State of matter
In physics, a state of matter or phase 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 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 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. 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, neutron-degenerate matter, and quark–gluon plasma.
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.
Four classical states
Solid
In a solid, constituent particles are closely packed together. The forces 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.In crystalline solids, the particles are packed in a regularly ordered, repeating pattern. There are various 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, and a face-centred cubic structure between 912 and. Ice has fifteen known crystal structures, or fifteen solid phases, which exist at various temperatures and pressures.
Glasses and other non-crystalline, amorphous solids without long-range order are not thermal equilibrium ground states; therefore they are described below as nonclassical states of matter.
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 of sublimation, and gases can likewise change directly into solids through deposition.
Liquid
A liquid is a nearly incompressible fluid that conforms to the shape of its container but retains a constant volume independent of pressure. The volume is definite if the temperature and pressure are constant. When a solid is heated above its melting point, it becomes liquid, given that the pressure is higher than the triple point of the substance. Intermolecular 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 being water, HO. The highest temperature at which a given liquid can exist is its critical temperature.Gas
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 enough kinetic energy so that the effect of intermolecular forces is small, 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 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, in which case the gas pressure equals the vapor pressure of the liquid.
A supercritical fluid is a gas whose temperature and pressure are above the critical temperature and critical 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 to extract caffeine in the manufacture of decaffeinated coffee.
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 either lightning, electric sparks, fluorescent lights, neon lights or in plasma televisions. The Sun's corona, some types of flame, 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 all ordinary matter in the universe is plasma, as it composes all stars.
Phase transitions
A state of matter is also characterized by phase 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 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, ferromagnetic states are demarcated by phase transitions and have distinctive properties.When the change of state occurs in stages the intermediate steps are called mesophases. Such phases have been exploited by the introduction of liquid crystal technology.
The state or phase of a given set of matter can change depending on pressure and temperature conditions, transitioning to other phases as these conditions change to favor their existence; for example, solid transitions to liquid with an increase in temperature. Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point, boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons 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 and the quark–gluon plasma are examples.
Non-classical states
Glass
is a non-crystalline or amorphous solid material that exhibits a glass transition when heated towards the liquid state. Glasses can be made of quite different classes of materials: inorganic networks, metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers.Thermodynamically, a glass is in a metastable state with respect to its crystalline counterpart. The conversion rate, however, is practically zero.
Crystals with some degree of disorder
A plastic crystal is a molecular solid with long-range positional order but with constituent molecules retaining rotational freedom; in an orientational glass this degree of freedom is frozen in a quenched disordered state.Similarly, in a spin glass magnetic disorder is frozen.
Liquid crystal states
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, the nematic phase consists of long rod-like molecules such as para-azoxyanisole, which is nematic in the temperature range. In this state the molecules flow as in a liquid, but they all point in the same direction 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, in liquid crystal displays.
Microphase separation
can undergo microphase separation to form a diverse array of periodic nanostructures, as shown in the example of the styrene-butadiene-styrene block copolymer shown at right. Microphase separation can be understood by analogy to the phase separation between oil and water. Due to chemical incompatibility between the blocks, block copolymers undergo a similar phase separation. However, because the blocks are covalently bonded to each other, they cannot demix macroscopically as water and oil can, and so instead the blocks form nanometre-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.