January 20, 2022

Gas or gas is one of the four fundamental states of matter (the others are solid state, liquid state, and plasma). Pure gas can consist of individual atoms (e.g. a noble gas such as neon), elemental molecules consisting of one type of atom (e.g. oxygen), or molecules of compounds formed of different atoms (e.g. . carbon dioxide). Gaseous mixtures contain various pure gases, such as air. Gas differs from liquid and solid in that there is a huge separation between individual particles. This separation usually makes colorless gases invisible to the human eye. The interactions of gas particles in the presence of electric and gravitational fields are considered negligible, which is represented by constant velocity vectors in the figure. One type of well-known gas is steam. The gaseous state of matter is located between the state of liquid and plasma, where plasma represents the upper temperature limit of gases. At the border of the lower end of the temperature scale lie degenerative quantum gases, which are gaining more and more attention. High-density atomic gases that are super-cooled to extremely low temperatures are classified according to their statistical behavior as either Bose gas or Fermi gas.

Elemental gases

The only chemical elements that are stable at standard temperature and pressure (STP) are multiatomic homonuclear molecules: hydrogen (H2), nitrogen (N2) and oxygen (O2); plus two halogens, fluorine (F2) and chlorine (Cl2). These gases, when grouped together with monatomically noble gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn), are called elemental gases. Alternatively, they are sometimes referred to as "molecular gases" to distinguish them from molecules that are also chemical compounds.


The word gas is a neologism originally used in the early 17th century by the Flemish chemist J.B. van Helmont. Van Helmont's word seems to have been simply a phonetic transcription of the Greek word χάος chaos - pronounced in Dutch as "loch" - in which case Van Helmont simply followed the established alchemical usage originally used in Paracelsus' works. According to Paracelsus' terminology, chaos means something like "ultra-raz."

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