The boundary on a phase diagram where two phases are in equilibrium is called

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

Answer: (A)

The boundary where two phases are in equilibrium on a phase diagram is called a phase boundary. Along this line, the two phases have equal chemical potential (Gibbs free energy) at the same temperature and pressure, so they can coexist. For example, the solid–liquid boundary shows where ice and water are in equilibrium, and the liquid–vapor boundary shows where water can be both liquid and vapor.

The triple point is a special condition where three phases coexist in equilibrium at once, not just two. The critical point is where the distinction between liquid and gas disappears, marking the end of the liquid–vapor boundary.