State of matter

Bromine in both liquid and gas state, encased inside acrylic in solid state

Helium's orange glow in its plasma state

A simplified phase diagram for water, showing whether solid ice, liquid water, or gaseous water vapor is the most stable at different combinations of temperature and pressure

In physics, a state of matter is one of the distinct forms in which matter can exist. Four states of matter are observable in everyday life: solid, liquid, gas, and plasma.

Different states are distinguished by the ways the component particles (atoms, molecules, ions and electrons) are arranged, and how they behave collectively. In a solid, the particles are tightly packed and held in fixed positions, giving the material a definite shape and volume. In a liquid, the particles remain close together but can move past one another, allowing the substance to maintain a fixed volume while adapting to the shape of its container. In a gas, the particles are far apart and move freely, allowing the substance to expand and fill both the shape and volume of its container. Plasma is similar to a gas, but it also contains charged particles (ions and free electrons) that move independently and respond to electric and magnetic fields.

Beyond the classical states of matter, a wide variety of additional states are known to exist. Some of these lie between the traditional categories; for example, liquid crystals exhibit properties of both solids and liquids. Others represent entirely different kinds of ordering. Magnetic states, for instance, do not depend on the spatial arrangement of atoms, but rather on the alignment of their intrinsic magnetic moments (spins). Even in a solid where atoms are fixed in position, the spins can organize in distinct ways, giving rise to magnetic states such as ferromagnetism or antiferromagnetism.

Some states occur only under extreme conditions, such as Bose–Einstein condensates and Fermionic condensates (in extreme cold), neutron-degenerate matter (in extreme density), and quark–gluon plasma (at extremely high energy).

The term phase is sometimes used as a synonym for state of matter, but it is possible for a single compound to form different phases that are in the same state of matter. For example, ice is the solid state of water, but there are multiple phases of ice with different crystal structures, which are formed at different pressures and temperatures.