Vapor pressure 40197 222145633 2008-06-27T19:49:01Z 216.185.75.182 /* See also */ '''Vapor pressure''' (also known as ''equilibrium vapor pressure'' or ''saturation vapor pressure''), is the [[pressure]] of a [[vapor]] in [[Thermodynamic equilibrium|equilibrium]] with its non-vapor [[Phase (matter)|phase]]s. All [[liquid]]s and [[solid]]s have a tendency to [[evaporate]] to a gaseous form, and all [[gas]]es have a tendency to [[Condensation|condense]] back into their original form (either liquid or solid). At any given [[temperature]], for a particular substance, there is a [[pressure]] at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of [[molecule]]s and [[atom]]s to escape from a liquid or a solid. A substance with a high vapor pressure at normal temperatures is often referred to as ''[[volatility (chemistry)|volatile]]''. The [[Kelvin equation]] shows how equilibrium vapor pressure depends on droplet size. An example is [[water vapor]] when air is [[saturation (chemistry)|saturated]] with water vapor. It is the vapor pressure usually found over a flat surface of liquid water, <ref>Babin, SM, [http://fermi.jhuapl.edu/people/babin/vapor/index.html ''Water Vapor Myths: A Brief Tutorial''] (revised 9/12/98), accessed 2007-07-08 </ref> and is a [[dynamic equilibrium]] where the rate of [[condensation]] of [[water]] equals the rate of [[evaporation]] of [[water]]. In general, the higher the temperature, the higher the vapor pressure. When [[air]] is at the saturation vapor pressure, it is said to be at the [[dew point]]. Thus, at saturation vapor pressure, air has a [[relative humidity]] of 100% and [[condensation]] occurs with any increase of water vapor content or a reduction in [[temperature]]. The international standard for saturation vapor pressure over water is given by the [[Goff-Gratch equation]]. Another more recent [[equation]] for [[water]] is the [[Arden Buck Equation]]. Assuming absolutely clean air, if water droplets have a high curvature, which is the case when they are smaller, they require relative humidities in excess of 100% (known as [[supersaturation]]) to be at an equilibrium vapor pressure. As droplets approach approximately [[20]] [[micrometers]], they can survive at 100% relative humidity. As the droplet grows larger by collision and [[Coalescence (meteorology)|coalescence]], it can survive longer because its curvature becomes smoother as the droplet grows. Of course, in actual practice in the Earth's atmosphere, the ability of water to condense into droplets is generally affected by the presence of [[hygroscopic]] dust particles ([[Condensation nuclei|Cloud Condensation nuclei]]). The relative humidity required for droplets to actually form can be significantly below the real saturation vapor pressure due to the solute effect. Finally, if the temperature becomes low enough in a cloud, as it does in [[nimbostratus]] and [[cumulonimbus]] clouds, microscopic ice crystals may also serve as condensation nuclei for the cloud in a process known as the [[Bergeron process]]. 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 where 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 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. ==Relation between vapor pressures and normal boiling points of liquids == [[Image:Vapor Pressure Chart.png|thumb|right|301 px|A typical vapor pressure chart for various liquids]] The higher the vapor pressure of a liquid at a given temperature, the lower the normal boiling point (i.e., the boiling point at atmospheric pressure) of the liquid. The vapor pressure chart to the right has graphs of the vapor pressures versus temperatures for a variety of liquids.<ref>{{cite book|author=Perry, R.H. and Green, D.W. (Editors)|title=[[Perry's Chemical Engineers' Handbook]]|edition=7th Edition|publisher=McGraw-Hill|year=1997|id= ISBN 0-07-049841-5}}</ref> As can be seen in the chart, the liquids with the highest vapor pressures have the lowest normal boiling points. For example, at any given temperature, [[propane]] has the highest vapor pressure of any of the liquids in the chart. It also has the lowest normal boiling point(-43.7 °C), which is where the vapor pressure curve of propane (the purple line) intersects the horizontal pressure line of one atmosphere ([[Atmosphere (unit)|atm]]) of absolute vapor pressure. Although the relation between vapor pressure and temperature is non-linear, the chart uses a logarithmic vertical axis in order to obtain slightly curved lines so that one chart can graph many liquids. ==Units of vapor pressure== The international [[SI]] unit for pressure is the [[pascal (unit)|pascal]] (Pa), equal to one [[newton]] per [[square meter]] (N·m<sup>-2</sup> or kg·m<sup>-1</sup>·s<sup>-2</sup>). The conversions to other pressure units are: {{Pressure Units}} ==Vapor pressure of solids== 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 (physics)|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, meaning it will cause most sealed containers to explode), 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]]. ==Water vapor pressure== {{ main|Vapor pressure of water}} Water, like all liquids, starts to boil when its vapor pressure reaches its surrounding pressure. At higher elevations the atmospheric pressure is lower and water will boil at a lower temperature. The boiling temperature of [[water]] for pressures around 100 [[Pascal (unit)|kPa]] can be approximated by <math>T_b = 100 + 0.0002772 \cdot (p - 101000) - 1.24 \cdot 10^{-9} \cdot (p - 101000)^2</math> where the temperature <math>T_b</math> is the boiling point temperature in degrees [[Celsius]] and the pressure <math>p</math> is in [[pascal (unit)|pascal]]s. One gets the vapor pressure by solving this equation for <math>p</math>. [[Image:Water vapor pressure graph.jpg|thumb|right|Graph of water vapor pressure versus temperature. Note that at the normal boiling point of 100°C, the vapor pressure equals the standard atmospheric pressure of 760 Torr.]] In meteorology, the international standard for the [[vapour pressure of water]] over a flat surface is given by the [[Goff-Gratch equation]]. ==Vapor pressure of mixtures== [[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: <math> p_\text{tot} = \sum_i p_i\chi_i </math> where ''p'' is vapor pressure, ''i'' is a component [[index (mathematics)|index]], and χ is a [[mole fraction]]. The term <math>p_i\chi_i</math> is the vapor pressure of component ''i'' in the mixture. 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 force]]s). 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 pure component. ==Examples of vapor pressures== {|style="text-align:center;" class="wikitable" ! Gas ! Vapor Pressure<br>(SI units) ! Vapor Pressure<br>(bar) ! Vapor Pressure<br>(mmHg) ! Temperature |- | [[Helium]] | 100 kPa | 1 | 750 | -269.15 °C |- | [[Propane]] | 2.2 MPa | 22 | 16500 | 55 °C |- | [[Butane]] | 220 kPa | 2.2 | 1650 | 20 °C |- | [[Carbonyl sulfide]] | 1.255 MPa | 12.55 | 9412 | 25 °C |- | [[Acetaldehyde]] | 98.7 kPa | 0.987 | 740 | 20 °C |- | [[Freon|Freon 113]] | 37.9 kPa | 0.379 | 284 | 20 °C |- | [[Methyl isobutyl ketone]] | 26.48 kPa | 0.02648 | 19.86 | 25 °C |- | [[Tungsten]] | 100 Pa | 0.001 | 0.75 | 3203 °C |- | [[Dioxygen]] | 54.2 MPa | 542 | 407936 | 20 °C |- | [[Dinitrogen]] | 63.2 MPa | 632 | 475106 | 20 °C |- |} == Usage of the term ''vapor pressure'' 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 equilibrium,<ref>[http://amsglossary.allenpress.com/glossary/search?id=vapor-pressure1 Glossary] (Developed by the [[American Meteorological Society]])</ref> and the '''equilibrium vapor pressure''' is specified as such. 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.<ref>[http://fermi.jhuapl.edu/people/babin/vapor/index.html A Brief Tutorial] (An article about the definition of equilibrium vapor pressure)</ref> ==See also== * [[Absolute humidity]] * [[Clausius-Clapeyron Equation]] * [[Partial pressure]] * [[Relative humidity]] * [[Relative volatility]] * [[Triple point]] * [[Vapor-liquid equilibrium]] * [[Vapor Pressure of Water at Various Temperatures]] * [[Volatility (chemistry)|Volatility]] ==References== {{Reflist}} ==External links== *[http://www.ilpi.com/msds/ref/vaporpressure.html MSDS Vapor Pressure] *[http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html#c2 Hyperphysics] *[http://www.envmodels.com/freetools.php?menu=pression Online vapor pressure calculation tool (Requires Registration)] [[Category:Chemical properties]] [[Category:Physical chemistry]] [[Category:Chemical engineering]] [[Category:Meteorology]] [[af:Dampdruk]] [[ast:Presión de vapor]] [[bs:Pritisak pare]] [[ca:Pressió de vapor]] [[de:Dampfdruck]] [[el:Τάση ατμών]] [[es:Presión de vapor]] [[eu:Lurrun-presio]] [[fa:فشار بخار]] [[fr:Pression de vapeur]] [[ko:증기압]] [[id:Tekanan uap]] [[is:Gufuþrýstingur]] [[it:Pressione di vapore]] [[he:לחץ אדים]] [[jv:Penetan uwab]] [[hu:Gőznyomás]] [[nl:Dampdruk]] [[ja:蒸気圧]] [[no:Damptrykk]] [[nn:Damptrykk]] [[pl:Ciśnienie pary nasyconej]] [[pt:Pressão de vapor]] [[ro:Presiune de vapori]] [[sr:Напон паре]] [[sh:Napon pare]] [[fi:Höyrynpaine]] [[sv:Ångtryck]] [[ta:ஆவியமுக்கம்]] [[th:ความดันไอ]] [[ur:بخاری دباؤ]] [[zh:蒸氣壓]]