CoachA phase can persist beyond its equilibrium boundary: liquid water cooled below without freezing (supercooling), heated above its boiling point without boiling (superheating), vapour compressed beyond without condensing (supersaturation). Such metastable states require the absence of nucleation sites — dust, scratches, dissolved gas, ice crystals — on which the new phase can start; a disturbance tips them suddenly and irreversibly to equilibrium.
Examples
Example 25.15 (The supercooled bottle)
Still water at in a clean bottle, knocked: ice crystals shoot through it and the temperature jumps to . The process is adiabatic and fast, so the sensible heat freezes a fraction of the water; the entropy created, , is positive as it must be. Clouds are full of supercooled droplets down to ; aircraft icing and the seeding of clouds both exploit that.