Traditionally, three states of matter were recognized: solid, which maintains a fixed volume and shape; liquid, which maintains a fixed volume but adopts the shape of its container; and gas, which occupies the entire volume available. Plasma, or ionized gas, is a fourth state and occurs at high temperatures.
Today, many other states of matter are often recognized, although there is no agreed-upon list of states, nor are there definite criteria for a new state. Some are of technological importance, such as the various liquid crystal states, the ferromagnetic state and the superconductive state. Others occur only under extreme laboratory conditions, such as the fermionic condensate and the quark-gluon plasma. This article gives a brief introduction to the main, generally accepted states, starting with the most familiar and proceeding to the most exotic.
The terms “state of matter” and phase are often used interchangeably. Strictly speaking, however, a phase is a region of a macroscopic physical system with uniform chemical composition and physical properties. For uniform systems there is one only region which may be called either a state or a phase (e.g. "solid state" or "solid phase"), but for non-uniform systems the numbers of states and phases may not be equal. For example, a bottle of salad dressing may separate into an oil-rich phase and a water-rich phase; there are then two phases, both of which are in the liquid state.
A solid has a stable, definite shape, and a definite volume. In a solid, the particles are packed closely together. They cannot move freely, they can only vibrate. The movement energy and temperature are very low. Solids can only change their shape by force, as when broken or cut.
In crystalline solids the particles (atoms, molecules, or ions) are arranged in an ordered three-dimensional structure. There are many different crystal structures, and the same substance can have more than one structure. For example, iron has a body-centred cubic structure at temperatures below 912°C, and a face-centred cubic structure between 912°C and 1394°C. Ice has fifteen known structures at various temperatures and pressures.
The shape of a liquid is not definite, but is determined by its container. The volume is definite if the temperature and pressure are constant. When a solid is heated above its melting point, it becomes liquid and the kinetic energy of its particles increases. These particles are usually farther apart than in the solid (with the noteworthy exception of water, H2O), and they can slide past each other easily. The highest temperature at which a given liquid can exist is its critical temperature.
A gas has no definite shape or volume, but occupies the entire container in which it is confined. The particles of a gas are far apart from each other, and can move around quickly. A liquid may be converted to a gas by heating at constant pressure to the boiling point, or else by reducing the pressure at constant temperature.
At temperatures below its critical temperature, a gas is also called a vapor, and can be liquefied by compression alone without cooling. A vapor can exist in equilibrium with a liquid (or solid), in which case the gas pressure equals the vapor pressure of the liquid (or solid).
A supercritical fluid (SCF) is a gas whose temperature and pressure are above the critical temperature and critical pressure respectively. It has the physical properties of a gas, but its high density confers solvent properties in some cases which lead to useful applications. For example, supercritical carbon dioxide is used to extract caffeine in the manufacture of decaffeinated coffee.
a solid → a liquid = melting (heat energy added) e.g. ice melts to water
a liquid → a gas = evaporation (heat energy added) e.g. water to water vapour
a solid → a gas = sublimation (heat energy added) e.g. dry ice (frozen CO2) to carbon dioxide
a gas → a liquid = condensation (heat energy removed) e.g. cloud to rain
a liquid → a solid = freezing (heat energy removed) e.g. water freezes to ice
a gas → a solid = deposition (heat energy removed) e.g. water vapour to frost
Liquid crystal states have properties intermediate between mobile liquids and ordered solids. For example, the nematic phase consists of long rod-like molecules such as para-azoxyanisole, which is nematic in the temperature range 118-136 °C. In this state the molecules flow as in a liquid, but they all point in the same direction (within each domain) and cannot rotate freely.
Other types of liquid crystals are described in the main article on these states. Several types have technological importance, for example in liquid crystal displays.
An amorphous or non-crystalline solid has a disordered structure like a liquid. However its molecules are relatively immobile so that is usually classed as a solid. Common examples are glass, rubber, and polystyrene and other polymers. Many amorphous solids soften into liquids when heated above their glass transition temperatures, at which the molecules become mobile.
In a ferromagnet -- for instance, solid iron -- the magnetic moment on each atom is aligned in the same direction (within a magnetic domain). If the domains are also aligned, the solid is a permanent magnet, which is magnetic even in the absence of an external magnetic field. The magnetization disappears when the magnet is heated to the Curie temperature, which for iron is 768°C.
An antiferromagnet has two networks of equal and opposite magnetic moments which cancel each other out, so that the net magnetization is zero. For example, in nickel(II) oxide (NiO), half the nickel atoms have moments aligned in one direction and half in the opposite direction.
In a ferrimagnet, the two networks of magnetic moments are opposite but unequal, so that cancellation is incomplete and there is a non-zero net magnetization. An example is magnetite (Fe3O4), which contains Fe2+ and Fe3+ ions with different magnetic moments.
Superconductors are materials which have zero electrical resistance, and therefore perfect conductivity. They also exclude all magnetic fields from their interior, a phenomenon known as the Meissner effect or perfect diamagnetism. Superconducting magnets are used as electromagnets in MRI machines.
The phenomenon of superconductivity was discovered in 1911, and for 75 years was only known in some metals and metallic alloys at temperatures below 30 K. In 1986 so-called high-temperature superconductivity was discovered in certain ceramic oxides, and has now been observed in temperatures as high as 164 K (which is still well below room temperature.)
In 1924, Albert Einstein and Satyendra Bose predicted the "Bose-Einstein condensate," sometimes referred to as the fifth state of matter. An example is helium-4 close to absolute zero in the superfluid state, discovered in 1937, in which it will attempt to 'climb' out of its container.
In the gas phase, the Bose-Einstein condensate remained an unverified theoretical prediction for many years. Finally in 1995, Wolfgang Ketterle and his team of graduate students produced such a condensate experimentally. A Bose-Einstein condensate is "colder" than a solid. It occurs when atoms have very similar (or the same) quantum levels. Substances with temperatures close to absolute zero (-273 °C) will exhibit the Bose-Einstein condensate.
A superfluid is a liquid, but exhibits so many other properties that many argue it is another state of matter.
Plasma occurs when the temperature is between 1000 degrees C and 1,000,000,000 degrees C. Some examples of plasma are the charged air around lightning and stars, including our own sun.
As a gas is heated, electrons begin to leave the atoms, resulting in the presence of free electrons, which are not bound to an atom or molecule, and ions, which are chemical species that contain unequal number of electrons and protons, and therefore possess an electrical charge. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields. At very high temperatures, such as those present in stars, it is assumed that essentially all electrons are "free," and that a very high-energy plasma is essentially bare nuclei swimming in a sea of electrons. Plasma is believed to be the most common state of matter in the universe.
This is a state of matter discovered at the CERN in 2000, in which the quarks that would normally make up protons and neutrons are freed and can be observed individually, similar to splitting molecules into atoms. This state of matter allows scientists to observe the properties of individual quarks, and not just theorize.