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In the solid state it is assumed that each molecule is performing oscillations of small amplitude about its fixed position of equilibrium. When heat is imparted to the solid, the amplitudes of these oscillations increase, and at a certain temperature the oscillations are so large that the molecules collide with each other, and break loose. This is the point of fusion. The process of solidification consists in the liquid molecules building themselves up again into a system of molecules oscillating about fixed points. The solid molecules exert considerable attractive forces upon each other; in separating them under the influence of these forces work is done, which is equivalent to the latent heat of fusion. Since the solid is very little compressible, there must also be repulsive forces between the molecules, which increase more rapidly than the attractive forces when the distances between the molecules are reduced. In its ordinary state, these two sets of forces are in equilibrium in a solid. In some cases the particles in a solid are rotating, as well as oscillating, about fixed positions.
The process of rebuilding the solid structure from the liquid takes place around definite points or nuclei. Small crystal aggregates are formed at a comparatively small number of points, and radiating masses of crystals shoot out from these centres until the whole mass is solid. Crystallisation does not usually begin at the freezing point unless solid is present; the liquid must be supercooled before solid appears. A solid, on the other hand, always fuses as soon as the melting point is reached, and cannot be permanently superheated. At the melting point, when both solid and liquid are present, there is a condition of kinetic equilibrium similar to that described in connection with a liquid in contact with its vapour.
The question as to the molecular weight of a solid has little or no significance, since the particles composing the solid crystal may not correspond with the molecular formula of the substance. In sodium chloride crystals, for example, the individual particles are charged sodium and chlorine atoms (ions), Na+ and Cl-, not molecules, NaCl, and a diamond crystal consists of single atoms of carbon linked together by strong forces to which the hardness and low volatility of the substance are due. The old idea that the solid state indicated a high molecular weight, and polymerisation, so that the molecules in diamond were Cn for example, is definitely incorrect. The solid state will be considered in detail later.
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