Phase

A phase is a physically distinct, homogeneous region of matter with uniform properties. Different phases can coexist while remaining separated by interfaces.

Phase transition: A transformation between phases caused by changing conditions such as temperature or pressure. It describes how a region of matter changes from one phase to another.
Thermodynamic equilibrium: A state in which macroscopic properties remain constant because driving gradients have vanished. Equilibrium determines which phases can persist under specified conditions.
Allotropy: The ability of an element to exist in different structural forms in the same physical state. It shows how one element can form distinct solid phases.
State of matter: A broad physical form of matter, such as solid, liquid, gas, or plasma. A state is broader than a phase; one state can contain several phases.
Water: A chemical compound of hydrogen and oxygen with formula H₂O. Its ice, liquid, and vapor provide familiar examples of distinct phases.
Phase diagram: A chart showing which phases are stable under different temperatures, pressures, or compositions. It maps the conditions under which this phase is stable.
Chemical potential: The change in a system’s free energy when a particle is added at fixed conditions. Equal chemical potentials characterize coexistence between phases of the same substance.
Polymorphism: The ability of a solid substance to crystallize in more than one structure. A material’s crystal polymorphs are distinct solid phases with different properties.
Mixture: A physical combination of substances that retain their chemical identities. A mixture can be homogeneous or divided into multiple phases.
Carbon: A chemical element that forms several structures, including diamond and graphite. Its solid forms demonstrate distinct phases with different atomic arrangements.

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