What do the phase boundaries on a phase diagram represent?

What do the phase boundaries on a phase diagram represent?

• A. The temperature range where a substance remains solid.
• B. The lines indicating the maximum density of a phase.
• C. The lines on a phase diagram that mark where two phases coexist, such as solid-liquid or liquid-gas.
• D. A region where matter cannot exist.

Answer: (C)

Phase boundaries on a phase diagram show the conditions where two phases are in equilibrium and can coexist. Along these lines, the system can be in either phase or a mixture of the two, because the two phases have the same chemical potential (or Gibbs free energy) at those temperatures and pressures. For example, the boundary between solid and liquid marks melting and freezing points at a given pressure, while the boundary between liquid and gas marks boiling and condensation points. Outside these lines, one phase becomes more stable than the other; between lines you can have mixtures of the two phases. Points where multiple boundaries meet (triple point) have all three phases in coexistence, and the end of the liquid–gas boundary at high pressure/temperature is the critical point where distinct liquid and gas phases disappear.

Phase boundaries on a phase diagram show the conditions where two phases are in equilibrium and can coexist. Along these lines, the system can be in either phase or a mixture of the two, because the two phases have the same chemical potential (or Gibbs free energy) at those temperatures and pressures. For example, the boundary between solid and liquid marks melting and freezing points at a given pressure, while the boundary between liquid and gas marks boiling and condensation points. Outside these lines, one phase becomes more stable than the other; between lines you can have mixtures of the two phases. Points where multiple boundaries meet (triple point) have all three phases in coexistence, and the end of the liquid–gas boundary at high pressure/temperature is the critical point where distinct liquid and gas phases disappear.