phasechanging

Phase changing refers to the transformation of a material from one thermodynamic phase to another, most commonly among solid, liquid, and gas, and in some cases toward plasma under extreme conditions. These transitions involve rearrangements of molecular structure and are accompanied by the absorption or release of latent heat. Phase changes occur at characteristic temperatures and pressures defined by phase equilibria and are central to thermodynamics and materials science.

Common transitions include melting (solid to liquid), freezing (liquid to solid), vaporization (liquid to gas) and condensation (gas to liquid), as well as sublimation (solid to gas) and deposition (gas to solid). At a given pressure, the transition takes place over a range where temperature remains near the phase-change point while heat is added or removed, allowing the phase boundary to move. Latent heat is the energy required to change the phase without changing temperature; for water, fusion requires about 334 kJ per kilogram and vaporization about 2260 kJ per kilogram at standard conditions.

Phase diagrams illustrate the stability of phases as functions of temperature and pressure, highlighting features such as the triple point, where three phases coexist, and the critical point, where the liquid-gas boundary ends. Kinetics of phase changes depend on nucleation and growth processes and can be influenced by impurities, interfaces, and confinement, leading to phenomena such as metastability and hysteresis.

Applications include phase-change materials used for thermal energy storage, which exploit high latent heat to absorb or release heat during charging and discharging. Phase changes underpin refrigeration cycles, boil-off and condensation in engines, and natural processes such as cloud formation and frost. Examples like ice melting at 0 °C and water boiling at 100 °C illustrate these concepts.