Figure 8.12 Top: chemical potential surfaces of the liquid and gas phases of H2O; the two phases are at equilibrium along the intersection (heavy curve). (The vertical scale for has an arbitrary zero.) Bottom: projection of the intersection onto the pT plane, generating the coexistence curve. (Based on data in Ref. [75].) 3.3 Explain why the chemical potential surfaces shown in Fig. 8.12 are concave downward; that is, why (/T)p becomes more negative with increasing T and (/p)T becomes less positive with increasing p