Below is a Figure from Denbigh's famous textbook. The Figure plots the chemical potential for two phases of a given substance against T and p. Denbigh himself points out that the Figure is incorrectly drawn. In particular, the curvature of the surface of chemical potential vs. T and p is incorrect
Show that this is the case for the curvature of the surface vs. T
(Hint.the temperature derivative of the entropy at constant pressure is related to what important,measurable physical quantity? Can this quantity ever be negative?And also recall how the curvature of a surface is related to the sign of a certain derivative.)