Phase Definition and Examples

Plasma
Plasma is a phase of matter. Roland Bordas / EyeEm / Getty Images

In chemistry and physics, a phase is a physically distinctive form of matter, such as a solid, liquid, gas, or plasma. A phase of matter is characterized by having relatively uniform chemical and physical properties. Phases are different from states of matter. The states of matter (e.g., liquid, solid, gas) are phases, but matter can exist in different phases yet remain in the same state of matter. For example, liquid mixtures can exist in multiple phases, such as an oil phase and an aqueous phase.

The term phase may also be used to describe equilibrium states on a phase diagram. When phase is used in this context, it's typically synonymous with a state of matter because the qualities that describe the phase include the organization of matter as well as variables such as temperature and pressure.

Phases of Matter

The distinct phases used to describe states of matter include:

  • Solid: Closely packed particles with a fixed volume and shape
  • Liquid: Fluid particles with a fixed volume but variable shape
  • Gas: Fluid particles with neither a fixed volume nor shape
  • Plasma: Charged particles with no fixed volume or shape
  • Bose-Einstein condensate: A diluted, cold boson gas
  • Mesophases: Intermediate phases between solid and liquid

There may be multiple phases within a single state of matter. For example, a bar of solid iron may contain multiple phases (e.g., martensite, austenite). An oil and water mixture is a liquid that will separate into two phases.

Interface

At equilibrium, there is a narrow space between two phases where the matter doesn't exhibit properties of either phase. This region, known as interface, may be very thin, yet can exert significant effects.