Lines on a phase diagram, the two phases on either side of a phase boundary are in equilibrium (coexist) at the phase-boundary.

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

Answer: (A)

Lines on a phase diagram are phase boundaries where two phases coexist in equilibrium. Along this line, the chemical potentials of the two phases are equal, allowing them to be in balance with each other at specific temperatures and pressures. Crossing the boundary favors one phase over the other, producing a phase change. The statement describes this coexistence along the boundary, so the term that fits is phase boundaries. A phase is one state of matter, and a phase diagram is the whole map, not just the line of coexistence. The triple point is a special condition where three phases are in equilibrium at a single point, not along a typical boundary line.