Phase changes occur along lines of equilibrium between phases on a phase diagram.

Phase changes occur along lines of equilibrium between phases on a phase diagram.

• A. Only at critical points
• B. True
• C. False
• D. Only at triple points

Answer: (B)

Phase diagrams map where solid, liquid, and gas are each stable, and the boundaries between those stability regions are lines of phase equilibrium. Along these boundaries, the two involved phases have equal chemical potential, so the system can exist as either phase and a small change in temperature or pressure pushes it to the other phase. That’s why phase changes like melting, freezing, vaporization, condensation, and sublimation occur when you cross these lines. Special points on the diagram, such as the triple point where three phases coexist and the critical point where liquid and gas become indistinguishable, don’t change the fact that phase changes happen along the entire lines of equilibrium. So the statement is true.

Phase diagrams map where solid, liquid, and gas are each stable, and the boundaries between those stability regions are lines of phase equilibrium. Along these boundaries, the two involved phases have equal chemical potential, so the system can exist as either phase and a small change in temperature or pressure pushes it to the other phase. That’s why phase changes like melting, freezing, vaporization, condensation, and sublimation occur when you cross these lines. Special points on the diagram, such as the triple point where three phases coexist and the critical point where liquid and gas become indistinguishable, don’t change the fact that phase changes happen along the entire lines of equilibrium. So the statement is true.