States of matter are the distinct forms that different phases of matter take on. Historically, the distinction is made based on
qualitative differences in bulk properties. Solid is the state in which matter maintains a fixed volume and shape; liquid is the
state in which matter maintains a fixed volume but adapts to the shape of its container; and gas is the state in which matter
expands to occupy whatever volume is available.
More recently, distinctions between states have been based on differences in molecular interrelationships. Solid is the
state in which intermolecular attractions keep the molecules in fixed spatial relationships. Liquid is the state in which
intermolecular attractions keep molecules in proximity, but do not keep the molecules in fixed relationships. Gas is that
state in which the molecules are comparatively separated and intermolecular attractions have relatively little effect on their
respective motions. Plasma is a highly ionized gas that occurs at high temperatures. The intermolecular forces created by
ionic attractions and repulsions give these compositions distinct properties, for which reason plasma is sometimes
described as a fourth state of matter.
Forms of matter that are not composed of molecules and are organized by different forces can also be considered different
states of matter. Fermionic condensate and the quark-gluon plasma are examples.
States of matter may also be defined in terms of phase transitions. A phase transition indicates a change in structure and
can be recognized by an abrupt change in properties. By this definition, a distinct state of matter is any set of states
distinguished from any other set of states by a phase transition. Water can be said to have several distinct solid states.
The appearance of superconductivity is associated with a phase transition, so there are superconductive states. Likewise,
liquid crystal states and ferromagnetic states are demarcated by phase transitions and have distinctive properties.
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