Editing State of matter (section)

== Four classical states ==

===Solid===
{{Main|Solid}}
[[File:Gif -AtomosSolido 01.gif|thumb|upright=0.75|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. The [[Bonding in solids|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 [[crystal|crystalline solids]], the particles (atoms, molecules, or ions) are packed in a regularly ordered, repeating pattern. There are various different [[crystal structure]]s, and the same substance can have more than one structure (or solid phase). For example, [[iron]] has a [[cubic crystal system|body-centred cubic]] structure at temperatures below {{convert|912|C|F}}, and a [[cubic crystal system|face-centred cubic]] structure between 912 and {{convert|1394|C|F}}. [[Ice]] has fifteen known crystal structures, or fifteen solid phases, which exist at various temperatures and pressures.<ref>
{{Cite book
 |author=M.A. Wahab
 |date=2005
 |title=Solid State Physics: Structure and Properties of Materials
 |publisher=Alpha Science
 |pages=1–3
 |isbn=978-1-84265-218-3
}}</ref>

[[Glass]]es and other non-crystalline, [[amorphous solid]]s without [[order and disorder (physics)|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 (chemistry)|sublimation]], and gases can likewise change directly into solids through [[deposition (phase transition)|deposition]].

===Liquid===
{{Main|Liquid}}
[[File:Gif -AtomosLiquid 03.gif|thumb|upright=0.75|Simple illustration of particles in the liquid state – they can flow and change shape.]]

A liquid is a nearly incompressible [[fluid]] that conforms to the shape of its container but retains a (nearly) 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 (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 being [[water]], H{{sub|2}}O. The highest temperature at which a given liquid can exist is its [[critical temperature]].<ref name="White">
{{Cite book
 |author=F. White
 |date=2003
 |title=Fluid Mechanics
 |page=4
 |publisher=McGraw-Hill
 |isbn=978-0-07-240217-9
}}</ref>

===Gas===

{{Main|Gas}}
[[File:Gif -AtomosGas 02.gif|thumb|upright=0.75|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 enough [[kinetic energy]] so that the effect of intermolecular forces is small (or zero for an [[ideal 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 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 temperature|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 [[supercritical fluid extraction|extract]] [[caffeine]] in the manufacture of [[decaffeination|decaffeinated]] coffee.<ref name=Turrell>
{{Cite book
 |author=G. Turrell
 |date=1997
 |title=Gas Dynamics: Theory and Applications
 |url=https://books.google.com/books?id=-6qF7TKfiNIC&pg=PA3
 |publisher=John Wiley & Sons
 |pages=3–5
 |isbn=978-0-471-97573-1
}}</ref>

===Plasma===
{{Main|Plasma (physics)}}
[[File:Plasma jacobs ladder.jpg|thumb|Artificial plasma produced in air by a [[Electric arc#Visual entertainment|Jacob's Ladder]]. The extremely strong [[potential difference]] between the two rods [[ionize]] 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 ionised 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 ionised 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 spark]]s, [[Fluorescent lamp|fluorescent lights]], [[Neon sign|neon lights]] or in [[Plasma display|plasma televisions]]. The [[solar corona|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, as 99% of all [[ordinary matter]] in the universe is plasma, as it composes all [[stars]].<ref>{{Cite web |date=Sep 7, 1999 |title=Plasma, Plasma, Everywhere |url=https://science.nasa.gov/science-news/science-at-nasa/1999/ast07sep99_1 |website=NASA Science}}</ref><ref>{{Cite book |last=Aschwanden, M. J. |title=Physics of the Solar Corona. An Introduction. |publisher=Praxis Publishing |year=2004 |isbn=978-3-540-22321-4}}</ref><ref>{{Cite book |last=Piel |first=Alexander |url=https://books.google.com/books?id=9cA0DwAAQBAJ&pg=PR8 |title=Plasma Physics: An Introduction to Laboratory, Space, and Fusion Plasmas |date=2017-09-07 |publisher=Springer |isbn=978-3-319-63427-2 |language=en}}</ref>