Phase Definition (Matter)

Chemistry Glossary Definition of Phase

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

Phase Definition

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 the same state of matter. For example, 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 more synonymous with a state of matter because the qualities that describe the phase include the organization of matter, as well variable such as temperature and pressure.

Types of Phases of Matter

The distinct phases used 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 dilute, cold boson gas
  • mesophases - such as an intermediate phase between a solid and liquid

But, 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.


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