In the ideal solution and ideal gas vapor approximation, each component's partial pressure equals its liquid mole fraction times its pure-component vapor pressure at the same temperature:These pressure formulas neglect the change in the pure-liquid reference between its saturation pressure and ; appreciable real gas behavior also requires a more general treatment. The ideal solution definition in terms of chemical potentials does not require either approximation.[ref]
Thermodynamics and Chemistry by Howard DeVoe. §9.4.1, printed pp. 248–249: partial-pressure form and its distinction from the general thermodynamic law; §9.4.2: ideal liquid mixture
A short derivation explains the reference choice. At vapor-liquid equilibrium, each component has the same chemical potential in the liquid and vapor. For an ideal gas mixture, chemical potential of an ideal gas giveswhere is a reference pressure. Approximate by and subtract the pure-component equation from the mixture equation:Taking the exponential function gives Raoult's law. Summing partial pressures and normalizing gives and ; this last step uses the ideal gas vapor assumption separately from the liquid model.
The law is named after François-Marie Raoult, who measured vapor pressures and freezing behavior of chemical solutions.[ref]
Thermodynamics and Chemistry by Howard DeVoe. Printed p. 380, Biographical Sketch: François-Marie Raoult; experimental work on dilute solutions
For the stipulated A/B ideal solution, put . At the fixed temperature , Raoult's law reduces toThe relative volatility is . For ,Hence every interior vapor-liquid equilibrium pair in this chosen model has A-enriched vapor; neither pure endpoint is enriched. Conversely,For example, corresponds to , and . Equal vapor mole fractions therefore need not mean equal liquid mole fractions.
These are fixed-temperature predictions. During boiling at a specified external pressure, the temperature instead adjusts until . The two stipulated vapor pressures specify only one temperature; they provide no boiling-temperature curve.
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Raoult's law is a principle in chemistry that relates to the vapor pressure of components in a solution. Specifically, it states that the partial vapor pressure of each component in an ideal solution is directly proportional to its mole fraction in the solution. In simpler terms, it means that the more of a particular component there is in a solution, the greater its contribution to the total vapor pressure of the solution.