Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid when subjected to an increase in heat given or pressure. The inverse process is solidification/freezing.
The conditions under which melting occurs correspond to the so-called melting point, which is characterized by a certain temperature and pressure value. Often reference is made to atmospheric pressure, so in this case the melting point can be considered synonymous with “melting temperature”.
During the melting process the volume may increase or decrease, depending on the substance under consideration and the pressure. This can be predicted by evaluating the slope of the solidification curve of the corresponding state diagram. For example in the case of water, for pressures less than 2000 atm this curve has a negative slope, therefore in this pressure range there is an increase in volume concomitant with the solidification process, whereas for pressures greater than 2000 atm the opposite occurs. In case of some materials such as glass and thermoplastic polymers, the transition from solid to liquid state happens much more gradually, passing from the so called softening point.
The fusion phenomenon is applied in different fields, where it is necessary to make a fluid material to define its characteristics of interest. Some applications are in the metalworking technique and in the production of glass.
- Given a certain pressure value, a substance has one and only one melting temperature.
- If the pressure remains constant during the melting process, the body temperature remains constant.
- The heat needed to melt a given substance of mass m is given by the product between melting heat and mass.
The conditions under which the melting takes place, correspond to the so-called melting point. The melting point is defined as a thermodynamic state, defined by a certain temperature (called melting temperature) and pressure (not necessarily equal to atmospheric pressure), at which the melting process takes place. For example in the case of water, for pressure values lower than 2000 atm this curve has a negative slope, so in this pressure range, there is an increase in volume in conjunction with the solidification process, while for pressures greater than 2000 atm the opposite occurs.
The melting point is sensitive to extremely large changes in pressure, but generally, this sensitivity is orders of magnitude less than that for the boiling point, because the solid-liquid transition represents only a small change in volume.
Only the crystalline solids have a definite melting point: amorphous solids, such as glass and thermoplastic polymers, do not have a well-defined melting point, but only a temperature range in which they become progressively softer until liquefy. In this case, we speak of a softening point. Furthermore, many crystalline solids do not have a precise melting temperature, at atmospheric pressure, because their decomposition temperature is lower than the melting temperature.
In most cases of practical interest, the value of the pressure is the atmospheric pressure, practically constant, and for this reason, the melting point is indicated with only the temperature.
During the melting substance absorbs a certain amount of heat, said the heat of fusion, that breaks the interatomic or intermolecular bonds that form the crystal lattice, and the temperature stops rising until the substance is not completely liquid: once the melting is over, the temperature starts again to rise.