Gas
Gas is one of the states of matter, in which the molecules move about unconstrained by each other and will expand into any available space. Any such fluid used as an anesthetic, as nitrous oxide; any such combustible fluid used as fuel.<ref>http://dictionary.reference.com/browse/gas</ref>
Gas is also short for gasoline; a volatile, flammable liquid mixture of hydrocarbons, obtained from petroleum, and used as fuel for internal-combustion engines, as a solvent, etc.<ref>http://dictionary.reference.com/browse/gasoline</ref>
Our atmosphere is mostly composed of gases.
Many gases are colorless, however, some are not. For instance, chlorine gas is greenish-yellow, and N2O4 is reddish-brown.
Category of Gases Category of Phases of Matter
- Snippet from Wikipedia: Gas
Gas is a state of matter with neither fixed volume nor fixed shape. It is a compressible form of fluid. 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, the latter of which provides the upper-temperature boundary for gases. Bounding the lower end of the temperature scale lie degenerative quantum gases which are gaining increasing attention. 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.