What is the purpose of a phase boundary in a phase diagram?

A phase boundary in a phase diagram serves the critical role of representing the conditions of temperature and pressure under which a substance transitions from one phase to another. These boundaries delineate the specific points where two phases coexist in equilibrium, indicating the exact values at which a phase change—such as melting, boiling, or sublimation—can occur.

At any point along a phase boundary, both phases are in equilibrium, meaning that the number of molecules transitioning from one phase to the other is equal. For example, at the solid-liquid boundary, the temperatures and pressures indicate where both solid and liquid states exist simultaneously. Understanding these boundaries is essential in thermochemistry as it helps predict how substances will behave under varying conditions, which is crucial for practical applications in chemistry and engineering.

The other options refer to aspects of thermodynamics or chemical reactions that do not strictly apply to the function of phase boundaries in the context of a phase diagram. While thermal equilibrium is indeed a concept related to phase transitions, it is the specific pressure and temperature values at these transitions that the phase boundaries delineate.