A vapor or vapour (see
spelling differences) is a substance in the gasphase at a temperaturelower 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 liquidor to a solidby increasing its pressure, without reducing the temperature.
waterhas 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 pressureis 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 pressureof 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
liquidor solidstate, below the critical temperatureof 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
cloudformation and condensation. It is commonly employed to carry out the physical processes of distillationand headspace extraction from a liquid sample prior to gas chromatography.
molecules of a vapor possess vibrational, rotational, and translational motion. These motions are considered in the kinetic theory of gases.
vapor pressureis 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.
normal boiling pointof a liquid is the temperatureat which the vapor pressureis 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 pressureof 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]
physical chemistrybehind distillationis 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.
water vaporis found near the earth's surface, and may condense into small liquid droplets and form meteorological phenomena such as fog, mistand haar.
Mercury-vapor lamps and sodium vapor lamps produce light from atoms in excited states.
Since it is in the gas phase, the amount of vapor present is quantified by the
partial pressureof the gas. Also, vapors obey the barometric formulain 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 limitand upper explosive limit of the flammable liquid.
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