 Dühring's rule
9 Estimating vapor pressure from molecular structure
10 Meaning in meteorology
11 See also
13 External links
Measurement and units
Vapor pressure is measured in the standard units of pressure. The
International System of Units
International System of Units (SI) recognizes pressure as a derived
unit with the dimension of force per area and designates the pascal
(Pa) as its standard unit. One pascal is one newton per square meter
(N·m−2 or kg·m−1·s−2).
Experimental measurement of vapor pressure is a simple procedure for
common pressures between 1 and 200 kPa. Most accurate results are
obtained near the boiling point of substances and large errors result
for measurements smaller than 1kPa. Procedures often consist of
purifying the test substance, isolating it in a container, evacuating
any foreign gas, then measuring the equilibrium pressure of the
gaseous phase of the substance in the container at different
temperatures. Better accuracy is achieved when care is taken to ensure
that the entire substance and its vapor are at the prescribed
temperature. This is often done, as with the use of an isoteniscope,
by submerging the containment area in a liquid bath.
Very low vapor pressures of solids can be measured using the Knudsen
effusion cell method.
In a medical context, vapor pressure is sometimes expressed in other
units, specifically millimeters of mercury (mmHg). This is important
for volatile anesthetics, most of which are liquids at body
temperature, but with a relatively high vapor pressure. Anesthetics
with a higher vapor pressure at body temperature will be excreted more
quickly, as they are exhaled from the lungs.
Estimating vapor pressures with Antoine equation
The Antoine equation is a mathematical expression of the
relation between the vapor pressure and the temperature of pure liquid
or solid substances. The basic form of the equation is:
displaystyle log P=A- frac B C+T
and it can be transformed into this temperature-explicit form:
displaystyle T= frac B A-log P -C
is the absolute vapor pressure of a substance
is the temperature of the substance
are substance-specific coefficients (i.e., constants or parameters)
is typically either
displaystyle log _ 10
displaystyle log _ e
A simpler form of the equation with only two coefficients is sometimes
displaystyle log P=A- frac B T
which can be transformed to:
displaystyle T= frac B A-log P
Sublimations and vaporizations of the same substance have separate
sets of Antoine coefficients, as do components in mixtures. Each
parameter set for a specific compound is only applicable over a
specified temperature range. Generally, temperature ranges are chosen
to maintain the equation's accuracy of a few up to 8–10 percent. For
many volatile substances, several different sets of parameters are
available and used for different temperature ranges. The Antoine
equation has poor accuracy with any single parameter set when used
from a compound's melting point to its critical temperature. Accuracy
is also usually poor when vapor pressure is under 10
Torr because of
the limitations of the apparatus used to establish the Antoine
The Wagner Equation gives "one of the best" fits to experimental
data but is quite complex. It expresses reduced vapor pressure as a
function of reduced temperature.
Relation to boiling point of liquids
Further information: Boiling point
A log-lin vapor pressure chart for various liquids
As a general trend, vapor pressures of liquids at ambient temperatures
increase with decreasing boiling points. This is illustrated in the
vapor pressure chart (see right) that shows graphs of the vapor
pressures versus temperatures for a variety of liquids. At the
normal boiling point of a liquid, the vapor pressure is equal to the
standard atmospheric pressure defined as 1 atmosphere (760
101 325 kPa).
For example, at any given temperature, methyl chloride has the highest
vapor pressure of any of the liquids in the chart. It also has the
lowest normal boiling point (−24.2 °C), which is where the
vapor pressure curve of methyl chloride (the blue line) intersects the
horizontal pressure line of one atmosphere (atm) of absolute vapor
Although the relation between vapor pressure and temperature is
non-linear, the chart uses a logarithmic vertical axis to produce
slightly curved lines, so one chart can graph many liquids. A nearly
straight line is obtained when the logarithm of the vapor pressure is
plotted against 1/(T+230) where T is the temperature in degrees
Celsius. The vapor pressure of a liquid at its boiling point equals
the pressure of its surrounding environment.
Raoult's law gives an approximation to the vapor pressure of mixtures
of liquids. It states that the activity (pressure or fugacity) of a
single-phase mixture is equal to the mole-fraction-weighted sum of the
components' vapor pressures:
displaystyle P_ tot =sum _ i Py_ i =sum _ i P_ i ^ sat x_ i ,
displaystyle p_ tot
is the mixture's vapor pressure,
displaystyle x_ i
is the mole fraction of component
in the liquid phase and
displaystyle y_ i
is the mole fraction of component
in the vapor phase respectively.
displaystyle P_ i ^ sat
is the vapor pressure of component
Raoult's law is applicable only to non-electrolytes (uncharged
species); it is most appropriate for non-polar molecules with only
weak intermolecular attractions (such as London forces).
Systems that have vapor pressures higher than indicated by the above
formula are said to have positive deviations. Such a deviation
suggests weaker intermolecular attraction than in the pure components,
so that the molecules can be thought of as being "held in" the liquid
phase less strongly than in the pure liquid. An example is the
azeotrope of approximately 95% ethanol and water. Because the
azeotrope's vapor pressure is higher than predicted by Raoult's law,
it boils at a temperature below that of either pure component.
There are also systems with negative deviations that have vapor
pressures that are lower than expected. Such a deviation is evidence
for stronger intermolecular attraction between the constituents of the
mixture than exists in the pure components. Thus, the molecules are
"held in" the liquid more strongly when a second molecule is present.
An example is a mixture of trichloromethane (chloroform) and
2-propanone (acetone), which boils above the boiling point of either
The negative and positive deviations can be used to determine
thermodynamic activity coefficients of the components of mixtures.
Vapor pressure of liquid and solid benzene
Equilibrium vapor pressure can be defined as the pressure reached when
a condensed phase is in equilibrium with its own vapor. In the case of
an equilibrium solid, such as a crystal, this can be defined as the
pressure when the rate of sublimation of a solid matches the rate of
deposition of its vapor phase. For most solids this pressure is very
low, but some notable exceptions are naphthalene, dry ice (the vapor
pressure of dry ice is 5.73 MPa (831 psi, 56.5 atm) at 20 degrees
Celsius, which causes most sealed containers to rupture), and ice. All
solid materials have a vapor pressure. However, due to their often
extremely low values, measurement can be rather difficult. Typical
techniques include the use of thermogravimetry and gas transpiration.
There are a number of methods for calculating the sublimation pressure
(i.e., the vapor pressure) of a solid. One method is to estimate the
sublimation pressure from extrapolated liquid vapor pressures (of the
supercooled liquid), if the heat of fusion is known, by using this
particular form of the Clausius–Clapeyron relation:
displaystyle ln ,P_ solid ^ S =ln ,P_ liquid ^ S - frac Delta H_
m R left( frac 1 T - frac 1 T_ m right)
displaystyle P_ solid ^ S
= Sublimation pressure of the solid component at the temperature
displaystyle T<T_ m
displaystyle P_ liquid ^ S
= Extrapolated vapor pressure of the liquid component at the
displaystyle T<T_ m
displaystyle Delta H_ m
= Heat of fusion
= Gas constant
= Sublimation temperature
displaystyle T_ m
= Melting point temperature
This method assumes that the heat of fusion is
temperature-independent, ignores additional transition temperatures
between different solid phases, and it gives a fair estimation for
temperatures not too far from the melting point. It also shows that
the sublimation pressure is lower than the extrapolated liquid vapor
pressure (ΔHm is positive) and the difference grows with increased
distance from the melting point.
Boiling point of water
Graph of water vapor pressure versus temperature. At the normal
boiling point of 100 °C, it equals the standard atmospheric pressure
Torr or 101.325 kPa.
Vapor pressure of water
Like all liquids, water boils when its vapor pressure reaches its
surrounding pressure. In nature, the atmospheric pressure is lower at
higher elevations and water boils at a lower temperature. The boiling
temperature of water for atmospheric pressures can be approximated by
the Antoine equation:
displaystyle log _ 10 P=8.07131- frac 1730.63 233.426+T_ b
or transformed into this temperature-explicit form:
displaystyle T_ b = frac 1730.63 8.07131-log _ 10 P -233.426
where the temperature
displaystyle T_ b
is the boiling point in degrees
Celsius and the pressure
is in Torr.
Main article: Dühring's rule
Dühring's rule states that a linear relationship exists between the
temperatures at which two solutions exert the same vapor pressure.
The following table is a list of a variety of substances ordered by
increasing vapor pressure (in absolute units).
Methyl isobutyl ketone
Estimating vapor pressure from molecular structure
Several empirical methods exist to estimate liquid vapor pressure from
molecular structure for organic molecules. Some examples are
SIMPOL, the method of Moller et al., and EVAPORATION.
Meaning in meteorology
In meteorology, the term vapor pressure is used to mean the partial
pressure of water vapor in the atmosphere, even if it is not in
equilibrium, and the equilibrium vapor pressure is specified
otherwise. Meteorologists also use the term saturation vapor pressure
to refer to the equilibrium vapor pressure of water or brine above a
flat surface, to distinguish it from equilibrium vapor pressure, which
takes into account the shape and size of water droplets and
particulates in the atmosphere.
Vapour pressure of water
Reid vapor pressure
Saturation vapor density
True vapor pressure
Vapor pressures of the elements (data page)
^ Růžička, K.; Fulem, M. & Růžička, V. "
Organic Compounds. Measurement and Correlation" (PDF).
^ a b What is the Antoine Equation? (Chemistry Department, Frostburg
State University, Maryland)
^ a b Sinnot, R.K. (2005). Chemical Engineering Design] (4th ed.).
Butterworth-Heinemann. p. 331. ISBN 0-7506-6538-6.
^ Wagner, W. (1973), "New vapour pressure measurements for argon and
nitrogen and a new method for establishing rational vapour pressure
equations", Cryogenics, 13 (8): 470–482,
^ Perry's Chemical Engineers' Handbook, 7th Ed. pp. 4–15
^ Perry, R.H.; Green, D.W., eds. (1997). Perry's Chemical Engineers'
Handbook (7th ed.). McGraw-Hill. ISBN 0-07-049841-5.
^ Petrucci, Ralph H.; Harwood, William S.; Herring, F.Geoffrey (2002).
General Chemistry (8th ed.). Prentice Hall. p. 484.
^ Dreisbach, R. R. & Spencer, R. S. (1949). "Infinite Points of
Cox Chart Families and dt/dP Values at any Pressure". Industrial and
Engineering Chemistry,. 41 (1). p. 176.
^ a b Moller B.; Rarey J.; Ramjugernath D. (2008). "Estimation of the
vapour pressure of non-electrolyte organic compounds via group
contributions and group interactions". Journal of Molecular Liquids.
143: 52. doi:10.1016/j.molliq.2008.04.020.
^ "Thermophysical Properties Of Fluids II – Methane, Ethane,
Propane, Isobutane, And Normal Butane" (page 110 of PDF, page 686 of
original document), BA Younglove and JF Ely.
^ "Thermophysical Properties Of Nitrous Oxide" (page 14 of PDF, page
10 of original document), ESDU.
^ Pankow, J. F.; et al. (2008). "SIMPOL.1: a simple group contribution
method for predicting vapor pressures and enthalpies of vaporization
of multifunctional organic compounds". Atmos. Chem. Phys. 8 (10):
^ "Vapour pressure of pure liquid compounds. Estimation by
^ Compernolle, S.; et al. (2011). "EVAPORATION: a new vapour pressure
estimation method for organic molecules including non-additivity and
intramolecular interactions". Atmos. Chem. Phys. 11 (18): 9431–9450.
^ Glossary Archived 2011-04-15 at the Wayback Machine. (Developed by
the American Meteorological Society)
^ A Brief Tutorial. jhuapl.edu (An article about the definition of
equilibrium vapor pressure)
Fluid Characteristics Chart
Online vapor pressure calculation tool (Requires Registration)
Vapor Pressures of Pure Liquid Organic Compounds