Four thermodynamic variables which may describe the system include temperature ("T"), pressure ("p"), mole fraction of component 1 (toluene) in the liquid phase ("x"1L), and mole fraction of component 1 in the vapour phase ("x"1V). However, since two phases are present () in equilibrium, only two of these variables can be independent (). This is because the four variables are constrained by two relations: the equality of the chemical potentials of liquid toluene and toluene vapour, and the corresponding equality for benzene.