Diffusion

An example of diffusion is the mixing of gases (for example, the spread of odors) or liquids (if you drop ink into water, the liquid will become uniformly colored after a while). Another example is connected with a solid body: the atoms of adjoining metals are mixed at the contact boundary. Particle diffusion plays an important role in plasma physics.

Usually, diffusion is understood as processes accompanied by the transfer of matter, however, sometimes other transfer processes are also called diffusion: thermal conductivity, viscous friction, etc.

The diffusion rate depends on many factors. So, in the case of a metal rod, thermal diffusion takes place very quickly. If the rod is made of synthetic material, thermal diffusion proceeds slowly. Diffusion of molecules in the general case proceeds even more slowly. For example, if a piece of sugar is lowered to the bottom of a glass of water and the water is not stirred, it will take several weeks before the solution becomes homogeneous. Even slower is the diffusion of one solid into another. For example, if copper is coated with gold, then diffusion of gold into copper will occur, but under normal conditions (room temperature and atmospheric pressure), the gold-bearing layer will reach a thickness of several microns only after several thousand years.

A quantitative description of diffusion processes was given by the German physiologist A. Fick ( English) in 1855

general description

All types of diffusion obey the same laws. The diffusion rate is proportional to the cross-sectional area of ​​the sample, as well as the difference in concentrations, temperatures or charges (in the case of relatively small values ​​of these parameters). Thus, heat will travel four times faster through a rod two centimeters in diameter than through a rod one centimeter in diameter. This heat will spread faster if the temperature difference per centimeter is 10°C instead of 5°C. The diffusion rate is also proportional to the parameter characterizing a specific material. In case of thermal diffusion this parameter is called thermal conductivity, in case of flow electric charges- electrical conductivity. The amount of a substance that diffuses during a certain time and the distance traveled by the diffusing substance are proportional square root diffusion time.

Diffusion is a process molecular level and is determined by the random nature of the movement of individual molecules. The diffusion rate is therefore proportional to the average velocity of the molecules. In the case of gases average speed there are more small molecules, namely, it is inversely proportional to the square root of the mass of the molecule and grows with increasing temperature. Diffusion processes in solids at high temperatures often find practical application. For example, certain types of cathode ray tubes (CRTs) use metallic thorium diffused through metallic tungsten at 2000°C.

If in a mixture of gases the mass of one molecule is four times greater than the other, then such a molecule moves twice as slowly compared to its movement in a pure gas. Accordingly, its diffusion rate is also lower. This difference in diffusion rates between light and heavy molecules is used to separate substances with different molecular weights. An example is isotope separation. If a gas containing two isotopes is passed through a porous membrane, the lighter isotopes penetrate the membrane faster than the heavier ones. For better separation, the process is carried out in several stages. This process has been widely used for the separation of uranium isotopes (separation of 235 U from the bulk of 238 U). Since this separation method is energy intensive, other, more economical separation methods have been developed. For example, the use of thermal diffusion in a gaseous medium is widely developed. A gas containing a mixture of isotopes is placed in a chamber in which a spatial temperature difference (gradient) is maintained. In this case, heavy isotopes are concentrated over time in the cold region.

Fick's equations

From the point of view of thermodynamics, the driving potential of any leveling process is the growth of entropy. At constant pressure and temperature, the role of such a potential is played by the chemical potential µ , causing the maintenance of the flow of matter. The flux of substance particles is proportional to the potential gradient

~

In most practical cases, the concentration is used instead of the chemical potential C. Direct Replacement µ on the C becomes incorrect in the case of high concentrations, since the chemical potential ceases to be related to the concentration according to the logarithmic law. If we do not consider such cases, then the above formula can be replaced by the following:

which shows that the flux density of matter J proportional to diffusion coefficient D[()] and the concentration gradient. This equation expresses Fick's first law. Fick's second law relates spatial and temporal changes in concentration (diffusion equation):

Diffusion coefficient D temperature dependent. In a number of cases, in a wide temperature range, this dependence is the Arrhenius equation.

An additional field applied parallel to the chemical potential gradient violates steady state. In this case, diffusion processes are described by the non-linear Fokker-Planck equation. Diffusion processes are of great importance in nature:

• Nutrition, respiration of animals and plants;
• The penetration of oxygen from the blood into human tissues.

Geometric description of the Fick equation

In the second Fick equation, on the left side is the rate of change of concentration over time, and on the right side of the equation is the second partial derivative, which expresses the spatial distribution of concentration, in particular, the convexity of the temperature distribution function projected onto the x-axis.

see also

• Surface diffusion is a process associated with the movement of particles occurring on the surface of a condensed body within the first surface layer of atoms (molecules) or over this layer.

Notes

Literature

• Bokshtein B.S. Atoms wander through the crystal. - M .: Nauka, 1984. - 208 p. - (Library "Quantum", Issue 28). - 150,000 copies.

Links

• Diffusion (video lesson, 7th grade program)
• Diffusion of impurity atoms on the surface of a single crystal

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Synonyms:

See what "Diffusion" is in other dictionaries:

- [lat. diffusio distribution, spreading] physical, chemical. the penetration of molecules of one substance (gas, liquid, solid) into another upon their direct contact or through a porous partition. Dictionary foreign words. Komlev N.G.,… … Dictionary of foreign words of the Russian language

Diffusion- is the penetration into the medium of particles of one substance of particles of another substance, which occurs as a result of thermal motion in the direction of decreasing the concentration of another substance. [Blum E.E. Dictionary of basic metallurgical terms. Yekaterinburg … Encyclopedia of terms, definitions and explanations of building materials

Modern Encyclopedia

- (from Latin diffusio spreading spreading, dispersion), the movement of particles of the medium, leading to the transfer of matter and the alignment of concentrations or to the establishment of an equilibrium distribution of concentrations of particles of a given type in the medium. In the absence of… … Big Encyclopedic Dictionary

DIFFUSION, the movement of a substance in a mixture from an area of ​​high concentration to an area of ​​low concentration, caused by the random movement of individual atoms or molecules. Diffusion stops when the concentration gradient disappears. Speed… … Scientific and technical encyclopedic dictionary

diffusion- and, well. diffusion f., German. Diffusion lat. diffusio spreading, spreading. Mutual penetration of adjoining substances into each other due to the thermal movement of molecules and atoms. Diffusion of gases, liquids. BAS 2. || trans. They are… … Historical dictionary gallicisms of the Russian language

Diffusion- (from the Latin diffusio distribution, spreading, dispersion), the movement of particles of the medium, leading to the transfer of matter and the alignment of concentrations or the establishment of their equilibrium distribution. Diffusion is usually determined by thermal motion ... ... Illustrated Encyclopedic Dictionary

The movement of particles in the direction of decreasing their concentration, due to thermal motion. D. leads to the alignment of the concentrations of the diffusing substance and the uniform filling of the volume with particles. ... ... Geological Encyclopedia

Introduction
1. The concept and patterns of diffusion
1.1 The concept of diffusion processes ……………………………………….. 5
1.2 Patterns of diffusion ………………………………………6
2. Use of diffusion processes
2.1 Diffusion in the processing of metals ……………………………………………8
2.2 Plasmolysis…………………………………………………………………… 11
2.3 Osmosis……………………………………………………………………………11
3. Application of diffusion in production…………………………………... 13
4. Application of diffusion in medicine. Apparatus "artificial kidney"…..15
5. Application of diffusion in engineering……………………………………………...16
Conclusion
List of used literature

Introduction

The topic of my term paper is: "Diffusion processes and their use in technology".

Diffusion is a fundamental phenomenon of nature. It underlies the transformations of matter and energy. Its manifestations take place at all levels of organization of natural systems on our planet, starting from the level of elementary particles, atoms and molecules, and ending with the geosphere. It is widely used in technology, in everyday life.
The essence of diffusion is the movement of particles of the medium, leading to the transfer of substances and the equalization of concentrations or to the establishment of an equilibrium distribution of particles of a given type in the medium. Diffusion of molecules and atoms due to their thermal motion. .
The diffusion process is one of the mechanisms for the manifestation of the second law of thermodynamics, according to which any system tends to move into a more equilibrium state, that is, a stable state characterized by an increase in entropy and a minimum of energy.
Diffusion is one of the most important technological processes in the manufacture of all types of electronic devices and microcircuits.

Diffusion is a fundamental process that underlies the functioning of living systems at any level of organization, from the level of elementary particles (electronic diffusion) to the biospheric level (circulation of substances in the biosphere).

The phenomenon of diffusion is widely used in practice. In everyday life - tea brewing, canning vegetables, making jams. In production - carburizing (... of steel parts, to increase their hardness and heat resistance), aluminizing and oxidation processes.

The purpose of this course work is to get acquainted with the concept of diffusion and diffusion processes, to analyze its use in production, technology, medicine. Taking into account the specifics of this topic and the range of issues raised, the structure of the work makes it possible to consistently answer theoretical questions in the first part, and in the second to learn the practical use of diffusion processes.

1. The concept and patterns of diffusion

1.1 Concept of diffusion processes

The process of penetration of particles (molecules, atoms, ions) of one substance between the particles of another substance due to chaotic motion is called diffusion. Thus, diffusion is the result of the chaotic movement of all particles of matter, of any mechanical action.

Since particles move in gases, and in liquids, and in solids, then diffusion is possible in these substances. Diffusion is the transfer of matter due to the spontaneous alignment of an inhomogeneous concentration of atoms or molecules of various types. If portions of various gases are let into the vessel, then after a while all gases are evenly mixed: the number of molecules of each type per unit volume of the vessel will become constant, the concentration will even out (Fig. 1)

Diffusion is explained as follows. First, between the two bodies, the interface between the two media is clearly visible (Fig. 1a). Then, due to their movement, individual particles of substances located near the boundary exchange places. The boundary between substances blurs (Fig. 1b). Having penetrated between the particles of another substance, the particles of the first begin to exchange places with the particles of the second, which are in ever deeper layers. The interface between substances becomes even more vague. Due to the continuous and random movement of particles, this process eventually leads to the fact that the solution in the vessel becomes homogeneous (Fig. 1c).

Fig.1. Explanation of the phenomenon of diffusion.

Diffusion of large particles suspended in a gas or liquid (for example, particles of Smoke or Suspension) is carried out due to their Brownian motion. In what follows, unless otherwise stated, we mean molecular diffusion.

Diffusion plays an important role in chemical kinetics and technology. When a chemical reaction occurs on the surface of a catalyst or one of the reactants (for example, coal combustion), diffusion can determine the rate of supply of other reactants and removal of reaction products, that is, it can be a determining (limiting) process. For evaporation and condensation, dissolution of crystals and crystallization, diffusion usually turns out to be decisive. The process of diffusion of gases through porous partitions or into a steam jet is used for isotope separation. Diffusion underlies numerous technological processes - adsorption, cementation, etc. Diffusion welding, diffusion metallization are widely used.

In liquid solutions, the diffusion of solvent molecules through semipermeable partitions (membranes) leads to the appearance of osmotic pressure, which is used in the physicochemical method of separation of substances.

1.2 Patterns of diffusion

The concentration difference is the driving force of diffusion. If the concentration is the same everywhere, there is no diffuse transfer of matter. The concentration equalization as a result of diffusion occurs only in the absence of external forces. If a concentration difference exists along with a temperature difference, in an electric field or under conditions where gravity is significant (with a large height difference), concentration equalization is not necessary. An example is the decrease in air density with height.

Let's turn to experience. Two glasses are filled with water, but one is cold and the other is hot. Dip the tea bags into the glasses at the same time. It is easy to see that in hot water, tea colors the water faster, diffusion proceeds faster. The diffusion rate increases with increasing temperature, since the molecules of interacting bodies begin to move faster.

Diffusion occurs most rapidly in gases, slower in liquids, and even slower in solids, which is due to the nature of the thermal motion of particles in these media. The trajectory of each gas particle is a broken line, because When particles collide, they change direction and speed of their movement. The disorder of motion leads to the fact that each particle gradually moves away from the place where it was, and its displacement along a straight line is much less than the path traveled along a broken line. Therefore, diffusion penetration is much slower than free movement (the rate of diffusion propagation of odors, for example, is much less than the speed of molecules). In liquids, in accordance with the nature of the thermal motion of molecules, diffusion is carried out by jumps of molecules from one temporary equilibrium position to another. Each jump occurs when energy is imparted to the molecule sufficient to break its bonds with neighboring molecules and move into the environment of other molecules (to a new energetically favorable position). On average, the jump does not exceed the intermolecular distance. Diffusion motion of particles in a liquid can be considered as motion with friction. The diffusion coefficient in a liquid increases with temperature, which is due to the “loosening” of the liquid structure during heating and the corresponding increase in the number of jumps per unit time.

In a solid body, several mechanisms can operate: exchange of places of atoms with vacancies (unoccupied nodes of the crystal lattice), movement of atoms along interstices, simultaneous cyclic movement of several atoms, direct exchange of places of two neighboring atoms, etc. The first mechanism prevails, for example, in the formation of substitutional solid solutions, the second - interstitial solid solutions. An increase in the number of defects (mainly vacancies) facilitates the movement of atoms in a solid, diffusion, and leads to an increase in the diffusion coefficient. The diffusion coefficient in solids is characterized by a sharp (exponential) dependence on temperature. Thus, the coefficient of diffusion of zinc into copper increases by a factor of 1014 as the temperature rises from 20 to 300°C.

All experimental methods for determining the diffusion coefficient contain two main points: bringing diffusing substances into contact and analyzing the composition of substances changed by diffusion. The composition (concentration of the diffused substance) is determined chemically, optically (by changing the refractive index or absorption of light), mass spectroscopically, by the method of labeled atoms, etc.

2. Use of diffusion processes

2.1 Diffusion in metal processing

Diffusion metallization is the process of diffusion saturation of the surface of products with metals or metalloids. Diffusion saturation is carried out in a powder mixture, gaseous medium or molten metal (if the metal has a low melting point).

Boriding - diffusion saturation of the surface of metals and alloys with boron to increase hardness, corrosion resistance, wear resistance is carried out by electrolysis in molten boron salt. Boriding provides a particularly high surface hardness, wear resistance, increases corrosion resistance and heat resistance. Boron steels have high corrosion resistance in aqueous solutions of hydrochloric, sulfuric and phosphoric acids. Boriding is used for cast iron and steel parts operating under friction conditions in an aggressive environment (in chemical engineering).

Aluminizing is a process of diffusion saturation of the surface layer with aluminum, carried out in powdered mixtures of aluminum or in molten aluminum. The goal is to obtain high heat resistance of the surface of steel parts. Aluminizing is carried out in solid and liquid media.

Siliconization - diffusion saturation with silicon is carried out in a gaseous atmosphere. The silicon-saturated layer of the steel part has not very high hardness, but high corrosion resistance and increased wear resistance in sea ​​water, nitric, hydrochloric in sulfuric acids. Siliconized parts are used in the chemical, pulp and paper and oil industries. To increase the heat resistance, siliconizing is used for products made of alloys based on molybdenum and tungsten, which have high heat resistance.

Diffusion processes in metals play a significant role. If two metals are brought into close contact by a beam of deposition or pressing of the powder of one metal with another and are subjected to sufficiently high temperatures, then each of these two metals will diffuse into the other. If one of the metals is liquid, then it simultaneously diffuses into the solid and dissolves it.

If we proceed from pure metals, then a whole gamut of phases of both metals is formed in the intermediate layer, usually separately from the mixture of boundary phases. The difference in concentrations in the individual layers is very different; the diffusion rate is therefore highly dependent on the structure of the lattice. In the case of a continuous series of solid solutions, the diffusion rate also depends on the composition of the bulk; thus, the diffusion of copper into nickel with a high melting point is much slower than the diffusion of nickel into copper. In the same metal, other metals diffuse, as the experiments of Gevez and Septs with lead showed, with the greater speed, the farther they are in their groups in the periodic system from each other (the further they are in their valency). By using a radioactive isotope of lead, it can also be established that homogeneous atoms exchange places particularly slowly. The fact of this autodiffusion clearly indicates the movement of metal atoms at high temperatures, on which crystallization and crystal growth are also based.

Diffusion is (literally) scattering, spreading, spreading. Physically characterizes the process of energy or matter transfer from a highly concentrated area to an area with a reduced concentration. The most common phenomenon that diffusion is associated with is the mixing of gas molecules (for example, when the scent of a perfume spreads in the air) or liquids. The same process can also be observed in solids. For example, if the end of the rod is electrically charged or heated, then the heat (or charge) will gradually spread from the hot to the cold area. Moreover, if you take a metal object, then the heat will spread quickly enough, and electricity- instantly. If the rod is made of synthetic material, then thermal propagation will proceed slowly, and electrical propagation will be even slower. Diffusion of solids occurs at an even lower rate.

It should be noted that this term (like many others) is used today not only in physics.

There is, for example, such a thing as the diffusion of innovations. This is a process by which the transfer of innovation to business entities through communication channels is carried out in time. In this case, diffusion is information dissemination, the speed and form of which depend on the Great importance also have features of perception of information by subjects conducting economic activities, as well as their ability to practical application received information. With the diffusion of an innovation that has already been mastered once and applied in another area, the number of consumers and producers increases in new places and conditions. The continuity of the process forms the boundaries and forms of distribution of innovations in a market economy.

Experts point out that in the context of economic activity diffusion has a cyclic character. At the same time, the implementation of the entire process of implementation, dissemination of innovations is divided into certain stages: fundamental and applied research, development and design, construction, development, as well as industrial production, marketing and sales.

Cultural diffusion is a concept that is used in the social sphere. It characterizes the process of mutual penetration of certain features from one social group into another when they come into contact. At the same time, diffusion may not leave any imprint on any of the interacting cultures. It may, however, happen that this penetration provokes a strong and equal (or one-sided) influence. The channels through which diffusion takes place are mainly tourism, war, trade, scientific conferences, fairs and exhibitions, exchange of specialists and students.

The dissemination of innovations in the social sphere can be carried out in two directions: horizontally or vertically.

Horizontal penetration (intergroup diffusion) is noted between individuals, groups, equal in status.

Vertical spread occurs between subjects with unequal status. This process is called stratification diffusion.

It is characterized by a pronounced symbolic polarization of the population. According to a number of culturologists, the middle class is considered an example of lifestyle and style today (both for the upper and lower classes).

Brings both positive and negative traits to society. Thus, the dissemination of lofty ideas about life to the middle and (in particular) the lower strata means, on the one hand, the enlightenment and democratization of the people. On the other hand, high culture in this case can be perceived primitively and vulgarly.

Absolutely all people have heard about such a concept as diffusion. This was one of the topics in 7th grade physics lessons. Despite the fact that this phenomenon surrounds us absolutely everywhere, few people know about it. What does it mean anyway? What is its physical meaning And how can you make life easier with it? Today we will talk about this.

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Diffusion in physics: definition

This is the process of penetration of molecules of one substance between the molecules of another substance. In simple terms, this process can be called mixing. During this mixing occurs the mutual penetration of the molecules of a substance between each other. For example, when making coffee, instant coffee molecules penetrate water molecules and vice versa.

The speed of this physical process depends on the following factors:

1. Temperature.
2. Aggregate state of matter.
3. External influence.

The higher the temperature of a substance, the faster the molecules move. Consequently, mixing process occurs faster at higher temperatures.

Aggregate state of matter - the most important factor. In each state of aggregation, the molecules move at a certain speed.

Diffusion can proceed in the following states of aggregation:

1. Liquid.
2. Solid.

Most likely, the reader will now have the following questions:

1. What are the causes of diffusion?
2. Where does it flow faster?
3. How is it applied in real life?

The answers to them can be found below.

Causes

Absolutely everything in this world has its own reason. And diffusion is no exception. Physicists are well aware of the reasons for its occurrence. And how to convey them to the average person?

Surely everyone has heard that molecules are in constant motion. Moreover, this movement is disorderly and chaotic, and its speed is very high. Thanks to this movement and the constant collision of molecules, their mutual penetration occurs.

Is there any evidence for this movement? Of course! Remember how quickly you started to smell perfume or deodorant? And the smell of the food your mom cooks in the kitchen? Remember how fast preparing tea or coffee. All this could not be, if not for the movement of molecules. We conclude that the main reason for diffusion is the constant movement of molecules.

Now only one question remains - what is the reason for this movement? It is driven by the desire for balance. That is, in the substance there are areas with high and low concentrations of these particles. And because of this desire, they are constantly moving from an area of ​​high concentration to a low concentration. They are constantly collide with each other, and interpenetration occurs.

Diffusion in gases

The process of mixing particles in gases is the fastest. It can occur both between homogeneous gases and between gases with different concentrations.

Vivid examples from life:

1. You smell the air freshener through diffusion.
2. You smell the cooked food. Note that you begin to feel it immediately, and the smell of the freshener after a few seconds. This is due to the fact that at high temperatures the speed of movement of molecules is greater.
3. Tears that arise when you cut onions. Onion molecules mix with air molecules, and your eyes react to this.

How does diffusion occur in liquids?

Diffusion in liquids proceeds more slowly. It can last from several minutes to several hours.

The brightest examples from life:

1. Preparation of tea or coffee.
2. Mixing water and potassium permanganate.
3. Preparing a solution of salt or soda.

In these cases, diffusion proceeds very quickly (up to 10 minutes). However, if an external influence is applied to the process, for example, stirring these solutions with a spoon, then the process will go much faster and take no more than one minute.

Diffusion when mixing thicker liquids will take much longer. For example, mixing two liquid metals can take several hours. Of course, you can do this in a few minutes, but in this case it will turn out poor quality alloy.

For example, diffusion when mixing mayonnaise and sour cream will take a very long time. However, if you resort to the help of external influence, then this process will not take even a minute.

Diffusion in solids: examples

In solids, the mutual penetration of particles proceeds very slowly. This process may take several years. Its duration depends on the composition of the substance and the structure of its crystal lattice.

Experiments proving that diffusion in solids exists.

1. Sticking of two plates of different metals. If you keep these two plates close to each other and under pressure, within five years there will be a layer between them having a width of 1 millimeter. This small layer will contain molecules of both metals. These two plates will be merged together.
2. A very thin layer of gold is applied to a thin lead cylinder. After that, this design is placed in an oven for 10 days. The air temperature in the furnace is 200 degrees Celsius. After this cylinder was cut into thin disks, it was very clearly seen that the lead penetrated into the gold and vice versa.

Examples of diffusion in the surrounding world

As you already understood, the harder the medium, the lower the rate of mixing of molecules. Now let's talk about where in real life you can get practical benefits from this. physical phenomenon.

The process of diffusion occurs in our life all the time. Even when we lie on the bed, a very thin layer of our skin remains on the surface of the sheet. It also absorbs sweat. It is because of this that the bed becomes dirty and needs to be changed.

So, the manifestation of this process in everyday life can be as follows:

1. When spreading butter on bread, it is absorbed into it.
2. When pickling cucumbers, salt first diffuses with water, after which salty water begins to diffuse with cucumbers. As a result, we get a delicious snack. Banks need to be rolled up. This is necessary so that the water does not evaporate. More precisely, water molecules should not diffuse with air molecules.
3. When washing dishes, the molecules of water and detergent penetrate the molecules of the remaining pieces of food. This helps them come off the plate and make it cleaner.

Manifestation of diffusion in nature:

1. The process of fertilization occurs precisely due to this physical phenomenon. The molecules of the egg and sperm diffuse, after which the embryo appears.
2. Soil fertilization. Through the use of certain chemicals or compost, the soil becomes more fertile. Why is this happening? The bottom line is that fertilizer molecules diffuse with soil molecules. After that, the diffusion process occurs between the molecules of the soil and the root of the plant. Thanks to this, the season will be more fruitful.
3. Mixing industrial waste with air greatly pollutes it. Because of this, within a radius of a kilometer, the air becomes very dirty. Its molecules diffuse with clean air molecules from neighboring areas. This is how the ecological situation in the city worsens.

The manifestation of this process in industry:

1. Siliconization is a process of diffusion saturation with silicon. It is carried out in a gaseous atmosphere. The silicon-saturated layer of the part has not very high hardness, but high corrosion resistance and increased wear resistance in sea water, nitric, hydrochloric and sulfuric acids.
2. Diffusion in metals plays an important role in the production of alloys. To obtain a high-quality alloy, it is necessary to produce alloys at high temperatures and with external influence. This will greatly speed up the diffusion process.

These processes occur in various industries:

1. Electronic.
2. Semiconductor.
3. Engineering.

As you understand, the process of diffusion can have both positive and negative effects on our lives. You need to be able to manage your life and maximize the benefits of this physical phenomenon, as well as minimize harm.

Now you know what is the essence of such a physical phenomenon as diffusion. It consists in the mutual penetration of particles due to their movement. Everything in life moves. If you are a student, then after reading our article you will definitely get a grade of 5. Good luck to you!

Among the numerous phenomena in physics, the diffusion process is one of the simplest and most understandable. After all, every morning, preparing himself fragrant tea or coffee, a person has the opportunity to observe this reaction in practice. Let's learn more about this process and the conditions for its occurrence in different states of aggregation.

What is diffusion

This word refers to the penetration of molecules or atoms of one substance between similar structural units of another. In this case, the concentration of penetrating compounds is leveled.

This process was first described in detail by the German scientist Adolf Fick in 1855.

The name of this term was derived from the Latin diffusio (interaction, dispersion, distribution).

Diffusion in liquid

The process under consideration can occur with substances in all three states of aggregation: gaseous, liquid and solid. To find practical examples this, you just have to look into the kitchen.

Stove-boiled borscht is one of them. Under the influence of temperature, the molecules of glucosin betanin (a substance due to which the beets have such a rich scarlet color) evenly react with water molecules, giving it a unique burgundy hue. This case is in liquids.

In addition to borscht, this process can also be seen in a glass of tea or coffee. Both of these drinks have such a uniform rich shade due to the fact that tea leaves or coffee particles, dissolving in water, evenly spread between its molecules, coloring it. The action of all popular instant drinks of the nineties is built on the same principle: Yupi, Invite, Zuko.

Interpenetration of gases

Odor-carrying atoms and molecules are in active motion and, as a result, are mixed with particles already in the air, and are fairly evenly dispersed throughout the volume of the room.

This is a manifestation of diffusion in gases. It is worth noting that the very inhalation of air also belongs to the process under consideration, as well as the appetizing smell of freshly prepared borscht in the kitchen.

Diffusion in solids

The kitchen table, on which the flowers stand, is covered with a bright tablecloth. yellow color. She received a similar shade due to the ability of diffusion to take place in solids.

The process of giving the canvas some uniform shade takes place in several stages as follows.

1. Particles of yellow pigment diffused in the dye tank towards the fibrous material.
2. Then they were absorbed by the outer surface of the dyed fabric.
3. The next step was again the diffusion of the dye, but this time into the fibers of the fabric.
4. In the final, the fabric fixed the pigment particles, thus becoming colored.

Diffusion of gases in metals

Usually, speaking about this process, consider the interaction of substances in the same aggregate states. For example, diffusion in solids, solids. To prove this phenomenon, an experiment is carried out with two metal plates pressed against each other (gold and lead). The interpenetration of their molecules takes quite a long time (one millimeter in five years). This process is used to make unusual jewelry.

However, compounds in different aggregate states are also capable of diffusing. For example, there is diffusion of gases in solids.

During the experiments, it was proved that a similar process occurs in the atomic state. To activate it, as a rule, a significant increase in temperature and pressure is required.

An example of such gaseous diffusion in solids is hydrogen corrosion. It manifests itself in situations where, arising in the process of some chemical reaction hydrogen atoms (H 2) under the influence of high temperatures (from 200 to 650 degrees Celsius) penetrate between the structural particles of the metal.

In addition to hydrogen, diffusion of oxygen and other gases can also occur in solids. This process, imperceptible to the eye, brings a lot of harm, because metal structures can collapse because of it.

Diffusion of liquids in metals

However, not only gas molecules can penetrate into solids, but also liquids. As in the case of hydrogen, most often this process leads to corrosion (if we are talking about metals).

A classic example of liquid diffusion in solids is the corrosion of metals under the influence of water (H 2 O) or electrolyte solutions. For most, this process is more familiar under the name of rusting. Unlike hydrogen corrosion, in practice it has to be encountered much more often.

Conditions for accelerating diffusion. Diffusion coefficient

Having dealt with the substances in which the process under consideration can occur, it is worth learning about the conditions for its occurrence.

First of all, the rate of diffusion depends on the state of aggregation of the interacting substances. The more in which a reaction occurs, the slower its rate.

In this regard, diffusion in liquids and gases will always be more active than in solids.

For example, if crystals of potassium permanganate KMnO 4 (potassium permanganate) are thrown into water, they will give it a beautiful crimson color within a few minutes. However, if you sprinkle KMnO 4 crystals on a piece of ice and put it all in the freezer, after a few hours, potassium permanganate will not be able to fully color the frozen H 2 O.

From the previous example, one more conclusion can be drawn about the conditions of diffusion. Apart from state of aggregation, the temperature also affects the rate of interpenetration of particles.

To consider the dependence of the process under consideration on it, it is worth learning about such a concept as the diffusion coefficient. This is the name of the quantitative characteristic of its speed.

In most formulas, it is denoted using the capital Latin letter D and in the SI system it is measured in square meters per second (m² / s), sometimes in centimeters per second (cm 2 / m).

The diffusion coefficient is equal to the amount of matter scattered through a unit surface over a unit of time, provided that the difference in densities on both surfaces (located at a distance equal to a unit length) is equal to one. The criteria that determine D are the properties of the substance in which the particle scattering process itself takes place, and their type.

The dependence of the coefficient on temperature can be described using the Arrhenius equation: D = D 0exp (-E/TR).

In the considered formula, E is the minimum energy required to activate the process; T - temperature (measured in Kelvin, not Celsius); R is the gas constant characteristic of an ideal gas.

In addition to all of the above, the diffusion rate in solids, liquids in gases is affected by pressure and radiation (induction or high-frequency). In addition, much depends on the presence of a catalytic substance; often it is it that acts as a trigger for the start of active dispersion of particles.

Diffusion equation

This phenomenon is a particular form of a differential equation with partial derivatives.

Its goal is to find the dependence of the concentration of a substance on the size and coordinates of the space (in which it diffuses), as well as time. In this case, the given coefficient characterizes the permeability of the medium for the reaction.

Most often, the diffusion equation is written as follows: ∂φ (r,t)/∂t = ∇ x .

In it φ (t and r) is the density of the scattering material at point r at time t. D (φ, r) is the generalized diffusion coefficient at the density φ at the point r.

∇ is a vector differential operator whose coordinate components are partial derivatives.

When the diffusion coefficient is density dependent, the equation is non-linear. When not - linear.

Having considered the definition of diffusion and the features of this process in different environments, it can be noted that it has both positive and negative sides.