Gas

Gas is one of the four fundamental states of matter. The others are solid, liquid, and plasma.^{[1][better source needed]} A pure gas is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom (elements such as oxygen) or from different atoms (compounds such as carbon dioxide). A gas mixture, such as air, contains a variety of pure gases. What distinguishes gases 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 gaseous state of matter occurs 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 increasing attention.^[4] 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.