States of matter are the distinct forms that matter can take. Three states of matter exist in most environments: solid, liquid, and gas . In extreme environments, other states are present, including plasma, Bose-Einstein condensates, and neutron stars. Furthermore, other states, such as quark-gluon plasmas, are believed to be possible. Much of the baryonic matter of the universe is in the form of hot plasma, both as rarefied interstellar medium and as dense stars.
Historically, the distinction is made based on qualitative differences in bulk properties. Solid is the state in which matter maintains a fixed volume and shape. Liquid is the state in which its volume varies only slightly, but adapts to the shape of its container. Gas is the state in which matter expands to occupy the volume and shape of its container. Each of the classical states of matter, unlike plasma, can transition directly into any of the other classical states.
In solids, the particles 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, particles are packed in a regularly ordered, repeating pattern. 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-centered cubic structure between 912 and 1394 °C. Ice has fifteen known crystal structures which exist at various temperatures and pressures.
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 .
A liquid is a 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 because 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, making the structure mobile. This means that the shape of a liquid is not definite, but determined by its container. The volume is usually greater than that of the corresponding solid, the most well known exception being water. The highest temperature at which a given liquid can exist is called its critical temperature .
The spaces between gas molecules are very big. Gas molecules have either very weak bonds or no bonds at all. The molecules in a gas can move freely and fast. 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 liquid may be converted to a gas by heating at constant pressure to the boiling point, or 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 without cooling. A vapor can exist in equilibrium with a liquid (or solid). In this 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. 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. This can be useful in several applications. For example, supercritical carbon dioxide is used to extract caffeine in the manufacture of decaffeinated coffee.