The lines on a phase diagram along which two phases coexist in equilibrium are called

• A. Phase Diagram
• B. Triple Point
• C. Critical Point
• D. Phase Boundaries

Answer: (D)

Two-phase coexistence in equilibrium is shown by lines on a phase diagram. These lines are phase boundaries, separating regions where different pairs of phases are stable. Along a phase boundary, the two phases have equal chemical potential, so they can exist together in balance as pressure and temperature vary along that line. The line between solid and liquid, for instance, marks where solid and liquid coexist; the line between liquid and gas marks where liquid and gas coexist; and the line between solid and gas marks where solid and gas coexist. The triple point is a single point where all three phases coexist, not a line, while the critical point is the end of the liquid–gas boundary beyond which liquid and gas become indistinguishable.