A phase boundary describes the interface two substances that can remain in contact indefinitely (that is to say, at equilibrium) without mixing, as when oil meets water or air meets stone.
The Gibbs Phase Rule relates the number of phases P which can remain in equilibrium with each other in a system of C chemical constituents given F degrees of freedom.
A degree of freedom could be the concentration of a constituent, or the temperature or pressure of the system. Chemical constituents are defined as distinct components that won't break down under the conditions of interest. For instance, water can be counted as a single constituent, unless conditions might break it down into hydrogen and oxygen, or dissociate to form hydronium and hydroxideions.
The relation is:
P + F = C + 2
The 2 comes from counting pressure and temperature.
The states of matter – solid, liquid, and gas – are referred to as phases. A transition from one state to another is called a phase change. The freezing and melting temperature for a given material at any pressure, is the same. At this temperature the liquid and solid phases coexist in phase equilibrium. The same also happens in phase transitions between liquid and gas. The heat transfer in a phase change is given by:
where L is the heat of fusion in case of transition between liquid and solid, or heat of vaporization in case of transition between liquid and gas.
When an intermediate phase of limited (or no) homogeneity range is located at or near a specific ratio of component elements that reflects the normal positioning of the component atoms in the crystal structure of the phase, it is often called a compound (or line compound).
Phase diagrams to be used by scientists are usually plotted in atomic percentage (or mole fraction), while those to be used by engineers are usually plotted in weight percentage.
Phase diagrams can be used in a manner similar to that used in the discussion of eutectic and eutectoid reactions to determine the microstructures expected to result from cooling an alloy through any of the other six types of reactions listed in Table 2.
Liquid crystals are another phase intermediate between solids and liquids; the molecules of such a substance have an orderly orientation and in some cases (that is, for smectic liquid crystals) even an orderly position in one direction, but are free to flow past one another.
In contrast, in the plasma phase the atoms are dissociated, i.e.
These phases are of great interest to astrophysics, because these high-pressure conditions are believed to exist inside stars that have used up their nuclear fusion "fuel", such as white dwarves and neutron stars.
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