States of Matter, in classical physics, three forms in which matter occurs—solid, liquid, and gas. Plasma, the collection of charged gaseous particles containing nearly equal numbers of negative and positive ions, is sometimes called the fourth state of matter (see Ion; Ionization).

Additional states of matter can occur at extremely low temperatures near absolute zero under laboratory conditions (see Cryogenics). These unusual states of matter result from quantum effects that occur at the level of atoms and particles. Liquid helium can take on a superfluid state in which all resistance to flow disappears. Atoms in a magnetically confined gas may behave like a single giant particle in a state called a Bose-Einstein condensate. A possible (but still unconfirmed) additional state of matter called supersolidity may occur when ultracold helium is in a solid state under very high pressure.

Unusual states of compressed matter can occur under the extreme gravitational pressures in white dwarf stars and in neutron stars. White dwarfs are the compressed cores of old stars that have lost their outer layers of gas. Matter in a white dwarf is in a degenerate electron state in which negatively charged electrons in the atoms at the core balance the inward pull of gravity. Even more exotic states of matter occur in neutron stars. Neutron stars are the collapsed cores of giant stars that have exploded as supernovas. Under a superdense iron crust, neutrons form a superfluid sea that has no resistance to movement. At the central core of some neutron stars may be a soup of quark matter made up of strange particles (strange is the name of a type of quark). Some astronomers think compressed quark stars made entirely of such quark matter may exist.

In the seconds following the big bang that scientists think started the universe, the enormous energy and density permitted matter to exist in special states. Quarks that materialized out of pure energy formed a quark-gluon plasma in which quarks were not yet bound together into other types of elementary particles. The four forces of nature that affect matter—electromagnetic, weak, strong, and gravitational—were unified and then began to separate from each other as the universe expanded and cooled.

The familiar states of matter—solid, liquid, and gas—can be distinguished by their basic properties. Solid matter is characterized by resistance to any change in shape, caused by a strong attraction between the molecules of which it is composed. In liquid form, matter does not resist forces that act to change its shape, because the molecules are free to move with respect to each other (see Molecule). Liquids, however, have sufficient molecular attraction to resist forces tending to change their volume. Gaseous matter, in which molecules are widely dispersed and move freely, offers no resistance to change of shape and little resistance to change of volume. As a result, a gas that is not confined tends to diffuse infinitely, increasing in volume and diminishing in density.

Most substances are solid at low temperatures, liquid at medium temperatures, and gaseous at high temperatures, but the states are not always distinct (see Temperature). The temperature at which any given substance changes from liquid to solid is its freezing point, the temperature at which it changes from solid to liquid is its melting point, and the temperature at which it changes from liquid to gas is its boiling point. The range of melting and boiling points varies widely. Helium remains a gas down to -268.9°C (-452°F), and tungsten remains a solid up to about 3420°C (about 6190°F).

For further discussion of the properties of matter in its different states, see Atom; Crystal; Fluid; Glass; Liquid Crystal; Thermodynamics; Vapor. See also Critical Point; Cryogenics.