Gas

Gas is a state of matter with neither fixed volume nor fixed shape. It is a compressible form of fluid, in contrast to a liquid. 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.