Liquids - 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 is of the same order as that associated with the attractive forces then the molecules are free to move (vibrate rotate etc) but remain associated with each other and the matter exists as a liquid. As opposed to a gas a liquid always assumes the shape of the vessel in which it is contained. A liquid is easily deformed and has a very low sheer modulus. Unless decomposition occurs, if energy is given to the liquid and its temperature raised it kinetic energy will eventually exceed the energy associated with its attractive forces and the liquid will boil and become a gas. Conversely if cooled the kinetic energy of the molecules will be reduced and the attractive forces will hold the molecules together rigidly and the material will solidify.

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