Melting [fusion]

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.

Melting laws

  • 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.

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