Physics:Gas

Gas is a state of matter with neither fixed volume nor fixed shape. It is a compressible form of fluid, in contrast to a liquid. A pure gas consists of individual atoms (e.g. a noble gas like neon), or molecules (e.g. oxygen (O₂) or carbon dioxide). Pure gases can also be mixed together such as in the air. What distinguishes gases from liquids and solids is the vast separation of the individual gas particles. This separation can make some gases invisible to the human observer.

The gaseous state of matter occurs between the liquid and plasma states,^[1] the latter of which provides the upper-temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases^[2] which are gaining increasing attention.^[3] High-density atomic gases super-cooled to very low temperatures are classified by their statistical behavior as either Bose gases or Fermi gases. For a comprehensive listing of these exotic states of matter, see list of states of matter.

The only chemical elements that are stable diatomic homonuclear molecular gases at STP are hydrogen (H₂), nitrogen (N₂), oxygen (O₂), and two halogens: fluorine (F₂) and chlorine (Cl₂). When grouped with the monatomic noble gases – helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn) – these gases are referred to as "elemental gases".

The word gas was first used by the early 17th-century Brabantian or Southern Netherlandish chemist Jan Baptist van Helmont.^[4] He identified carbon dioxide, the first known gas other than air.