Gases – Gases, liquids and solids are physical forms of matter and in general, unless decomposition occurs, all substances can exist in any of the three forms. All particulate matter from molecules upward, attract one another by electric, magnetic and gravitational forces the electrical forces being the dominant forces. In addition all matter at any given temperature contains kinetic energy, involving rotational, vibrational and translational energy forms.  If the kinetic energy of the molecules greatly exceeds that associated with the attractive forces then the molecules are free to move away from each other and the matter exists as a gas. If the temperature is reduced the kinetic energy of the molecules is also reduced and eventually the attractive forces become dominant and the molecules coalesce and the gas condenses to a liquid. Gas molecules are free to move randomly in any direction and will expand to fill whatever volume in which they are contained.

Further Information on this Topic can be found in the books of R.P.W. Scott by following the listed links