The Solid State Solids have both definite volume and definite shape. The geometric stability of a solid: is not due to any difference in compactness between the solid and liquid states, for the density of a substance in the solid state may actually be less than that of the corresponding liquid, as in the case of ice and water. is to ascribes rather to the fact that sructural units, instead of being in random motion like the molecules of liquid or gas, are confined to definite positions of equilibrium within the crystal of solid, positions about which the particles may vibrate but which they cannot readily leave.

A crystalline is one in which the constituent structural units are arranged in definite geometrical configuration characteristic of the substance. This geometric configuration repeat itself with definite periodicities in three dimensions, resulting thus in substance having long range order. crystalline substance such as ice, sodium chloride, or naphtalene melt sharply at a constant and definite temperature. A Amorphous possesing many of the attributes of solid, such as definite shape, a certain rigidity, and hardness, do not show under test a definite configurational arrangement or order extending for any distance. For that reason they are not considered to be true solids but rather highly supercooled liquids of very high viscosity. amorphous substance like glass or asphalt gradually and over a temperature interval. under certain conditions an amorphous substance may acquire crystalline characteristic. Thus glass may crystallize on long standing or heating. Again, natural rubber when stretched exhibits a differnt pattern on examination with X-rays, an indication of the production of order and a definite configurational arrangement.

Crystallization ◦ A pure liquid on being cooled at constant pressure suffers a decrease in the average translational energy of its molecules, and hence its temperature drops until the freezing point is reached. ◦ At this temperature attractive forces of the molecules are sufficient to overcome the translational energy, and the molecules are force to arranged themselves in geometric pattern whic is characteristic for each substances. ◦ When crystallization starts, heat is evolved. This heat evolution arrest further temperature drop, and the temperature of solid and liquids remain constant as long as both phase are present. ◦ Further removal of heat results merely in the crystalliation of more liquid, until finally the whole mass solidifies, only then does the temperature begin to fall again and cooling.The mount of heat evolved per mole of substance is called the heat of crystallization of the substance.

Fusion ◦ The reverse of crystallization is the fusion or melting of the solid. ◦ As the pure solid is heated, its average vibrational energy increases, until at the melting point some particles are vibrating with sufficient energy to overcome the confining forces. The solid then begin to fuse. ◦ For given pressure the temperature at which this occurs is the same as the crystallization temperature. ◦ To accomplis further fusion, heat must be supplied to compensate for the loss of the particles with high energy. The amount of heat which must be absorbed to accomplish the transition of one mole of solid to one mole of liquid is known the heat of fusion. ◦ This amount of heat is equal in magnitude, but is opposite in sign of the heat of crystallization of the substance.

Crystallography ◦ the experimental science of the arrangement of atoms in solids. The Crystal Systems Properties of Crystals ◦ Polymorphism ◦ The Crystal Lattice X-Ray Crystallography The Heat Capacity of Solids ◦ Characteristic Einstein temperature ◦ Characteristic Debye temperature ◦ Debye third law