Can a substance exist as more than one phase at the same temperature and pressure?

• A. No, only one phase exists at a given T and P.
• B. Yes, at a phase boundary such as melting or boiling, solid and liquid can coexist.
• C. Yes, always.
• D. Only at extremely high pressure.

Answer: (B)

At a boundary between phases, two phases can be in equilibrium at the same temperature and pressure. This is why a substance can exist as more than one phase exactly where the boundary lies on the temperature–pressure diagram.

For example, at the melting point of ice (0°C at 1 atm), solid ice and liquid water can coexist. The temperature stays at 0°C while both phases are present, until all ice has melted (or all water freezes). Similarly, at the boiling point (100°C at 1 atm), liquid water and water vapor can coexist, with heat transfer occurring at a constant temperature until all liquid has boiled away.

These coexistence points are why phase diagrams have lines where two phases are in equilibrium. Away from those lines, typically only one phase is stable.