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{{Other uses}}
{{short description|Substances in the gas phase at a temperature lower than its critical point}}
{{Use American English|date=July 2019}}
[[File:Nitrogen dioxide gas.jpg|thumb|An ampule of [[nitrogen oxide]] vapor: brown [[nitrogen dioxide]] and colorless [[dinitrogen tetroxide]], in [[Nitrogen dioxide#Basic thermal properties|equilibrium]]]]
In physics, a '''vapor''' ([[American English]]) or '''vapour''' ([[Commonwealth English]]; [[American and British English spelling differences#-our, -or|see spelling differences]]) is a substance in the [[gas]] phase at a [[temperature]] lower than its [[critical temperature]],<ref name=Petrucci>R. H. Petrucci, W. S. Harwood, and F. G. Herring, ''General Chemistry'', Prentice-Hall, 8th ed. 2002, p. 483–86.</ref> which means that the vapor can be [[condensation|condensed]] to a [[liquid]] by increasing the [[pressure]] on it without reducing the temperature of the vapor. A vapor is different from an [[aerosol]].<ref name=Cheng2014/> An aerosol is a suspension of tiny particles of liquid, solid, or both within a gas.<ref name=Cheng2014>{{cite journal|last1=Cheng|first1=T.|title=Chemical evaluation of electronic cigarettes|journal=Tobacco Control|volume=23|issue=Supplement 2|year=2014|pages=ii11–ii17|issn=0964-4563|doi=10.1136/tobaccocontrol-2013-051482|pmc=3995255|pmid=24732157}}</ref>

For example, water has a critical temperature of {{convert| 647|K|C F}}, which is the highest temperature at which liquid water can exist at any pressure. In the [[Earth's atmosphere|atmosphere]] at ordinary temperatures gaseous water (known as [[water vapor]]) will condense into a liquid if its [[partial pressure]] is increased sufficiently.

A vapor may co-exist with a liquid (or a solid). When this is true, the two phases will be in equilibrium, and the gas-partial pressure will be equal to the equilibrium [[vapor pressure]] of the liquid (or solid).<ref name=Petrucci/>

== Properties ==
[[Image:phase-diag2.svg|thumb|upright=1.3|The vapor-liquid critical point in a pressure-temperature [[phase diagram]] is at the high-temperature extreme of the liquid–gas phase boundary (the dotted green line gives the anomalous behaviour of water).]]
''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. (For example, water has a critical temperature of 374&nbsp;°C (647 K), which is the highest temperature at which liquid water can exist.) If the vapor is in contact with a liquid or solid phase, the two phases will be in a state of [[thermodynamic equilibrium|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 technology|headspace extraction]] from a liquid sample prior to [[gas chromatography]].

The constituent [[molecule]]s of a vapor possess vibrational, rotational, and translational motion. These motions are considered in the [[kinetic theory of gases]].

==Vapor pressure==
{{Main|Vapor pressure}}
[[File:Binary Boiling Point Diagram new.svg|left|thumb|Liquid–vapor equilibrium]]
[[File:Vapor being used in a cloud chamber.jpg|thumb|If the [[vapor pressure]] exceeds the [[List of types of equilibrium|equilibrium]] value, it becomes [[supersaturated]] and [[condenses]] on any available [[nucleation]] sites e. g. [[particles]] of dust. This principle is used in [[cloud chamber]]s, where particles of [[radiation]] are visualized because they [[nucleate]] formation of water droplets.]]
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 [[atmosphere (unit)|normal atmospheric pressure]].<ref name=Petrucci/>

For two-phase systems (e.g., two liquid phases), the vapor pressure of the individual phases are equal.  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.<ref>Thomas Engel and Philip Reid, ''Physical Chemistry,'' Pearson Benjamin-Cummings, 2006, p.194</ref>

== Examples ==
[[File:Crepuscular Rays Beam through the Mist Blown from Takkakaw Falls.jpg|thumb|Invisible water vapor condenses to form visible water droplets called mist]]

* [[Perfume]]s contain chemicals that vaporize at different temperatures and at different rate in scent accords, known as [[note (perfumery)|note]]s.
* [[Earth's atmosphere|Atmospheric]] [[water vapor]] is found near the earth's surface, and may condense into small liquid droplets and form meteorological phenomena, such as [[fog]], [[mist]], and [[haar (fog)|haar]].
* [[Mercury-vapor lamp]]s and [[sodium vapor lamp]]s produce light from atoms in [[excited state]]s.
* [[Flammable liquid]]s do not burn when ignited.<ref>{{Cite book|url=https://books.google.com/books?id=fvrgJx81f3EC&q=Flammable+liquids+do+not+burn+when+ignited+rather+it+is+the+vapor+cloud&pg=PA41|title=Fundamentals of Fire Protection for the Safety Professional|last1=Ferguson|first1=Lon H.|last2=Janicak|first2=Christopher A.|date=2005-09-01|publisher=Government Institutes|isbn=9781591919605|language=en}}</ref> 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.
[[Electronic cigarette|E-cigarettes]] produce [[aerosol]]s, not vapors.<ref name=Cheng2014/>

==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.

==See also==
{{Wiktionary|vapor|vapour}}
* {{annotated link|Contrail|aka=vapor trail}}
* {{annotated link|Dilution (equation)}}
* {{annotated link|Evaporation}}
* {{annotated link|Henry's law}}
* {{annotated link|Vaporizer (disambiguation)}}

==References==
{{Reflist}}

{{State of matter}}

{{Authority control}}

[[Category:Gases]]
[[Category:Pressure]]
[[Category:Chemical properties]]

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