Candela Rocío Barbisan
Chemical engineer
Temperature at which a solid is capable of disintegrating its crystal lattice.
To better understand this important concept, it is necessary to understand what fusion is, the process of which involves the physical change that a substance undergoes, from its transformation from a solid to a liquid state. Well, without great effort, our mind goes back to the ice that we removed from the freezer, and it is simple and clear to take it as an example: frozen water, once it is exposed to the environment, where the temperature is higher than in the freezerundergoes an absorption of energy in the form of heat in order to change to a liquid state.
Thus, suppose the ice was at -15ºC in the freezer. Once it is removed from there, it must first be heated up to the temperature at which it undergoes the change of state, which, as we well know, will be 0ºC. Once it reaches that temperature, heat energy will continue to be delivered to it that will allow the change of state to a constant temperature. If the container containing the liquid water, now at 0 °C, remains in the same place, the water will continue to heat up until it reaches equilibrium with room temperature. Therefore, in the above process there are three important stages: first, heating from -15 ºC to 0ºC, then the change of state and, finally, heating to room temperature. Now, in this case we will focus only on the change of state, there, the heat delivered during that single stage, is the heat of fusion, specifically defined as the energy that one kg of a substance absorbs until it melts. In the case of water it is: 3.34 x 105 J/kg.
It should be noted that the melting temperature and the freezing temperature for a substance coincide (in general) if the pressure is the same, that is, if the physical change occurs in reverse, said substance will freeze at the same temperature and the heat that will be released when solidifying a kg of said substance will be the heat of freezing, therefore, both concepts are closely related.
normal melting point
Analogous to the normal boiling point, in this case it is defined as the temperature at which the liquid and solid phases come into equilibrium for an atmospheric pressure of 101.3 kPa. Although the term “normal” refers to a pressure, it differs from the boiling point in the lesser degree of affect that the pressure has on the melting temperature.
The normal melting point is an intensive property of a substance, it does not change with respect to the amount of matter, and it is used to characterize crystalline solids, since it allows to identify their degree of purity and quality. As with boiling point, they are tabulated values, such as:
Water: 0ºC
Ethanol: -114.1ºC
Methanol: -97.6ºC
Hydrogen: -259.2ºC
To understand the differences between these values we must refer to the type of compound and forces involved that define the binding energies. In this line, it is known that a mixture has a lower melting point than its pure components. Surely you have noticed on the news that, on many routes, table salt is placed on top of a snowy surface, this is done precisely to lower the freezing point, making the solution now made up of salt and water have a lower melting point and therefore it melts. In these cases, the melting point of the mixture will also depend on the concentration of the solute.
industrial casting processes
The determination of the melting temperature of different substances allowed the development of different industrial processes that today give rise to many of the things we use. The casting or casting process is based on placing a solid material in a container and exposing it to high temperatures so that it melts, once the liquid is derived into a mold where it solidifies. A clear example of this is the casting of metals such as aluminum. In these cases, the metals are melted in smelting furnaces and then taken to the molds that allow the manufacture of parts that would not otherwise be possible.
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