Faasimuutuse

Faasimuutuse, Finnish for phase transition, is the process by which a substance changes its state of matter—such as from solid to liquid (melting), liquid to gas (boiling), or solid to gas (sublimation)—in response to changes in temperature, pressure, or other thermodynamic variables. Transitions occur at specific boundaries on a phase diagram and involve changes in properties like enthalpy, volume, and symmetry.

Phase transitions are commonly categorized as first-order, which involve latent heat and discontinuities in enthalpy and density (for example melting, freezing, vaporization, condensation, sublimation); and continuous or second-order transitions, where enthalpy changes continuously but response functions such as heat capacity or susceptibility diverge (as in a ferromagnetic to paramagnetic transition at the Curie point). There are also more complex or multicritical transitions in materials with multiple order parameters.

Thermodynamically, at a phase boundary the Gibbs free energies of the coexisting phases are equal. The Clapeyron equation describes how pressure and temperature are related along a coexistence line: dP/dT = ΔH/(TΔV). Transitions can proceed via nucleation and growth, requiring energy to form new-phase nuclei; metastable states exist under conditions like supercooling or superheating, and heterogeneous nucleation lowers the energy barrier.

Common examples include the melting of ice at 0°C and the boiling of water at 100°C at one atmosphere, as well as the sublimation of dry ice. Faasimuutukset are central to fields such as materials science, geoscience, meteorology, and energy storage, where phase-change materials are used for temperature regulation and energy management.