Latent heat is the amount of heat that a substance needs to absorb or release to change its physical state or phase, keeping the temperature constant during this process. The heat supplied or released does not manifest itself, it does not “feel” as is usual in a temperature variation; hence the term latent heat. latent heat of fusion
The latent word comes from the Latin word “latens” which means hidden, so latent heat behaves as if it did not exist when the temperature was kept constant. But it does exist: what happens is that it is totally consumed in the phase change or physical state.
Taking water as an example, the phase changes are as follows: ice (solid) melts into liquid water (liquid) in a process called melting; and liquid water in turn vaporizes to become water vapor (gas) in a process called vaporization. latent heat of fusion
On the other hand, water vapor is cooled to liquid water in a process called condensation; and liquid water turns back to ice in a process called solidification. latent heat of fusion
Each of these phase changes is associated with a latent heat, which can be positive (absorption) or negative (release).
Latent heat of fusion
It is the amount of heat that must be supplied to a given quantity of solid substance to change its physical state from solid to liquid. During fusion there is no change in temperature, so while the solid melts or melts the liquid that is generated from it has the same temperature as the whole solid.
Generally, this heat is determined at the normal melting point, which is the temperature at which the solid begins to melt under atmospheric pressure. At this temperature there is an equilibrium or simultaneous presence of the solid state and the liquid state at the same time . latent heat of fusion
The solid state is characterized by the presence of a compact structure due to the existence of high-energy chemical bonds , which give it structural rigidity. For the transformation of a solid to liquid, these interactions must be broken, so energy is consumed (absorbed) for that purpose.
A typical example is when ice cream is consumed, made up of water, sugar and other substances. It is common to observe that ice cream, if it is not consumed quickly, begins to melt, that is, to melt. This occurs when the temperature of the ice cream reaches the melting point of ice. latent heat of fusion
Some examples of latent heats of fusion expressed in J / g will be listed; that is, the energy that a gram of solid must absorb to melt at the melting point:
– Mercury 11.8
Ice absorbs a lot of heat not because its interactions are stronger, but because it is capable of exceptionally dissipating heat between its crystals. latent heat of fusion
Latent heat of vaporization
It is the amount of heat that a given amount of substance absorbs to change from a liquid state to a gaseous state at the normal boiling point . That is, it is the heat that the liquid absorbs when it boils and turns into a gas without an increase in temperature. latent heat of fusion
Within the liquid the molecules of the substances can interact by the formation of hydrogen bonds and by forces of attraction between the molecules. In order to pass a substance from the liquid state to the gaseous state, these forces must be overcome, so heat must be supplied.
Sensible heat is what the liquid absorbs only to increase its temperature, which will depend on its specific heat. Meanwhile, the latent heat of vaporization is used to transform it directly into steam, a process that is much easier at the boiling point .
Examples latent heat of fusion
Below are listed examples of some liquids with their respective latent heats of vaporization expressed again in J / g:
-Acetic acid 402
-Ethyl alcohol 846
-Carbon dioxide 574
Note the enormous latent heat of liquid water: 1 g of liquid water (about 1 mL) needs to absorb 2256 J to vaporize. Liquid water dissipates the heat it receives even better compared to ice.
Latent heat of solidification
It is the heat that a certain amount of substance must give off to go from its liquid state to its solid state at the point of solidification or freezing. Again, until the liquid has completely solidified, the temperature will remain constant.
The solidification process is the reverse of the fusion process, so the values of the latent heat of solidification and those of the latent heat of fusion are the same, but with opposite signs.
The molecules of a substance in the liquid state move with a certain freedom due to the energy they possess. Therefore, to pass to the solid phase, the molecules must release energy in the form of heat, which allows greater interaction between the molecules of the substance. latent heat of fusion
The interaction between the molecules favors the formation of the chemical bonds of the solid phase. An example of this occurs when liquid water is placed in the ice-forming containers in the refrigerator freezer. The freezer extracts heat from the liquid water and it solidifies to become ice.
Some examples of latent heats of solidification (in J / g) for some liquids are listed below:
Latent heat of condensation
It is the amount of heat that must be given off or released from a certain amount of substance, to go from the gaseous state to the liquid state. The gas that condenses and the liquid formed maintain the same temperature during condensation.
Condensation is the opposite of vaporization. The latent heat of condensation has the same value as the latent heat of vaporization, but with the opposite sign, and at the boiling point. An example of this is when cooking and steam condenses on the inside of the pots. latent heat of fusion
Gaseous molecules move freely driven by the energy they possess. Therefore, they must release energy to allow the intermolecular interactions typical of the liquid state to establish between the molecules of the substance.
In nature there are many other examples of the phenomenon of water condensation. Water vapor rises in the atmosphere and condenses in the clouds in the form of water droplets.
The presence of water droplets on car windows is also observed, as a result of water condensation due to a drop in temperature that occurs in the early morning, constituting the so-called dew.
Finally, for some vapors their respective latent heats of condensation expressed, once again, in J / g will be listed:
-Acetic acid (-402)
-Ethyl alcohol (-846)