Gases, liquids and solids

The three most common states, or phases, of matter, gases, liquids and solids are very familiar (Walton, 1976). Phases that are not so well known are plasmas and liquid crystals (although these are both found in electrical and electronic devices in everyday use). All these states are generally distinguished by the degree of translational and orientational order of the constituent molecules. On this basis some phases may be further subdivided. For example, solids, consisting of a rigid arrangement of molecules, can be crystalline or amorphous. In an amorphous solid (a good example is a glass), the molecules are fixed in place, but with no pattern in their arrangement. As shown in figure 1.1, the crystalline solid state is characterized by long-range translational order of the constituent molecules (the molecules are constrained to occupy specific positions in space) and long-range orientational order (the molecules orient themselves with respect to each other). The molecules are, of course, in a constant state of thermal agitation, with a mean translational kinetic energy of 3kT/2 (k is Boltzmann's constant, T is temperature; kT/2 for each component of their velocity). However, this energy is considerably less than that associated with the chemical bonds in the material and the motion does not disrupt the highly ordered molecular arrangement.