atmospheric pressure. 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. Examples 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. 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 flammable limit (LFL) and upper flammable limit (UFL), of the flammable liquid. E-cigarettes produce aerosols, not vapors. 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 References Gases Pressure Chemical properties ca:Vapour fr:Vapeur gl:Vapour