Today we are going to talk about one of the thermodynamic processes that usually occurs in nature. It's about the reverse sublimationIt occurs when an exothermic state change from a gas to a solid occurs without first converting to its liquid phase. It has other names, such as regressive sublimation or deposition.
In this article we are going to tell you all the characteristics, how it happens and how important reverse sublimation is.
Key features
It is an exothermic process since the gaseous particles must lose energy in the form of heat and give it to the environment. In this way, it is achieved that the product of this reaction has less energy than that of the reactants. In such a way that it cools down enough to that can form crystals, solidify or freeze on a surface. This reverse sublimation process can be seen where there is a sufficiently icy surface so that crystals can be deposited on it directly.
When we speak of deposition, we are not referring to the fact that the particles are deposited from the gas phase without actually wetting the surface. Normally we find reverse sublimation phenomena on icy objects such as frost that is deposited on the leaves during winter. We can detect this deposition since it is formed by a thin layer of crystals, although it can also be an apparent dust or clay.
Thanks to the control of this process New multilayer materials can be obtained where each layer consists of a specific solid that is deposited by both physical and chemical processes. To better understand this phenomenon, you can consult information about condensation and sublimation and about the states of water.
The role of reverse sublimation
It is, as its name suggests, a compound process sublimation. It does not start from a solid evaporating, but from a gas that solidifies or freezes. It can be quite surprising to think that a gas can be cooled to such an extent that it does not even need to pass through it has been liquid in the first place.
Let's see what role the surface plays in reverse sublimation. When a gas is highly disordered and diffuse, it begins to rearrange its particularities and establish itself as a solid when temperatures drop. This rearrangement is thermodynamically difficult to carry out. And it is that it needs a type of support that is capable of receiving the gas particles so that they can be concentrated. Once the particles are concentrated, they can interact with each other to exchange heat with the colder surface.
This is how they lose energy thanks to the surface acting as a heat exchanger. As the particles exchange heat with the cooler surface, they slow down and the first crystalline nuclei form. These nuclei serve as a depositing ground for other groups of particles and the rest of the surrounding gas. Thanks to this structure, reverse sublimation can begin to form. The end result of this process is the formation of a solid crystal layer on the surface. To learn more about the states of water, you can visit our article on .
Conditions for reverse sublimation to occur
For this process to take place, first of all there must be several conditions. The first is that the surface of contact with the particles must have a temperature below its freezing point. This means that The gas must be supercooled so that as soon as it touches the surface, its entire stability can be disturbed.
On the other hand, if the surface is cool enough, the high temperature of the gas can be transferred more quickly to make all the particles adapt to the structure on the surface. There are various reverse sublimation methods where the contact surface does not even have to be cold for a reaction to occur. In the technology industry, a lot of work is done with this process and it is called chemical vapor deposition by combustion.
Examples
Let's look at the main examples of this type of process. When we take a beer out of the refrigerator, the glass turns white. The bottle provides enough surface area for the water vapor molecules to collide, quickly losing all their energy. If the glass covering the beer is black, the white color will be much more noticeable. We can scratch it with a fingernail to see that the vapor has turned into solid.
Sometimes this process is such that the beer becomes covered in a white frost. The effect lasts for a short time since as minutes go by it condenses and becomes damp in the hand.
Another example is frost. Just as it occurs on the walls of a beer bottle, the frost that deposits on the inside walls of some refrigerators also undergoes this process. These layers of ice crystals can also be observed on tuna at ground level. This freezing doesn't fall from the sky like snow does. The air is simply so cold that when it hits the surface of plants, it freezes directly. They go from a gaseous state to a solid state. For more information on similar phenomena, see the Cirrus.
Physical and chemical deposition
So far, we've only talked about water. However, it can also occur with other substances or compounds. Let's suppose we have a chamber containing gaseous gold particles. We can introduce a resistant, icy object into this chamber, and layers of gold will be deposited on top of it. The same would occur with other metals or compounds, provided that an increase in pressure is not required to create a vacuum.
On the other hand, we have chemical deposition. If there is a chemical reaction between the gas and the surface, it is a chemical deposition. This is commonly used for polymer coating in industry. Thanks to chemical deposition, surfaces such as diamond, tungsten, nitrides, carbides, silicon, graphene, etc. are treated.
As you can see, reverse sublimation is a natural process that humans benefit from for various uses in industry. I hope that with this information you can learn more about reverse sublimation and how it happens.