Melting

Melting, the process of transition from a solid (amorphous or crystalline) state to a liquid state. Each substance melts at a quite definite temperature for it, which is called the melting point. This temperature depends on external pressure. The melting point is indicated either in degrees Celsius or in degrees of the absolute scale (from absolute zero, which as is known corresponds to -273°). During melting, the volume of a body usually increases: a decrease in volume is an exception (bismuth, ice). The lowest melting temperature is that of helium (-272.1°), one of the highest is that of tantalum carbide (+3,830°). Mercury (solid) melts at -38.9°; sodium at +98°, tin at 232°, silver at 961°, platinum at 1,771°, tungsten, from which electric lamp filaments are made, at 3,390° and graphite at 3,550°.

When heating a solid body, the temperature rises to a certain value (melting point; fig. 1, a) and remains constant, while the melting _heat is absorbed until all the substance has melted. After this, the temperature begins to rise again (fig. 1, b). The heat that goes into melting a substance while the temperature remains constant is called the latent heat of melting and for water is for example equal to 80 small calories per 1 g of ice. In the reverse process of melting - solidification - the temperature drops, then stops at a certain level (melting point or solidification point; fig. 2, d) and after the entire liquid has solidified, it begins to drop again (fig. 2, b'). Sometimes the phenomenon of supercooling of a liquid to a temperature below the melting point is observed (fig. 2, c); by introducing a small crystal of the same substance into the solution, supercooling can be stopped: the liquid solidifies. In a solid crystalline body, there are certain quasi-elastic forces binding its ions or molecules with their centers of equilibrium in the crystal lattice. At the melting point, these forces become equal to zero.