Gas

This page is about the physical properties of gas as a state of matter. For the uses of gases, and other meanings, see Gas (disambiguation).

Gas phase particles (atoms, molecules, or ions) move around freely

As a noun in the English language, a gas is one of three classical states of matter^[1]. 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 from within the gas. A pure gas may be comprised of individual atoms (e.g. a noble gas or atomic gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). A gas mixture would contain a variety of pure gases much like the air that we breathe. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles as seen in the model to the right. This separation usually makes a colourless gas invisible to the human observer.

This article provides background information on the gaseous state of matter found between the liquid and plasma states^[2], the latter of which provides the upper temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases^[3] which are gaining increased attention these days^[4]. High density atomic gases super cooled to incredibly low temperatures are classified by their statistical behavior as either a bose gas or a fermi gas. For a comprehensive listing of these exotic states of matter see list of states of matter.

The journey begins with a brief review of the physical characteristics of gases. From that introduction, the article diverges and views gases from two distinct perspectives, macro-, or the system point of view, and micro-scopic, or particle viewpoint. Simplified models or gas laws are introduced next. These models characterize ideal gas behavior. From modeling, the article explores the historical connection to gases including prominent scientists along the path. The final section introduces gas terms which link to more detailed applications involving gases. Among these advanced applications are energy considerations like thermodynamics, and movement through a gas, gas flows, or aerodynamics.