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Water condenses into visible droplets after evaporating from a cup of hot tea

A vapour (English spelling) or vapor (American-English Spelling) is a substance in the gas phase at a temperature lower than its critical point.[1] This means that the vapour can be condensed to a liquid by increasing its pressure without reducing the temperature.

For example, water has a critical temperature of 374 °C (647 K), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water (known as water vapour) will condense to liquid if its partial pressure is increased sufficiently.

A vapour may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas partial pressure will equal the equilibrium vapour pressure of the liquid (or solid).[1]


The vapour-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.)

Vapour 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 °C (647 K), which is the highest temperature at which liquid water can exist.) If the vapour 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 vapour.

Vapour 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 vapour possess vibrational, rotational, and translational motion. These motions are considered in the kinetic theory of gases.

Vapour pressure[edit]

Liquid-Vapour Equilibrium

The vapour pressure is the equilibrium pressure from a liquid or a solid at a specific temperature. The equilibrium vapour 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 vapour pressure is equal to one atmosphere (unit).[1]

For two-phase systems (e.g., two liquid phases), the vapour pressure of the individual phases are equal. In the absence of stronger inter-species attractions between like-like or like-unlike molecules, the vapour pressure follows Raoult's Law, which states that the partial pressure of each component is the product of the vapour pressure of the pure component and its mole fraction in the mixture. The total vapour pressure is the sum of the component partial pressures.[2]


Water vapour is responsible for humidity

Measuring vapour[edit]

Since it is in the gas phase, the amount of vapour present is quantified by the partial pressure of the gas. Also, vapours obey the barometric formula in a gravitational field just as conventional atmospheric gases do.

Flammable liquids[edit]

Flammable liquids do not burn when ignited.[citation needed] It is the vapour cloud above the liquid that will burn if the vapour's concentration is between the Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL) of the flammable liquid.

See also[edit]


  1. ^ a b c R. H. Petrucci, W. S. Harwood, and F. G. Herring, General Chemistry, Prentice-Hall, 8th ed. 2002, p. 483-6.
  2. ^ Thomas Engel and Philip Reid, Physical Chemistry, Pearson Benjamin-Cummings, 2006, p.194