On a phase diagram, what does the line representing a phase boundary indicate?

On a phase diagram, what does the line representing a phase boundary indicate?

• A. The line where two phases are in equilibrium
• B. The point where all three phases coexist
• C. The region where only one phase exists
• D. The point where gas condenses at zero pressure

Answer: (A)

The line on a phase diagram represents the conditions where two phases are in equilibrium and can coexist. At every point along that boundary, the two phases have the same chemical potential at a given temperature and pressure, so neither one naturally converts into the other. If you move off the line into the region where one phase is more stable, the other phase disappears and you’re left with just that single phase. The special case where all three phases coexist occurs at a single point where those boundaries meet, not along a line. And a boundary describing condensation at zero pressure isn’t a valid phase-coexistence line, since a liquid phase isn’t stable at zero pressure.

The line on a phase diagram represents the conditions where two phases are in equilibrium and can coexist. At every point along that boundary, the two phases have the same chemical potential at a given temperature and pressure, so neither one naturally converts into the other. If you move off the line into the region where one phase is more stable, the other phase disappears and you’re left with just that single phase. The special case where all three phases coexist occurs at a single point where those boundaries meet, not along a line. And a boundary describing condensation at zero pressure isn’t a valid phase-coexistence line, since a liquid phase isn’t stable at zero pressure.