A simplified phase diagram forwater, showing whether solid ice, liquid water, or gaseous water vapor is the most stable at different combinations of temperature and pressure
Inphysics, astate of matter orphase of matter is one of the distinct forms in whichmatter can exist. Four states of matter are observable in everyday life:solid,liquid,gas, andplasma.
Different states are distinguished by the ways the component particles (atoms,molecules,ions andelectrons) 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 andvolume. 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 asferromagnetism orantiferromagnetism.
The termphase is sometimes used as asynonym 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 multiplephases of ice with differentcrystal structures, which are formed at different pressures and temperatures.
Simple illustration of particles in the solid state – they are closely packed to each other.
In a solid, constituent particles (ions, atoms, or molecules) are closely packed together. Theforces between particles are so strong that the particles cannot move freely but can only vibrate. As a result, a solid has a stable, definite shape, and a definite volume. Solids can only change their shape by an outside force, as when broken or cut.
Incrystalline solids, the particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern. There are various differentcrystal structures, and the same substance can have more than one structure (or solid phase). For example,iron has abody-centred cubic structure at temperatures below 912 °C (1,674 °F), and aface-centred cubic structure between 912 and 1,394 °C (2,541 °F).Ice has fifteen known crystal structures, or fifteen solid phases, which exist at various temperatures and pressures.[1]
Solids can be transformed into liquids by melting, and liquids can be transformed into solids by freezing. Solids can also change directly into gases through the process ofsublimation, and gases can likewise change directly into solids throughdeposition.
Simple illustration of particles in the liquid state – they can flow and change shape.
A liquid is a nearly incompressiblefluid that conforms to the shape of its container but retains a (nearly) constant volume independent of pressure. The volume is definite if thetemperature andpressure are constant. When a solid is heated above itsmelting point, it becomes liquid, given that the pressure is higher than thetriple point of the substance. Intermolecular (or interatomic or interionic) forces are still important, but the molecules have enough energy to move relative to each other and the structure is mobile. This means that the shape of a liquid is not definite but is determined by its container. The volume is usually greater than that of the corresponding solid, the best known exception beingwater, H2O. The highest temperature at which a given liquid can exist is itscritical temperature.[2]
Simple illustration of particles in the gas state – in reality these particles will be much further apart.
A gas is a compressible fluid. Not only will a gas conform to the shape of its container but it will also expand to fill the container.
In a gas, the molecules have enoughkinetic energy so that the effect of intermolecular forces is small (or zero for anideal gas), and the typical distance between neighboring molecules is much greater than the molecular size. A gas has no definite shape or volume, but occupies the entire container in which it is confined. A liquid may be converted to a gas by heating at constant pressure to theboiling point, or else by reducing the pressure at constant temperature.
At temperatures below itscritical temperature, a gas is also called avapor, 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 thevapor pressure of the liquid (or solid).
Asupercritical fluid (SCF) is a gas whose temperature and pressure are above the critical temperature andcritical pressure respectively. In this state, the distinction between liquid and gas disappears. A supercritical fluid has the physical properties of a gas, but its high density confers solvent properties in some cases, which leads to useful applications. For example,supercritical carbon dioxide is used toextractcaffeine in the manufacture ofdecaffeinated coffee.[3]
Artificial plasma produced in air by aJacob's Ladder. The extremely strongpotential difference between the two rodsionize particles in the air, creating a plasma.
A gas is usually converted to a plasma in one of two ways, either from a huge voltage difference between two points, or by exposing it to extremely high temperatures. Heating matter to high temperatures causes electrons to leave the atoms, resulting in the presence of free electrons. This creates a so-called partially ionized plasma. 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. This forms the so-called fully ionized plasma.
The plasma state is often misunderstood, and although not freely existing under normal conditions on Earth, it is quite commonly generated by eitherlightning,electric sparks,fluorescent lights,neon lights or inplasma televisions. TheSun's corona, some types offlame, and stars are all examples of illuminated matter in the plasma state. Plasma is by far the most abundant of the four fundamental states; 99% of allordinary matter in the universe is plasma, as it composes allstars.[4][5][6]
This diagram shows the nomenclature for the different phase transitions.
A state of matter is also characterized byphase transitions. A phase transition indicates a change in structure and can be recognized by an abrupt change in properties. A distinct state of matter can be defined as any set ofstates distinguished from any other set of states by aphase transition. Water can be said to have several distinct solid states.[7] The appearance of superconductivity is associated with a phase transition, so there aresuperconductive states. Likewise,ferromagnetic states are demarcated by phase transitions and have distinctive properties.When the change of state occurs in stages the intermediate steps are calledmesophases. Such phases have been exploited by the introduction ofliquid crystal technology.[8][9]
The state orphase of a given set of matter can change depending onpressure andtemperature conditions, transitioning to other phases as these conditions change to favor their existence; for example, solid transitions to liquid with an increase in temperature. Nearabsolute zero, a substance exists as asolid. As heat is added to this substance it melts into a liquid at itsmelting point, boils into a gas at itsboiling point, and if heated high enough would enter aplasma state in which theelectrons are so energized that they leave their parent atoms.
Forms of matter that are not composed of molecules and are organized by different forces can also be considered different states of matter.Superfluids (likeFermionic condensate) and thequark–gluon plasma are examples.
Schematic representation of a random-network glassy form (left) and ordered crystalline lattice (right) of identical chemical composition.
Glass is a non-crystalline oramorphous solid material that exhibits aglass transition when heated towards the liquid state. Glasses can be made of quite different classes of materials: inorganic networks (such as window glass, made ofsilicate plus additives), metallic alloys,ionic melts,aqueous solutions, molecular liquids, andpolymers.Thermodynamically, a glass is in ametastable state with respect to its crystalline counterpart. The conversion rate, however, is practically zero.
Crystals with some degree of disorder
Aplastic crystal is a molecular solid with long-range positional order but with constituent molecules retaining rotational freedom; in anorientational glass this degree of freedom is frozen in aquenched disordered state.
Similarly, in aspin glass magnetic disorder is frozen.
Liquid crystal states have properties intermediate between mobile liquids and ordered solids. Generally, they are able to flow like a liquid but exhibit long-range order. For example, thenematic phase consists of long rod-like molecules such aspara-azoxyanisole, which is nematic in the temperature range 118–136 °C (244–277 °F).[10] 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. Like a crystalline solid, but unlike a liquid, liquid crystals react to polarized light.
Other types of liquid crystals are described in the main article on these states. Several types have technological importance, for example, inliquid crystal displays.
Copolymers can undergo microphase separation to form a diverse array of periodic nanostructures, as shown in the example of thestyrene-butadiene-styrene block copolymer shown at right. Microphase separation can be understood by analogy to the phase separation betweenoil and water. Due to chemical incompatibility between the blocks, block copolymers undergo a similar phase separation. However, because the blocks arecovalently bonded to each other, they cannot demix macroscopically as water and oil can, and so instead the blocks formnanometre-sized structures. Depending on the relative lengths of each block and the overall block topology of the polymer, many morphologies can be obtained, each its own phase of matter.
Ionic liquids also display microphase separation. The anion and cation are not necessarily compatible and would demix otherwise, but electric charge attraction prevents them from separating. Their anions and cations appear to diffuse within compartmentalized layers or micelles instead of freely as in a uniform liquid.[11]
Magnetically ordered states
Transition metal atoms often havemagnetic moments due to the netspin of electrons that remain unpaired and do not form chemical bonds. In some solids the magnetic moments on different atoms are ordered and can form a ferromagnet, an antiferromagnet or a ferrimagnet.
In aferromagnet—for instance, solidiron—the magnetic moment on each atom is aligned in the same direction (within amagnetic domain). If the domains are also aligned, the solid is a permanentmagnet, which is magnetic even in the absence of an externalmagnetic field. Themagnetization disappears when the magnet is heated to theCurie point, which for iron is 768 °C (1,414 °F).
Anantiferromagnet has two networks of equal and opposite magnetic moments, which cancel each other out so that the net magnetization is zero. For example, innickel(II) oxide (NiO), half the nickel atoms have moments aligned in one direction and half in the opposite direction.
In aferrimagnet, 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 ismagnetite (Fe3O4), which contains Fe2+ and Fe3+ ions with different magnetic moments.
Aquantum spin liquid (QSL) is a disordered state in a system of interacting quantum spins which preserves its disorder to very low temperatures, unlike other disordered states. It is not a liquid in physical sense, but a solid whose magnetic order is inherently disordered. The name "liquid" is due to an analogy with the molecular disorder in a conventional liquid. A QSL is neither aferromagnet, where magnetic domains are parallel, nor anantiferromagnet, where the magnetic domains are antiparallel; instead, the magnetic domains are randomly oriented. This can be realized e.g. bygeometrically frustrated magnetic moments that cannot point uniformly parallel or antiparallel. When cooling down and settling to a state, the domain must "choose" an orientation, but if the possible states are similar in energy, one will be chosen randomly. Consequently, despite strong short-range order, there is no long-range magnetic order.
Superfluids and condensates
Liquid helium in a superfluid phase creeps up on the walls of the cup in aRollin film, eventually dripping out from the cup
Bose–Einstein condensates and superfluids
Velocity in a gas ofrubidium as it is cooled: the starting material is on the left, and Bose–Einstein condensate is on the right.
Bose–Einstein condensation was predicted in 1925 byAlbert Einstein, based on the particle statistics developed by him andSatyendra Nath Bose.[12] Bose-Einstein condensation occurs whenbosonic particles are cooled close toabsolute zero, −273.15 °C (−459.67 °F). At a specific temperature, a large fraction of them suddenly occupies the same lowest energyquantum state.[12] The effect is termedcondensation in analogy with thecondensation of water, with which it shares some similarities.[13]
In 1937, it was discovered thathelium-4, the most common isotope ofhelium, forms a superfluid below thelambda temperature of 2.17 K (−270.98 °C; −455.76 °F). The state is described assuperfluid because it has zeroviscosity and flows without friction. In this state it will attempt to "climb" out of its container.[14] It also has infinitethermal conductivity so that notemperature gradient can form in a superfluid. Placing a superfluid in a spinning container will result inquantized vortices. These properties are explained by the theory that helium-4 atoms form a Bose–Einstein condensate in the superfluid state.[citation needed]
In the gas phase, the Bose–Einstein condensate remained an unverified theoretical prediction for many years. However in 1995, the research groups ofEric Cornell andCarl Wieman, ofJILA at theUniversity of Colorado at Boulder, produced the first such condensate experimentally withrubidium atoms. Independently of Cornell and Wieman,Wolfgang Ketterle also produced a Bose–Einstein condensate in a gas ofsodium atoms in 1995.[13]
Afermionic condensate is similar to the Bose–Einstein condensate but composed offermions. While thePauli exclusion principle prevents individual fermions from occupying the same quantum state, pairs of fermions can combine to form composite particles that behave like bosons. These pairs can then occupy the same state, forming a condensate analogous to a Bose–Einstein condensate. Examples of fermionic condensates include superconductors and the superfluid phase ofhelium-3, a rare isotope of helium. Fermionic condensate has also been observed in ultracoldlithium-6.[15]
Superconductors are materials which have zeroelectrical resistivity, and therefore perfect conductivity. This is a distinct physical state which exists at low temperature, and the resistivity increases discontinuously to a finite value at a sharply defined transition temperature for each superconductor.[16] A superconductor also excludes all magnetic fields from its interior, a phenomenon known as theMeissner effect or perfectdiamagnetism.[16]Superconducting magnets are used as electromagnets inmagnetic resonance imaging 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-calledhigh-temperature superconductivity was discovered in certainceramic oxides, and has now been observed in temperatures as high as 164 K.[17]
Under extremely high pressure, as in the cores of dead stars, ordinary matter undergoes a transition to a series of exotic states of matter collectively known asdegenerate matter, which are supported mainly by quantum mechanical effects. In physics, "degenerate" refers to two states that have the same energy and are thus interchangeable. Degenerate matter is supported by thePauli exclusion principle, which prevents twofermionic particles from occupying the same quantum state. Unlike regular plasma, degenerate plasma expands little when heated, because there are simply no momentum states left. Consequently, degenerate stars collapse into very high densities. More massive degenerate stars are smaller, because the gravitational force increases, but pressure does not increase proportionally.
Electron-degenerate matter is found insidewhite dwarf stars. Electrons remain bound to atoms but are able to transfer to adjacent atoms.Neutron-degenerate matter is found inneutron stars. Vast gravitational pressure compresses atoms so strongly that the electrons are forced to combine with protons via inverse beta-decay, resulting in a superdense conglomeration of neutrons. Normally freeneutrons outside an atomic nucleus willdecay with a half life of approximately 10 minutes, but in a neutron star, the decay is overtaken by inverse decay. Cold degenerate matter is also present in planets such asJupiter and in the even more massivebrown dwarfs, which are expected to have a core withmetallic hydrogen. Because of the degeneracy, more massive brown dwarfs are not significantly larger. In metals, the electrons can be modeled as a degenerate gas moving in a lattice of non-degenerate positive ions.
In regular cold matter,quarks, fundamental particles of nuclear matter, are confined by thestrong force intohadrons that consist of 2–4 quarks, such as protons and neutrons. Quark matter or quantum chromodynamical (QCD) matter is a group of phases where the strong force is overcome and quarks are deconfined and free to move. Quark matter phases occur at extremely high densities or temperatures, and there are no known ways to produce them in equilibrium in the laboratory; in ordinary conditions, any quark matter formed immediately undergoes radioactive decay.
Strange matter is a type ofquark matter that is suspected to exist inside some neutron stars close to theTolman–Oppenheimer–Volkoff limit (approximately 2–3solar masses), although there is no direct evidence of its existence. In strange matter, part of the energy available manifests asstrange quarks, a heavier analogue of the commondown quark. It may be stable at lower energy states once formed, although this is not known.
Quark–gluon plasma is a very high-temperature phase in whichquarks become free and able to move independently, rather than being perpetually bound into particles, in a sea ofgluons, subatomic particles that transmit thestrong force that binds quarks together. This is analogous to the liberation of electrons from atoms in a plasma. This state is briefly attainable in extremely high-energy heavy ion collisions inparticle accelerators, and allows scientists to observe the properties of individual quarks. Theories predicting the existence of quark–gluon plasma were developed in the late 1970s and early 1980s,[18] and it was detected for the first time in the laboratory at CERN in the year 2000.[19][20] Unlike plasma, which flows like a gas, interactions within QGP are strong and it flows like a liquid.
At high densities but relatively low temperatures, quarks are theorized to form a quark liquid whose nature is presently unknown. It forms a distinctcolor-flavor locked (CFL) phase at even higher densities. This phase issuperconductive for color charge. These phases may occur inneutron stars but they are presently theoretical.
Color-glass condensate is a type of matter theorized to exist in atomic nuclei traveling near the speed of light. According to Einstein's theory of relativity, a high-energy nucleus appears length contracted, or compressed, along its direction of motion. As a result, the gluons inside the nucleus appear to a stationary observer as a "gluonic wall" traveling near the speed of light. At very high energies, the density of the gluons in this wall is seen to increase greatly. Unlike the quark–gluon plasma produced in the collision of such walls, the color-glass condensate describes the walls themselves, and is an intrinsic property of the particles that can only be observed under high-energy conditions such as those atRelativistic Heavy Ion Collider (RHIC) and possibly at theLarge Hadron Collider (LHC) as well.
Very high energy states
Various theories predict new states of matter at very high energies. An unknown state has created thebaryon asymmetry in the universe, but little is known about it. Instring theory, aHagedorn temperature is predicted for superstrings at about 1030 K, where superstrings are copiously produced. AtPlanck temperature (1032 K), gravity becomes a significant force between individual particles. No current theory can describe these states and they cannot be produced with any foreseeable experiment. However, these states are important incosmology because the universe may have passed through these states in theBig Bang.
A supersolid is a spatially ordered material (that is, a solid or crystal) with superfluid properties. Similar to a superfluid, a supersolid is able to move without friction but retains a rigid shape. Although a supersolid is a solid, it exhibits so many characteristic properties different from other solids that many argue it is another state of matter.[21]
In a string-net liquid, atoms have apparently unstable arrangement, like a liquid, but are still consistent in overall pattern, like a solid. When in a normal solid state, the atoms of matter align themselves in a grid pattern, so that the spin of any electron is the opposite of the spin of all electrons touching it. But in a string-net liquid, atoms are arranged in some pattern that requires some electrons to have neighbors with the same spin. This gives rise to curious properties, as well as supporting some unusual proposals about the fundamental conditions of the universe itself.
Metals, like potassium, in the chain-melted state appear to behave as liquids and solids at the same time. This is a result of being subjected to high temperature and pressure, leading the chains in the potassium to dissolve into a liquid while the crystals remain solid.[22]
Aquantum Hall state gives rise to quantizedHall voltage measured in the direction perpendicular to the current flow. Aquantum spin Hall state is a theoretical phase that may pave the way for the development of electronic devices that dissipate less energy and generate less heat.
Photonic matter is a phenomenon wherephotons interacting with a gas develop apparent mass, and can interact with each other, even forming photonic "molecules". The source of mass is the gas, which is massive. This is in contrast to photons moving in empty space, which have norest mass, and cannot interact.
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^Shao, Y.; Zerda, T.W. (1998). "Phase Transitions of Liquid Crystal PAA in Confined Geometries".Journal of Physical Chemistry B.102 (18):3387–3394.doi:10.1021/jp9734437.
^Álvarez, V.H.; Dosil, N.; Gonzalez-Cabaleiro, R.; Mattedi, S.; Martin-Pastor, M.; Iglesias, M. & Navaza, J.M.: Brønsted Ionic Liquids for Sustainable Processes: Synthesis and Physical Properties. Journal of Chemical & Engineering Data 55 (2010), Nr. 2, S. 625–632.doi:10.1021/je900550v
^Heinz, Ulrich; Jacob, Maurice (16 February 2000). "Evidence for a New State of Matter: An Assessment of the Results from the CERN Lead Beam Programme".arXiv:nucl-th/0002042.
