Solids - 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 much less that associated with the attractive forces then the molecules will be locked together by interactive forces and the matter will exists as a solid and the molecules will be closely packed together. A solid is not easily deformed and has high Young’s Modulus and high sheer modulus values. If energy is given to the solid by heating and its temperature raised it kinetic energy will eventually exceed the energy associated with its attractive forces and the solid will melt. Providing decomposition does not occur due to thermal degradation on further heating more energy will cause the liquid to boil and become a gas.

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