Gas

 

Gas is one of the four fundamental states of matter (the others being solidliquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble 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. What distinguishes a gas from liquids and solids is the vast separation of the individual gas particles. This separation usually makes a colorless gas invisible to the human observer. The interaction of gas particles in the presence of electric and gravitational fields are considered negligible, as indicated by the constant velocity vectors in the image.

The gaseous state of matter is found 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 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 word gas was first used by the early 17th-century Flemish chemist Jan Baptist van Helmont.[4] He identified carbon dioxide, the first known gas other than air.[5] Van Helmont’s word appears to have been simply a phonetic transcription of the Ancient Greek word χάος Chaos – the g in Dutch being pronounced like ch in “loch” (voiceless velar fricative, IPA |x]) – in which case Van Helmont was simply following the established alchemical usage first attested in the works of Paracelsus. According to Paracelsus’s terminology, chaos meant something like “ultra-rarefied water”.[6]

An alternative story[7] is that Van Helmont’s word is corrupted from gahst (or geist), signifying a ghost or spirit. This was because certain gases suggested a supernatural origin, such as from their ability to cause death, extinguish flames, and to occur in “mines, bottom of wells, churchyards and other lonely places”. In contrast, French-American historian Jacques Barzun[8] speculated that Van Helmont had borrowed the word from the German Gäscht, meaning the froth resulting from fermentation.

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