We call the melting point (or melting temperature) the temperature at which a substance changes from a solid to a liquid state. Melting is the process by which an element changes from the liquid phase to the solid phase.
The melting point is usually 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 increases, the melting point decreases, and the melting point range becomes wider.
To better understand what has been said, let's consider a block of iron and suppose we heat it. Initially, its temperature begins to rise until it reaches a temperature at which the iron block begins to melt (liquid state). The temperature at which this process occurs is called the melting point.
The fusion process of an element occurs at a constant temperature, as with any phase change. The heat released during the fusion period is called the latent heat of fusion.
Factors that influence the melting point of a substance
The melting point of a substance depends on several fundamental factors that influence the amount of thermal energy needed to break the interactions that maintain its structure in a solid state.
1. Intermolecular forces
The strength of the forces holding the molecules or atoms together in a solid determines how much energy is needed to break them apart and turn the substance into a liquid. Some of the most important forces are:
- Ionic bonds : Present in compounds such as table salt (NaCl), these are extremely strong interactions between oppositely charged ions. This gives ionic solids very high melting points.
- Van der Waals forces : These are weak interactions between nonpolar molecules. Solids formed by these forces (such as noble gases in the solid state) have low melting points because the attractive forces are easily overcome with minimal heat.
- Hydrogen bonds : These are intermolecular forces stronger than Van der Waals forces, present in compounds such as water (H₂O) or acetic acid. They raise the melting point compared to similar substances without this type of bond.
- Dipole-dipole interactions : These occur in polar molecules and affect the melting point depending on the strength of the dipole. The stronger the attraction between dipoles, the higher the temperature needed to melt the substance.
2. Crystal structure
The way in which atoms or molecules are organized in a solid directly influences its melting point:
- Solids with strong crystal lattices : Materials such as metals, ionic crystals, and covalent solids (such as diamond and quartz) have highly organized and strong structures, which require a great deal of energy to break them down. This gives them very high melting points.
- Amorphous substances : Materials such as glass and some polymers do not have a well-defined crystalline structure, so they do not have a precise melting point. Instead of melting abruptly, they soften progressively as the temperature increases.
3. Pressure
Pressure also plays an important role in the melting point of a substance:
- In most solids, an increase in pressure causes an increase in the melting point, since the particles are more compressed and require more energy to move freely.
- However, there are exceptions. In the case of ice (solid H₂O), increasing pressure lowers its melting point. This occurs because ice has a less dense structure than liquid water, so applying pressure favors the transition to liquid.
Difference between melting point and freezing point
The main difference between melting point and freezing point lies in the process of state change they describe:
- Melting point : The temperature at which a substance changes from a solid to a liquid when heat is applied. At this point, the particles gain enough energy to overcome the intermolecular forces that held them in a rigid structure.
- Freezing point : The temperature at which a substance changes from a liquid to a solid state when it loses heat. At this point, the particles reduce their kinetic energy and arrange themselves into a more stable structure, forming a solid.
In many pure substances, the melting point and freezing point occur at the same temperature, but in opposite directions. For example, pure water melts and freezes at 0°C under normal atmospheric pressure. However, factors such as pressure or the presence of impurities can change the freezing point without affecting the melting point (for example, salt lowers the freezing point of water, but not its melting point).
How is the melting point determined?
Melting point measurement is performed using a device called a Thiele melting point meter or in a melting block. The process is as follows:
- The element is introduced into a capillary tube to place it in the Thiele
- The capillary tube is heated slowly until it changes phase.
- The temperature recorded at the phase change corresponds to the melting temperature.
Eutectic point
The eutectic point is the minimum temperature at which a mixture of two or more solid components can completely melt, transforming into a liquid, leaving no solid phase. This phenomenon occurs when the mixture has a specific, fixed composition of the substances involved.
At the eutectic point, the components of the mixture melt at a single temperature, lower than the melting points of the pure substances separately.
When eutectic conditions are reached, both components of the mixture are in equilibrium between the solid and liquid phases, meaning that the substances melt and crystallize at the same temperature. This melting point is important in industry and in the manufacture of metal alloys, since eutectic alloys are used to obtain specific characteristics, such as hardness or thermal conductivity, that cannot be achieved by melting pure metals individually.
In some cases, the eutectic point involves not just two components, but can be a mixture of three or more substances, each with its own melting point.
A classic example of a eutectic alloy is the combination of lead and tin, which has a eutectic point at a lower temperature than either metal alone. This makes eutectic alloys useful for applications such as soldering, where a low melting point is necessary to avoid damaging other components.
Examples of the melting temperature of some materials
We report the values of the 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 |