When we study the phase transition processes, that is, the phase changes of a substance, we see that for this to happen it is necessary to supply or remove heat from the substance in question. In our daily lives, we can observe the phase change of water that evaporates from a garment laid out on a clothesline or from an ice cube melting when exposed to the environment.
We can then define the phase transition as the internal reorganization of the molecules of a substance, causing significant changes in its properties. At the level of recall about phase transitions, we have:
gas to liquid →condensation
liquid to gas →vaporization
liquid to solid →solidification
solid to liquid →melt
solid to gas →sublimation
gas to solid →sublimation
We have seen that physical processes that take place in closed systems conserve the total energy of the system. In phase transition processes, such as melting and evaporation, the temperature remains constant even though heat is being supplied to the system. To understand where this energy is going, let's look at what's going on microscopically.
We can associate a potential energy for each particle of substance as the energy needed to put it in that position. If we want to change their internal arrangement, we need to do some kind of work on the particles. Therefore, we can associate a potential energy with the arrangement of the atoms and molecules of the substance.
When heat is supplied, atoms and molecules tend to vibrate more intensely, increasing the temperature, which is a measure of the average kinetic energy of the particles. During the process of fusion or vaporization, the temperature remains constant, but the arrangement of atoms and molecules is changed.
The potential energy of each changes, and the variation of this potential energy is the heat given up or taken away from the substance.
The measure of energy spent, per unit of mass, is the latent heat of fusion or vaporization. The greater the latent heat, the greater the variation in potential energy due to the modification of the atomic or molecular arrangement of that substance.
In this way, the total energy is conserved in the phase transition processes. The energy supplied or withdrawn is transformed into kinetic energy or into potential energy (internal rearrangement of atoms).
By Domitiano Marques
Graduated in Physics
Source: Brazil School - https://brasilescola.uol.com.br/fisica/conservacao-energia-nas-transicoes-fase.htm