Binodal Curve or Coexistence Curve: It is a curve (like the bell-like one shown above) defining the region of composition and temperature in a phase diagram for a binary mixture across which a transition occurs from miscibility of the components to conditions where single-phase mixtures are metastable or unstable. Binodal compositions are defined by pairs of points on the curve of Gibbs energy of mixing vs composition that have common tangents, corresponding to compositions of equal chemical potentials of each of the two components in two phases.