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States of matter |
In physical science, bulk matter can exist in various states (or forms) which may depend on temperature and pressure. One state of matter has physical properties that are qualitatively different from another state of matter.
Traditionally, three states of matter are 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.
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 only one 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.
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The particles (ions, atoms or molecules) are packed closely together. The forces between particles are strong enough so 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 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 (or solid phase). 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 crystal structures, or fifteen solid phases which exist at various temperatures and pressures.
The volume is definite if the temperature and pressure are constant. When a solid is heated above its melting point, it becomes liquid. 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, with the noteworthy exception of water, H2O. The highest temperature at which a given liquid can exist is its critical temperature.
In a gas, the molecules have still more energy, so that the effect of intermolecular forces is small (or zero for an ideal gas), and the molecules are far apart from each other and can move around quickly. 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 (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.1 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 it 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.
Transition metal atoms often have magnetic moments due to the net spin of electrons which 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 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.)
Close to absolute zero, some liquids form a second liquid state described as superfluid because it has zero viscosity or infinite fluidity. This was discovered in 1937 for helium which forms a superfluid below the lambda temperature of 2.17 K. In this state it will attempt to 'climb' out of its container.2. It also has infinite thermal conductivity so that no temperature gradient can form in a superfluid.
These properties are explained by the theory that the common isotope helium-4 forms a Bose-Einstein condensate (see next section) in the superfluid state. More recently, Fermionic condensate superfluids have been formed at even lower temperatures by the rare isotope helium-3 and by lithium-6.3
In 1924, Albert Einstein and Satyendra Bose predicted the "Bose-Einstein condensate," sometimes referred to as the fifth state of matter. As mentioned above, an example is helium-4 in the superfluid state.
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 may occur when atoms have very similar (or the same) quantum levels, at temperatures very close to absolute zero (-273 °C).
Plasmas or ionized gases can exist at temperatures starting at several thousand degrees C. Some examples of plasma are the charged air produced by lightning, and stars such as 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.
A plasma can be considered as a gas of highly ionized particles, but the powerful interionic forces lead to distinctly different properties, so that it is usually considered as a different phase or state of matter.
This is a state of matter discovered at the CERN in 20004, 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.
Under extremely high pressure, ordinary matter undergoes a transition to a series of exotic states of matter collectively known as degenerate matter. These are of great interest to astrophysics, because these high-pressure conditions are believed to exist inside stars that have used up their nuclear fusion "fuel", such as white dwarves and neutron stars.
A supersolid is a spatially ordered material (that is, a solid or crystal) with superfluid properties. A supersolid is a solid, but exhibits so many other properties that many argue it is another state of matter.5
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 which would require some electrons to have neighbors with the same spin. This gives rise to some curious properties, as well as supporting some unusual proposals about the fundamental conditions of the universe itself.
One of the metastable states of strongly non-ideal plasma is Rydberg matter, which forms upon condensation of excited atoms. These atoms can also turn into ions and electrons if they reach a certain temperature.
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This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (September 2008) |
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