What is the latent heat of fusion?

On a daily basis, we tend to use expressions such as “the water is hot”, “the water has already boiled” or “the water has evaporated”. However, they are not phrases that should be used lightly, since each one of them has its own connotation, associated with the changes of state experienced by the different bodies or substances.

There are many substances that, after being exposed to heat, undergo changes of state, that is, the well-known transition from solid to liquid. A good example of this would be to put a piece of ice in a pot and, in turn, place it on the hot burner. Immediately, it will be possible to see how the ice begins to melt little by little, because it has reached the melting point of water or 0 °C.

It is a change caused by the increase in temperature on the solid object. Of course, not all elements melt at the same level of specific heat as ice. In the case of iron, we have that it has a melting point of 1535 °C, while aluminum must reach 660 °C and salt 801 °C.

Now, if the molten liquid continues to be exposed to heat, the temperature will rise until it reaches its maximum boiling point (100 °C). At this moment, the water will have boiled and, therefore, the change from liquid to gaseous state will begin, in which the substance becomes a vapor and is integrated into the atmosphere.

Each of these changes from solid to liquid, and from liquid to gas, require a specific amount of energy to achieve their transformation, which is known as latent heat. 

In addition, among these phenomena is that of the enthalpy of vaporization of water, in which the liquid becomes gaseous, because said liquid body retains the same pressure as the vapor, after being subjected to an amount of energy.

Know the types of heat

Before knowing the types, it is appropriate to specify the concept of heat. It is a process experienced between two bodies with different temperatures, that is, the transfer of energy from a body with a higher temperature to another with a lower temperature. 

Said energy is of the thermal type, since it is the total energy accumulated in the molecules of a body with a high temperature. In addition, it is important to bear in mind that, since it is the transfer of energy , its measurement can be done in the traditional heat units: joules, calories and kilocalories.

Regarding the classification of heat, we can say that there are several types, determined according to the temperature and the solid or liquid state of the given body. Next, we explain them in detail.

Fusion heat: it is the moment in which a solid body changes its state to liquid.

Heat of vaporization: is the change from liquid to gaseous state.

Latent heat: It is that energy necessary for a body to pass from one state to another. It is a type of heat responsible for carrying out said transition from solid to liquid or from liquid to gas. However, this heat only fulfills the function of the phase change, but is not capable of increasing the temperature of the body.

Sensible heat: It is the one capable of generating an alteration in the temperature of the body without modifying the composition of its molecular structure , that is, although this type of heat provides caloric energy, it does not generate a phase change in the state of the body. In fact, when talking about sensible and latent heat, this is the main difference.

Specific heat: Its purpose is to increase the body temperature by only one degree. This heat is applicable to the mass of a plastic, metal and wood composite. Similarly, it is possible to calculate the specific heat of ice, the specific heat of water, among other elements.

To contextualize a little more about the types of heat, we will continue with the initial example. By exposing the ice cubes (solid state) to heat, the temperature of said body will rise until it melts (liquid state). This change of state would be the melting point, produced by the latent heat of fusion of water, which is the constant temperature of 0 ºC, during the transition process. On the contrary, if the ice thaws at room temperature without the intervention of a heat source, we would be in the presence of the melting temperature.

Continuing with the water at the melting point, if it continues to be exposed to heat, the temperature will rise to a maximum of 100 ºC and it will reach its boiling point. Said temperature will once again remain stable, thanks to the latent heat of vaporization. However, after boiling, a second phase or evaporation will begin, in which the water changes to a gaseous state, while maintaining the latent heat of vaporization of the water.

On the other hand, to explain sensible heat, we can imagine that we heat any type of element in a liquid state, but only let it heat up a little, without its structure being altered, that is, it does not pass into a gaseous state.

Each type of heat has different variants, which make its function unique when in contact with a solid or liquid body. In fact, scientists conclude that latent heats follow particular parameters for each of the substances, so the value is associated with the pressure evidenced during the state transition .

Let us always remember that each body, substance or element has a structure with a distinctive molecular composition and, therefore, cannot react in the same way with respect to the action of fusion heat, vaporization heat and latent heat.

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