In
physics, a substance is said to undergo a
phase transition when it passes through a discontinuous change of state. For example, when water
freezes that is a phase transition, because there is a sharply defined
temperature, the freezing point, at which the water turns to ice (assuming that the
pressure is kept constant). At this temperature water and ice can
coexist and are said to be in
equilibrium. Above the freezing point ice will melt. Water can be "
supercooled" to below the freezing point, but it will be thermodynamically
unstable and will freeze if disturbed.
The most familiar phase transitions are the changes between the solid, liquid and gaseous states which occur when the temperature or pressure of a substance is changed. Another type of phase transition, which is studied in mineralogy, is the order-disorder phase transition where the atoms of a mineral form an orderly pattern at low temperatures but are arranged randomly at high temperatures.