In elementary applications, Raoults law is generally valid when the liquid phase is either nearly pure or a mixture of similar substances. Raoults law may be adapted to non-ideal solutions by incorporating two factors that account for the interactions between molecules of different substances.  The first factor is a correction for gas non-ideality, or deviations from the ideal-gas law. It is called the fugacity coefficient (). The second, the activity coefficient , is a correction for interactions in the liquid phase between the different molecules.
This modified or extended Raoult's law is then written as