phaseequilibrium

Phase equilibrium refers to a condition in which two or more phases of a substance coexist in thermodynamic balance, with no net conversion of material between them. In a mixture or compound, phase equilibrium is achieved when the chemical potential of each component is the same in all coexisting phases, so there is no driving force for mass transfer.

Phase diagrams illustrate the stable phases of a system as functions of temperature, pressure, and composition. Regions on the diagram represent phases, while lines or surfaces separate coexistence conditions where two or more phases are in equilibrium. A triple point is where three phases coexist in equilibrium. For single-component systems, common boundaries include solid–liquid (melting/freezing), liquid–gas (evaporation/condensation), and solid–gas (sublimation). The position of these boundaries shifts with pressure and temperature, described by thermodynamic relations such as the Clausius–Clapeyron equation.

In a pure substance at a given pressure, the solid–liquid and liquid–gas boundaries mark the conditions under which ice and water, or water and vapor, are simultaneously stable. For water, at 1 atmosphere the melting point is near 0 °C and the boiling point near 100 °C, while the triple point occurs at about 0.01 °C and 611 pascals.

In multicomponent systems, phase equilibrium requires equality of the chemical potentials of each component across all coexisting phases, which leads to more complex phase diagrams with features such as liquid–liquid immiscibility and solid solutions. Phase equilibrium concepts underpin distillation, crystallization, alloy design, and many natural processes in meteorology and geology.