Solids retain their volume and shape. In them, the average kinetic energy of structural units is much less than the potential energy of their interaction. When a solid crystal is heated, the amplitude of thermal vibrations of structural units increases. This leads to a weakening of interstructural bonds and, as one of the final results, to the destruction of the crystal lattice. The body becomes liquid, that is, a melting process occurs.
Melting
This is an equilibrium phase transition of a solid crystalline body into a liquid state, accompanied by the absorption of heat.
Further heating of a solid body causes its evaporation, characterized by the separation of molecules that have the greatest energy from the surface of the body. In addition to liquids, solids can also evaporate directly without an intermediate transition through the liquid state, that is, sublimation occurs.
Sublimation
Equilibrium phase transition from the solid phase to the gaseous phase, bypassing the liquid state.
General concept
Sublimation in physics is the process of transition of a substance from a solid to a gaseous state, bypassing the liquid state. In another way it is called sublimation of the substance. This process is accompanied by the absorption of energy (in physics this energy is called “heat of sublimation”). The process is very important and has wide application in experimental physics.
Desublimation, on the contrary, is the process of transition of a substance from a gaseous to a solid state. Another name for this process is “deposition”. It is completely the opposite of sublimation. During deposition, energy is released rather than absorbed, and in very large quantities. Desublimation is also very important, but it is much more difficult to give an example of its targeted use by a person, especially in everyday life.
Sublimation (sublimation)
Details Category:
SUBLIMATION
, sublimation, the transition of a solid into a vapor state and back (from vapor to solid), bypassing the liquid phase. Sublimation is characteristic only of such solid “volatile” bodies, the vapors of which have significant pressure even at a temperature below the boiling point of these bodies. In technology, sublimation is used to clean solids from impurities and contaminants. In view of the fact that high temperature often (especially in the case of complex organic compounds) causes decomposition of the product, tarring and significant loss, the use of a relatively low temperature during sublimation preserves the product and increases its yield; to lower the temperature even further, the sublimation process is usually carried out under reduced pressure and, to speed up the process, often in an atmosphere of some indifferent gas, for example, nitrogen. The process of sublimation itself consists of heating the sublimated body to a temperature at which its vapors begin to be released; from this point on, maintain a constant temperature throughout the process; the released vapors are cooled, and they turn back into a solid product, but already cleared of impurities. Maintaining a constant temperature throughout the entire process is a very important condition for sublimation, since temperature fluctuations have a detrimental effect on the process: lowering it slows down sublimation, and increasing it often leads to decomposition of the product.
Therefore, heating, with very rare exceptions, is carried out not over bare fire, but with the help of some intermediate body (heat transmitter), which makes it easier to regulate the temperature; Such heat transmitters are bodies that do not change from an increase in temperature: sand, oil, heated air, low-melting alloys, cast iron shavings, a concentrated solution of magnesium chloride. In some cases, heating is carried out with steam. Single sublimation does not always immediately give a sufficiently pure product; in such cases they resort to secondary and third V.; each subsequent sublimation is carried out under different temperature conditions and often using other inert gases; That. fractional, or fractionated, sublimation is carried out, similar to the fractionated distillation of liquids. Fractional sublimation achieves a faster and more thorough separation of a mixture of solids.
The main parts of the sublimation apparatus are: 1) a cube in which the raw product is heated and converted into a vapor state, and 2) a chamber in which the vapor is cooled and converted into a solid. The solid mass loaded into the cube is heated unevenly: the particles adjacent to the heating surface of the cube are subject to the most intense heating, while the rest are heated relatively weakly, and, due to the fact that the solid mass b. h is a poor conductor of heat; uneven heating of the entire mass occurs, which is also associated with significant heat consumption. To avoid this, it is necessary to place the solid substance in a low layer in the cube; the cube is equipped with a stirrer, etc. With continuous stirring, a thin layer of the substance is heated quite evenly. The volume of the chamber significantly exceeds the volume of the cube. To speed up the process of turning steam into a solid, they resort to artificial cooling of the vapor by spraying the chamber with water or lowering its temperature with cooled salt brine or cooled air; The method of cooling depends on the ease with which condensation of products from vapor occurs.
The design of the apparatus depends on the physical and chemical properties of the sublimated body and on the ease with which this body undergoes decomposition, but regardless of this, all types of apparatus used in sublimation can be divided into two large groups: apparatus operating without reduced pressure (old type) , and devices operating under reduced pressure (with vacuum). The latter have currently acquired the greatest importance, because they require less time to purify the product and give a greater yield. In the past, the sublimation of salicylic acid was carried out in cylindrical boilers with a hemispherical bottom, lined with lead, and without a stirrer; as a result, the substance sintered into a thick lump that conducts heat poorly; while the upper parts of this lump did not have time to warm up to the required temperature, the lower parts overheated and decomposed, thereby reducing the yield of the product. The sublimation cube was connected to the cooling chamber by a series of narrow tubes, not insulated on the outside, which caused premature cooling of the vapors in these tubes, the release of solid matter and clogging of the tubes. The chamber itself was small in volume and had a vertical rather than horizontal position. As a result, the products of vapor condensation during cooling were layered on top of each other and did not form clearly visible crystals; the vertical position and insufficient dimensions of the chamber did not provide sufficient circulation of vapors inside the chamber for cooling; as a result, complete condensation was not achieved.
A modern apparatus for the sublimation of salicylic acid (Fig. 1) consists of a cube in
with a flat bottom, equipped with a stirrer
driven
by a transmission;
in the lid of this cube (also flat to prevent vortex movements of rising vapors) a hole is made, surrounded by a funnel (hole )
, through which the substance undergoing sublimation is loaded;
The bottom of the cube is surrounded by a metal jacket, which is filled with a substance that serves as a heat transmitter, so that the bottom of the cube does not come into direct contact with the flame anywhere. An inert gas is admitted into the cube through an annular tube r
.
To monitor and regulate the sublimation process, control instruments are placed in this ring tube: a thermometer to determine the temperature of the cube and a vacuum gauge to determine the degree of vacuum; A wide pipe leads from the cube into chamber d
, which has the shape of a horizontally elongated cylinder.
On the side of the chamber lying opposite the pipe connecting it to the cube, there is a door e
through which purified salicylic acid is removed from the chamber.
The side surface of the chamber is equipped with two square hermetically sealed windows for
monitoring the progress of condensation. Apparatuses for the sublimation of benzoic acid and camphor are constructed similarly. When purifying naphthalene, additional heating of the raw product with superheated water steam is used. Sulfur is purified in two ways: distillation (to obtain sulfur in pieces - cutting sulfur) and sublimation (to obtain sulfur in the form of dust - sulfur color). In practice, usually both cleaning methods are carried out together in the same apparatus; By adjusting the temperature of the process, they direct it towards distillation or sublimation as desired. Lumpy, unrefined sulfur is loaded into a cylindrical boiler heated by a naked fire, where the sulfur melts, heavy impurities settle to the bottom, and liquid sulfur flows through a pipe into a cast-iron horizontally located retort, equipped with its own firebox.
At a temperature not exceeding 144°, liquid sulfur evaporates; the vapor passes through the pipe from the retort into the chamber, where it cools and settles along the walls in the form of a sulfur color. The sulfur deposited on the walls is swept away several times during the process. If the temperature is raised above 144°, then the sulfur collects at the bottom of the chamber in liquid form and flows through the holes into the molds, from where, after cooling, the cutting sulfur is extracted. To purify ammonium chloride, sublimation is also used. The raw, unrefined mass is loaded into an iron cauldron with a flat bottom; the boiler (cube) is lined with refractory bricks from the bottom and sides, and the top is covered with a slightly convex lid made of iron or lead; the boiler is heated with bare fire. The moisture contained in the crude ammonium chloride is removed first, for which purpose, at the beginning of sublimation, a hole is opened in the lid of the cube. When all the water has evaporated and the first vapors of ammonium chloride appear, close the hole and continue heating. Be careful not to heat too much as this may cause organic matter to char and contaminate the product. The sublimation of ammonium chloride takes a long time: a load of half a ton is sublimated within five days. At the end of the process, remove the lid on which sublimated ammonium chloride has been deposited in a layer about 10 cm thick, and remove the pure product.
The sublimation of iodine presents some features. Unpurified iodine contains 10-25% impurities and water. To purify iodine, cast iron cauldrons of relatively small capacity with a lead lid are used; several such boilers communicate with one ceramic chamber; Ceramic cauldrons with polished lids are more often used; These boilers are heated in a sand bath. The load of each boiler is relatively small - about 10 kg. When heated, iodine sublimes and collects on the lid of the boiler in the form of leaves, which are cleaned from the lids from time to time.
Let us also point out the purification of white arsenic. Sublimation is carried out in an apparatus schematically shown in Fig. 2; white arsenic is loaded into a cast iron boiler
, the capacity of which is designed for 150 kg of crude arsenic anhydride.
The boiler is
equipped with a high, narrow cap
made
of sheet iron and is heated directly in the flame of a coal firebox.
Upon reaching the sublimation temperature, arsenic begins to sublimate and is deposited on the iron cap in the form of a transparent vitreous body - arsenic glass; the other part of the vapor passes through pipe b
into chamber
c
, where it is deposited on the walls in the form of a powder - purified arsenic anhydride.
Source: Martens. Technical encyclopedia. Volume 4 - 1928
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Process description
Almost anything can serve as catalysts for sublimation in physics. Sometimes substances sublimate (that’s what this process is called in physics) when they reach a certain temperature. As a rule, we are talking about temperatures above average, but there are some exceptions when substances “expand” at negative values.
Sometimes oxygen can be a catalyst for this process. In such cases, the substance will change into a gaseous substance upon contact with air. By the way, this technique is often used by directors in science fiction films. Great, isn't it?!
For desublimation, the catalysts are exactly the same, but you need to catch one pattern: all parameters, with the exception of some special chemical reactions, will have a negative sign. That is, if during sublimation the bulk of the processes occur at positive temperatures, then during deposition, on the contrary, low ones will appear.
It is also worth noting that the transition occurs sequentially. Each period of time has its own transition.
Many scientists even divide it into stages, but this need not be done. Let’s apply this to distillation and to its reverse process. This is what allows physicists to control the process and use it even in everyday life.
How to sublimate effectively and why it is useful
Sublimation is useful because it relieves the psyche, allows a person to take a broader look at the situation and find a safe and positive form of activity to splash out their feelings.
Small instructions:
- Relax. Life around you does not stop while you are focused on your condition.
- Try to dream. Go through all the options that come to mind. One day you will catch on to something and realize that this is what you want to strive for.
- Meditate. Deep meditation, when carried out correctly, will help you get to the bottom of your inner “I” and find out what this “I” wants to do and what solution to problems it will prefer.
- Fall in love. And remember your condition. With the same feeling of euphoria, you need to start fulfilling your chosen goal.
Examples
There are many examples of sublimation in physics, but there are also quite a lot of examples of the reverse process. Both categories are worth considering.
So, examples of fitting:
- Dry ice.
This is probably the most common example of the process. Surely everyone has seen or held it in their hands at least once. At one time, dry ice was an extremely popular subject for filming videos on YouTube. Almost every person has seen at least one such video. It is worth noting that ice is used not only for entertainment purposes. It also has quite wide application in everyday life. - Drying clothes in the cold.
Absolutely every housewife hangs out her laundry in the cold in winter. It would seem that it should return frozen, but it returns completely dry. This is due to the fact that sublimation of water molecules has occurred. This is the most obvious example of the use of sublimation in physics.
It's time to move on to deposition. It is advisable to consider examples:
- Frost.
This is the most obvious example of desublimation in nature, which absolutely everyone has associated with. The process occurs when there is an extremely sharp cooling and the dew point passes too quickly. This phenomenon is widespread. You can see frost in late autumn and winter. It is most clearly visible in October-November, when there is still very little snow. - Pattern on the windows in winter.
Yes, it turns out that it is desublimation that creates our New Year's atmosphere. Intricate patterns arise due to the huge difference between indoor and outdoor temperatures.
What is it for?
The process of sublimation, in addition to the fact that it is often found in nature, is widespread in everyday matters. This is due to convenience, as well as the low toxicity of things prone to this process. So, here are some examples of its use in everyday life:
- Drying clothes. As mentioned above, water molecules simply erode, bypassing one of the states of aggregation. This drying method still remains almost the most popular.
- Color printers. Solid colored paint particles immediately transform into a gaseous state under the influence of pressure and temperature. Despite the fact that this method is a thing of the past, in some places it is common in our time.
- Moth repellents and aromatic leaves. It is not uncommon to find one of these items in your closet. Such plates do not simply dissolve, as many people think, but turn into a gas state and transfer odor.
In addition, sublimation is widely used in a variety of physical experiments. It is interesting to note that in chemistry the ability of substances to sublimate is often the fundamental reason for a qualitative reaction.
Crystal-gas transition diagram
$p=BT^{frac{3}{2}}a^{-frac{alpha{}w_0}{kT}} (3),$ where:
- $w_0$ – sublimation energy;
- $B$ is a constant characterizing the substance.
Here, point $B (T=Θ)$ located on the graph (Fig. 1) corresponds to a two-phase state - saturated vapor located above the crystalline substance. The steam is in a state of dynamic equilibrium with the solid phase.
Figure 1. Graph. Author24 - online exchange of student work
As the temperature increases, the crystal becomes a gas. An increase in pressure causes the gas to desublimate and enter the solid phase. The crystal-gas phase transition diagram is similar to the liquid-gas transition diagram. Points below and to the right of the curve (which correspond to lower pressure and higher temperature) indicate that the substance is in the gas state.
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Where else does this term appear?
The term “sublimation” can be found not only in physics and chemistry. It is also relevant in psychology. In this science, its interpretation is completely different: it is a way to “let off steam” by radically changing your type of activity.
The term is also used in the printing industry. In this field of activity, the definition changes: sublimation printing is one of the ways to transfer an image to any surface using paint that goes through a sublimation process. Simply put, it is one of the ways to print on any surface.
