Vapor

Vapor or vapour (see spelling differences) is the gas phase component of another state of matter (e.g. liquid or solid) which does not completely fill its container. It is distinguished from the pure gas phase by the presence of the same substance in another state of matter. Hence when a liquid has completely evaporated, it is said that the system has been completely transformed to the gas phase.

Properties
The terms vapor and gas are frequently but incorrectly used interchangeably. A vapor refers to a gas phase in a state of equilibrium with identical matter in a liquid or solid state below its boiling point.

The term gas refers to a compressible fluid phase, as in common usage. 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. The atmospheric boiling point of a liquid is the temperature at which the vapor pressure is equal to one atmosphere (unit). See the article on vapor pressure for more information on this topic.

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

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, or the vapor pressure is equal to the sum of the product of the vapor pressure of the pure compound and its mole fraction in the mixture for all of the constituents.

The constituent atoms or molecules of a vapor possess vibrational, rotational, and translational motion. More information can be found under the entry of the Kinetic theory of gases. 

Examples

 * Perfumes contain chemicals that vaporize at different temperatures. The top note vaporizes first followed by the heart note. The slowest to vaporize is the base note.
 * Fog, mist and haar are meteorological designations for visible near surface water vapor.

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.