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Gibbs energy-composition-pressure surface for the fixed temperature at which the critical-point composition is equimolar. The isobaric tangent line corresponds to a reduced pressure of approximately 0.75 and has a slope equal to the difference in chemical potentials of the equilibrium states, µ₁-µ₂ at T and P. The Gibbs energy is differenced and plotted positively downward. There is a clear change of curvature across the yellow-red boundary corresponding to the change from metastable to unstable conditions.