A vapor or vapour (see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature. [R.H.Petrucci, W.S.Harwood and F.G.Herring, "General Chemistry", 8th edition (Prentice-Hall 2002), p.486] 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 its partial 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). [Petrucci et al. p.483]


"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

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). [Petrucci et al. p.484]

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 vapour pressure is the sum of the component partial pressures. [Thomas Engel and Philip Reid, "Physical Chemistry" (Pearson Benjamin-Cummings 2006) p.194]

The physical chemistry behind distillation is based on manipulating the equilibrium occurring between the liquid and vapor phases of a molecule in solution.


*Perfumes contain chemicals that vaporize at different temperatures and at different rate in scent accords known as notes.
*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.
*Mercury-vapor lamps and sodium vapor lamps produce light from atoms in excited states.

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 explosive limit and upper explosive limit of the flammable liquid.

ee also

*Water vapor
*Dilution (equation)
*Vapor pressure
*Vapor Trail
*Gas phase
*Henry's Law
*Raoult's Law


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Look at other dictionaries:

  • Vapor — Va por, n. [OE. vapour, OF. vapour, vapor, vapeur, F. vapeur, L. vapor; probably for cvapor, and akin to Gr. ? smoke, ? to breathe forth, Lith. kvepti to breathe, smell, Russ. kopote fine soot. Cf. {Vapid}.] [Written also {vapour}.] [1913… …   The Collaborative International Dictionary of English

  • Vapor — puede referirse a lo siguiente: Vapor (estado): aquel gas que se puede condensar por presurización a temperatura constante o por enfriamiento a presión constante. Vapor de agua. El vapor como fuente de propulsión o de generación de energía… …   Wikipedia Español

  • vapor — VAPÓR, vapoare, s.n. Navă pusă în mişcare de un motor propriu. – Din ngr. vapori. Trimis de bogdanrsb, 27.04.2006. Sursa: DEX 98  VAPÓR s. (mar.) navă, vas, (rar) bastiment. (vapor de mare tonaj.) Trimis de siveco, 05.08.2004. Sursa: Sinonime … …   Dicționar Român

  • vapor — (Del lat. vapor, ōris). 1. m. Fluido gaseoso cuya temperatura es inferior a su temperatura crítica. Su presión no aumenta al ser comprimido, sino que se transforma parcialmente en líquido; p. ej., el producido por la ebullición del agua. 2. Gas… …   Diccionario de la lengua española

  • Vapor — Va por, v. i. [imp. & p. p. {Vapored}; p. pr. & vb. n. {Vaporing}.] [From {Vapor}, n.: cf. L. vaporare.] [Written also {vapour}.] [1913 Webster] 1. To pass off in fumes, or as a moist, floating substance, whether visible or invisible, to steam;… …   The Collaborative International Dictionary of English

  • Vapor — Va por, v. t. To send off in vapor, or as if in vapor; as, to vapor away a heated fluid. [Written also {vapour}.] [1913 Webster] He d laugh to see one throw his heart away, Another, sighing, vapor forth his soul. B. Jonson. [1913 Webster] …   The Collaborative International Dictionary of English

  • vapor — |ô| s. m. 1. Fluido como fumo que pela ação do calor se desprende dos corpos úmidos e que não é senão a água ou umidade que se transforma. 2. Exalação de corpos sólidos resultante de decomposição ou de combustão. 3.  [Por extensão] Fumo.… …   Dicionário da Língua Portuguesa

  • vapor — [vā′pər] n. [ME vapour < Anglo Fr < MFr vapeur < L vapor < IE base * wep , to give off vapors] 1. a) visible particles of moisture floating in the air, as fog, mist, or steam b) any cloudy or imperceptible exhalation, as smoke or… …   English World dictionary

  • vapor — vàpōr (vapȏr) m <G vapóra> DEFINICIJA 1. rij. para 2. reg. parobrod 3. reg. svaki veći brod ETIMOLOGIJA tal. vapore ← lat. vapor: para …   Hrvatski jezični portal

  • vapor — late 14c., from Anglo Fr. vapour, from L. vaporem (nom. vapor) exhalation, steam, heat, of unknown origin. Vapors fit of fainting, hysteria, etc. is 1660s, from medieval notion of exhalations from the stomach or other organs affecting the brain …   Etymology dictionary

  • Vapor — (lat.), Dampf …   Pierer's Universal-Lexikon

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