to occur energy has to be added to the solid in the form of heat (States, 2003). The geometric organization of molecules in a solid is generally a crystal lattice structure, with each molecule in a well-defined space relative to the other molecules (Sisler, VanderWerf and Davidson, 1967, 62). A crystal is a solid body bounded by symmetrically arranged plane surfaces (faces), giving crystalline solids a definite geometric form, and considerable mechanical strength and rigidity. The angles between the corresponding faces of the crystals of a given substance are the same in all crystals of that substance. Many solids can be identified by their crystal structure. By increasing the temperature of a solid to its melting point, which is specific for each substance, the energy of some molecules will become sufficient to overcome the binding forces of the crystal and the solid begins to melt. As further heat is applied, more molecules will break from the crystal and melt until all the crystal passes into the liquid state.
Some materials, such as glass, glue, and tar do not have a crystalline form, so they are classified as pseudo-solids (Sisler, VanderWerf and Davidson, 1967, 63). Pseudo-solids do not have fixed melting points, and the corresponding liquids do not solidify at a constant temperature. They can be regarded as undercooled liquids of high velocity, because under the right conditions, they can be solidified.
The temperature at which the solid state and the liquid state of a given substance exist in equilibrium is known as the melting point of the solid or the freezing point of the solid (Sisler, VanderWerf and Davidson, 1967, 63). Applying more heat will cause the substance to melt more completely, and withdrawing heat will cause the substance to solidify. The number of calories needed to convert a unit weight of a solid substance into a liquid at the melting point is known as the heat of fusion. The heat of cryst...