The melting point (or melting temperature) is the temperature at which a substance changes from a solid to a liquid state. Fusion is a property in which an element passes from the liquid phase to the solid phase.
The melting point is generally specified at standard atmospheric pressure (1 atmosphere or 100,000 pascals) and for a pure substance.
The melting point of a pure element is always higher than that of an impure substance. For this reason, melting points are often used to characterize organic and inorganic compounds and to determine their purity. If the presence of other components is increased, the melting point is lowered and the melting point range is broader.
To better understand what has been said, let us consider a block of iron and suppose that we heat it. Initially, its temperature begins to rise until it reaches a temperature where the iron block begins to melt (liquid state). The temperature at which this process takes place is called the melting temperature.
The melting process of an element occurs at a constant temperature, as in any phase change. The heat given in the period of fusion is called the latent heat of fusion.
How does pressure influence the melting point?
The melting temperature is affected by the pressure acting on the solid.
In general, an increase in pressure decreases the melting temperature in those substances that contract from the solid state to the liquid state (ice) and causes it to rise in those substances that expand in the same passage.
It should be noted that pressure is a physical property that also depends on volume. Reducing the volume reduces the pressure and increases the melting temperature.
Is the melting point the same as the freezing point?
In theory, the melting point of a solid should be equal to the freezing point of the liquid. In practice, small differences are observed between these values.
How is the melting point determined?
The measurement of the melting point is carried out with a device called Thiele or in a melting block. The process is carried out as follows:
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The element is inserted into a capillary tube to be placed in the Thiele
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The capillary tube is heated slowly until the moment it changes phase.
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The temperature recorded in the phase change corresponds to the melting temperature.
What is the eutectic point?
The eutectic point is like the minimum temperature at which a mixture of solids can melt (generally, two solids), which have a fixed composition.
The eutectic point determines the melting point of one substance with another.
Examples of the melting temperature of some materials
We report the values of melting temperatures and boiling points of some substances determined at a pressure of 1 atm.
|
Chemical substance |
Melting point in degrees Celsius |
Boiling point in degrees Celsius |
|
Water |
0 |
100 |
|
Hydrogen |
-259.19 |
-252.92 |
|
Carbon |
3526.8 |
4026.8 |
|
Aluminum |
660.27 |
2518.8 |
|
Mercury |
-38.77 |
356.68 |