Editing Raoult's law (section)

== Principles ==
[[File:Raoultov zakon.png|thumb|Vapor pressure of a binary solution that obeys Raoult's law. The black line shows the total vapor pressure as a function of the mole fraction of component B, and the two green lines are the partial pressures of the two components.]]

Raoult's law is a phenomenological relation that assumes ideal behavior based on the simple microscopic assumption that intermolecular forces between unlike molecules are equal to those between similar molecules, and that their molar volumes are the same: the conditions of an ideal solution. This is analogous to the [[ideal gas law]], which is a limiting law valid when the interactive forces between molecules approach zero, for example as the concentration approaches zero. Raoult's law is instead valid if the physical properties of the components are identical. The more similar the components are, the more their behavior approaches that described by Raoult's law. For example, if the two components differ only in [[isotope|isotopic]] content, then Raoult's law is essentially exact.

Comparing measured vapor pressures to predicted values from Raoult's law provides information about the true relative strength of [[intermolecular forces]]. If the vapor pressure is less than predicted (a negative deviation), fewer molecules of each component than expected have left the solution in the presence of the other component, indicating that the forces between unlike molecules are stronger. The converse is true for positive deviations.

For a solution of two liquids A and B, Raoult's law predicts that if no other gases are present, then the total vapor pressure <math>p</math> above the solution is equal to the weighted sum of the "pure" vapor pressures <math>p_\text{A}</math> and <math>p_\text{B}</math> of the two components.  Thus the total pressure above the solution of A and B would be

: <math>p = p_\text{A}^\star x_\text{A} + p_\text{B}^\star x_\text{B}.</math>

Since the sum of the mole fractions is equal to one, 
: <math>p = p_\text{A}^\star (1 - x_\text{B}) + p_\text{B}^\star x_\text{B} = p_\text{A}^\star + (p_\text{B}^\star - p_\text{A}^\star) x_\text{B}.</math>

This is a linear function of the mole fraction <math>x_\text{B}</math>, as shown in the graph.