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.

In order for a phase change to occur, a certain temperature must be reached (at a certain pressure), and a certain amount of heat energy must be added of removed from the material. This is called the latent heat of a material. For instance, the latent heat of fusion (melting) of water is about 80 cal/g, and its fusion (melting) point is 0 deg. C. This means that water at 0 deg. C must have 80 cal per gram of substance taken away from it in order to freeze, or ice at 0 deg. C must have 80 cal per gram of substance added to it in order to melt. If the latent heat of the substance is not met, the substance will remain at its equilibrium temperature with proportional amounts of the two phases of matter the substance is changing to and from, depending on the heat energy added.

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