Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 13:34

When a crystalline solidA state of matter having a specific shape and volume and in which the particles do not readily change their relative positions. melts, it loses its rigid form and adopts the shape of its container. At the same time there is usually an increase in volume of a few percent. On the molecular level we can interpret this as a breakdown in the regular structure of the solid. As temperatureA physical property that indicates whether one object can transfer thermal energy to another object. rises toward the melting pointThe temperature at which a solid becomes a liquid. Also called freezing point., the molecules vibrate more and more strongly. Above the meltingThe process of a liquid forming from a solid. point, these vibrations are so energetic that they overcome the forces holding the molecules in the crystal latticeAn orderly, repeating arrangement of points in 3-D space in which each p;oint has surroundings identical to every other point. A crystal's constituent atoms, molecules, and ions are arranged about each lattice point.. The molecules no longer vibrate around an average position but begin to slide past each other. The regular arrangement of the crystal disappears, but the molecules have not escaped each other’s attractive influence. The very small volume change which occurs on melting shows that the molecules have moved apart to only a very limited extent and that there can be only a few gaps caused by the less-regular packing. This view is confirmed by the experimental fact that liquids, as opposed to gases, are very difficult to compress. Even at the bottom of the deepest oceans, under pressures of thousands of atmospheres, the densityThe ratio of the mass of a sample of a material to its volume. of water is only minutely larger than at the surface.