Absolutely all people have heard about such a concept as diffusion. This was one of the topics in physics lessons in the 7th grade. Despite the fact that this phenomenon surrounds us absolutely everywhere, few people know about it. What does it mean anyway? What is it physical meaning, and how can you make life easier with its help? 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 molecules of another substance. In simple terms, this process can be called mixing. During this mixing occurs the mutual penetration of 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 a substance.
3. External influence.

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

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

Diffusion can occur 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 happen 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 understand perfectly well the reasons for its occurrence. How can we convey them to the average person?

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

Is there any evidence of this movement? Certainly! Remember how quickly you began to smell perfume or deodorant? And the smell of the food your mother is preparing in the kitchen? Remember how quickly preparing tea or coffee. All this could not have happened 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 caused this movement? It is driven by the desire for balance. That is, in a substance there are areas with high and low concentrations of these particles. And thanks to this desire, they constantly move from an area of ​​high concentration to a low concentration. They are constantly collide with each other, and mutual penetration 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 food being cooked. Note that you begin to feel it immediately, but the smell of the freshener after a few seconds. This is explained by the fact that at high temperatures the speed of movement of molecules is greater.
3. The tears you get when chopping onions. The onion molecules mix with air molecules, and your eyes react to this.

How does diffusion occur in liquids?

Diffusion in liquids is slower. It can last from a few minutes to several hours.

The most striking examples from life:

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

In these cases, diffusion occurs 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 will 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 work low 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 occurs 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. Adhesion 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 1 millimeter wide. This small layer will contain molecules of both metals. These two plates will be fused together.
2. A very thin layer of gold is applied to a thin lead cylinder. After which this structure is placed in an oven for 10 days. The air temperature in the oven is 200 degrees Celsius. After this cylinder was cut into thin discs, it was very clearly visible that the lead had penetrated into the gold and vice versa.

Examples of diffusion in the environment

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 lives constantly. 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 you spread butter on bread, it gets absorbed into it.
2. When pickling cucumbers, the salt first diffuses with the water, after which the salt water begins to diffuse with the cucumbers. As a result, we get a delicious snack. Banks need to be rolled up. This is necessary to ensure that the water does not evaporate. More precisely, water molecules should not diffuse with air molecules.
3. When washing dishes, molecules of water and detergent penetrate into 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. By using certain chemicals or compost, the soil becomes more fertile. Why is this happening? The idea is that fertilizer molecules diffuse with soil molecules. After which the process of diffusion occurs between the molecules of the soil and the plant root. Thanks to this, the season will be more productive.
3. Mixing industrial waste with air greatly pollutes it. Because of this, the air within a kilometer radius becomes very dirty. Its molecules diffuse with molecules of clean air from neighboring areas. This is how the environmental situation in the city is deteriorating.

Manifestation of this process in industry:

1. Siliconization is the process of diffusion saturation with silicon. It is carried out in a gas atmosphere. The silicon-saturated layer of the part does not have very high hardness, but has 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 manufacture of alloys. To obtain a high-quality alloy, it is necessary to produce alloys at high temperatures and with external influences. This will significantly speed up the diffusion process.

These processes occur in various industries:

1. Electronic.
2. Semiconductor.
3. Mechanical 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 the harm.

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

Lesson objectives:

Educational: consolidate students’ knowledge on a given topic, teach them to understand and describe the behavior of molecules of a substance in various states of aggregation, explain the significance of the diffusion process in nature and human life.

Educational: continue to develop students’ ability to think scientifically.

Educational: to instill in students the ability to compare phenomena seen in nature with acquired knowledge about various physical laws.

Key terms:

State of matter is a state of matter that can be characterized by a set of certain properties (for example, preservation or inability to preserve volume, shape, etc.).

Diffusion

The concept of the state of aggregation of matter.

The world around us is complex and changeable. At the same time, we are able to notice that the limitless diversity of the world is not so limitless after all. We often see the same substances in different states.

The simplest example by which I can prove the veracity of my words is water. It is easiest to see in different states - it is steam or fog, it is ice or snow, it is liquid running from the tap in the kitchen. Whatever the characteristics of water in one form or another, it always remains water - its composition does not change. These are the same 2 hydrogen molecules and 1 oxygen molecule.

If we continue to use the example we took, we can see that these 3 states of water depend on certain external conditions. Thus, water freezes at 0 degrees, turning into ice, and water boils at 100 degrees, turning into steam. This photo clearly demonstrates all 3 states of water:

Rice. 1: 3 physical states of water

So, what conclusions can we draw after thinking carefully about the example we have given? They will be like this:

The state of aggregation of a substance is a state of a substance that can be characterized by a set of certain properties (for example, preservation or inability to preserve volume, shape, etc.) under certain conditions.

Not only water can be in three states of aggregation: solid, liquid and gaseous. This is inherent in all substances.

Sometimes, to the three above states of aggregation, a fourth is added – plasma. You can get an idea of ​​what plasma looks like from the following figure:

Rice. 2: plasma lamp

but you will learn about plasma in more detail in physics and chemistry lessons in high school.

Diffusion process

As we all have already learned, all substances consist of tiny particles - ions, atoms, molecules, which are in constant motion. It is this movement that causes the process of diffusion to occur.

Diffusion is a process involving the mutual penetration of molecules of substances into the spaces between molecules in other substances.

Let's take a closer look at diffusion in various states of aggregation.

Diffusion in gases

Let's give examples of the process of diffusion in gases together. Variants of manifestation of this phenomenon may be as follows:

Spreading the scent of flowers;

Tears over chopping onions;

A trail of perfume that can be felt in the air.

The gaps between particles in the air are quite large, the particles move chaotically, so the diffusion of gaseous substances occurs quite quickly.

Let's watch a video demonstrating this process:

Diffusion in liquids.

Particles of substances in liquids, and these are most often ions of substances, interact with each other quite strongly. At the same time, the distance between the ions is quite large, which allows the particles to mix easily.

The following video picture shows how the diffusion process occurs in liquids. Paint particles, falling on the surface of the water, easily diffuse, that is, penetrate into the water.

Rice. 3: Paint particles spread in the water.

You can observe the same process, but in dynamics, in the video using the example of the dissolution of potassium permanganate crystals:

Diffusion in solids.

Solids can have different structures and consist of molecules, atoms or ions. In any case, regardless of what microparticles the body consists of, the interaction of these particles with each other is very strong. Despite the fact that they, these particles, still move, these movements are very insignificant. The spaces between the particles are small, making it difficult for other substances to penetrate between them. The process of diffusion in solids is very slow and invisible to the naked eye.

Let's watch a video about it:

Having learned about the peculiarities of the diffusion process in various states of aggregation, we saw that the process is not equally fast. What does the rate of diffusion depend on? We already have one answer to this question - the rate of the diffusion process depends on the state of aggregation of the substance.

You and I also know that particles of substances begin to move faster with increasing temperature. Does this mean that the diffusion process will also accelerate with increasing temperature? The answer is obvious. To confirm, let's watch the video:

The intensity of the diffusion of one substance into another also depends on the concentration of these substances and on external influences (for example, if you simply drop a solution of iodine into water and if you also mix it, the rate at which the solution acquires a uniform color will be different).

conclusions

1. The state of aggregation of a substance is a state of a substance that can be characterized by a set of certain properties (for example, preservation or inability to preserve volume, shape, etc.) under certain conditions. Not only water can be in three states of aggregation: solid, liquid and gaseous. This is inherent in all substances.

2. Diffusion is a process consisting in the mutual penetration of molecules of substances into the spaces between molecules in other substances.

3. The rate of diffusion depends on: temperature, concentration, external influences, and the state of aggregation of the substance.

It is difficult to overestimate the process of diffusion in human life. For example, the penetration of oxygen through the thinnest wall of the alveoli into the capillaries of the lungs occurs precisely due to diffusion. The walls of the alveoli are very thin; from a physical point of view, the alveolar wall is a semi-permeable membrane. The concentration of oxygen in atmospheric air is much higher than its concentration and capillary blood, which is why oxygen penetrates through the semi-permeable membrane - where there is less of it. Thanks to diffusion we breathe.

This process also partially ensures the penetration of nutrients from the digestive system into the blood and the effect of many medications.

The figure schematically shows how nutrients are absorbed in the human intestine.

Rice. 4: small intestine of a mammal

Bibliography

Lesson on the topic: “Diffusion in gases, liquids, solids”, author Selezneva A.M., Municipal Educational Institution Secondary School No. 7, Boyarka, Kyiv region.

Peryshkin A.V. “Physics 7th grade”, Moscow, Bustard, 2006

Rodina N. A., Gromov S. V., “Physics”, M., Mir, 2002

Edited and sent by Borisenko I.N..

Worked on the lesson:

Municipal educational institution Zaozernaya secondary school with in-depth study of individual subjects No. 16

Topic: “Diffusion in living and inanimate nature.”

Completed:

student of class 8A Zyabrev Kirill.

Physics teacher: Zavyalova G.M.

Biology teacher: Zyabreva V.F.

Tomsk – 2008

I. Introduction. ………………………………………………………… 3

II. Diffusion in living and inanimate nature.

1. History of the discovery of the phenomenon. ……………………………………. 4

2. Diffusion, its types. ………………………………………….. 6

3. What does the rate of diffusion depend on? ……………………….. 7

4. Diffusion in inanimate nature. ……………………………... 8

5. Diffusion in living nature. ………………………………… 9

6. Use of diffusion phenomena. …………………………. 16

7. Design of individual diffusion phenomena. …………… 17

III. Conclusion. …………………………………………………... 20

IV. Used Books. ……………………………………. . 21

I. Introduction.

There are so many amazing and interesting things happening around us. Distant stars are shining in the night sky, a candle is burning in the window, the wind carries the aroma of blooming bird cherry trees, an aging grandmother follows you with her gaze…. I want to know a lot, try to explain it myself. After all, many natural phenomena are associated with diffusion processes, which we talked about recently at school. But they said so little!

Goals of work :

1. Expand and deepen knowledge about diffusion.

2. Model individual diffusion processes.

3. Create additional computer-based material for use in physics and biology lessons.

Tasks:

1. Find the necessary material in the literature, the Internet, study and analyze it.

2. Find out where diffusion phenomena occur in living and inanimate nature (physics and biology), what significance they have, and where they are used by humans.

3. Describe and design the most interesting experiments on this phenomenon.

4. Create animated models of some diffusion processes.

Methods: analysis and synthesis of literature, design, modeling.

My work consists of three parts; the main part consists of 7 chapters. I studied and processed materials from 13 literary sources, including educational, reference, scientific literature and Internet sites, and also prepared a presentation made in the Power Point editor.

II. Diffusion in living and inanimate nature.

II .1. The history of the discovery of the phenomenon of diffusion.

When observing a suspension of flower pollen in water under a microscope, Robert Brown observed a chaotic movement of particles arising “neither from the movement of the liquid nor from its evaporation.” Suspended particles 1 µm in size or less, visible only under a microscope, performed disordered independent movements, describing complex zigzag trajectories. Brownian motion does not weaken over time and does not depend on the chemical properties of the medium; its intensity increases with increasing temperature of the medium and with a decrease in its viscosity and particle size. Even a qualitative explanation of the causes of Brownian motion was possible only 50 years later, when the cause of Brownian motion began to be associated with impacts of liquid molecules on the surface of a particle suspended in it.

The first quantitative theory of Brownian motion was given by A. Einstein and M. Smoluchowski in 1905-06. based on molecular kinetic theory. It was shown that random walks of Brownian particles are associated with their participation in thermal motion along with the molecules of the medium in which they are suspended. Particles have on average the same kinetic energy, but due to their greater mass they have a lower speed. The theory of Brownian motion explains the random movements of a particle by the action of random forces from molecules and friction forces. According to this theory, the molecules of a liquid or gas are in constant thermal motion, and the impulses of different molecules are not the same in magnitude and direction. If the surface of a particle placed in such a medium is small, as is the case for a Brownian particle, then the impacts experienced by the particle from the molecules surrounding it will not be exactly compensated. Therefore, as a result of “bombardment” by molecules, a Brownian particle comes into random motion, changing the magnitude and direction of its speed approximately 1014 times per second. From this theory it followed that by measuring the displacement of a particle over a certain time and knowing its radius and the viscosity of the liquid, Avogadro’s number can be calculated.

The conclusions of the theory of Brownian motion were confirmed by measurements by J. Perrin and T. Svedberg in 1906. Based on these relationships, Boltzmann's constant and Avogadro's constant were experimentally determined. (Avogadro's constant denoted by NA, the number of molecules or atoms in 1 mole of a substance, NA=6.022.1023 mol-1; name in honor of A. Avogadro.

Boltzmann constant, physical constant k, equal to the ratio of the universal gas constant R to Avogadro's number N A: k = R / N A = 1.3807.10-23 J/K. Named after L. Boltzmann.)

When observing Brownian motion, the position of the particle is recorded at regular intervals. The shorter the time intervals, the more broken the trajectory of the particle will look.

The laws of Brownian motion serve as a clear confirmation of the fundamental principles of molecular kinetic theory. It was finally established that the thermal form of motion of matter is due to the chaotic movement of atoms or molecules that make up macroscopic bodies.

The theory of Brownian motion played an important role in the substantiation of statistical mechanics; the kinetic theory of coagulation (mixing) of aqueous solutions is based on it. In addition, it also has practical significance in metrology, since Brownian motion is considered as the main factor limiting the accuracy of measuring instruments. For example, the limit of accuracy of the readings of a mirror galvanometer is determined by the vibration of the mirror, like a Brownian particle bombarded by air molecules. The laws of Brownian motion determine the random movement of electrons, which causes noise in electrical circuits. Dielectric losses in dielectrics are explained by random movements of the dipole molecules that make up the dielectric. Random movements of ions in electrolyte solutions increase their electrical resistance.

Trajectories of Brownian particles (Perrin experiment scheme); The dots mark the positions of particles at equal time intervals.

Thus, DIFFUSION, OR BROWNIAN MOTION – This random movement of tiny particles suspended in a liquid or gas, occurring under the influence of impacts from environmental molecules; open

R. Brown in 1827

II. 2. Diffusion, its types.

A distinction is made between diffusion and self-diffusion.

Diffusion is the spontaneous penetration of molecules of one substance into the spaces between the molecules of another substance. In this case, the particles are mixed. Diffusion is observed for gases, liquids and solids. For example, a drop of ink is mixed in a glass of water. Or the smell of cologne spreads throughout the room.

Diffusion, like self-diffusion, exists as long as there is a density gradient of the substance. If the density of any one and the same substance is not the same in different parts of the volume, then the phenomenon of self-diffusion is observed. Self-diffusion called the process of density equalization(or concentration proportional to it) the same substance. Diffusion and self-diffusion occur due to the thermal movement of molecules, which, in nonequilibrium states, creates flows of matter.

Mass flux density is the mass of a substance ( dm), diffusing per unit time through a unit area ( dS pl), perpendicular to the axis x :

(1.1)

The phenomenon of diffusion obeys Fick's law

(1.2)

where is the modulus of the density gradient, which determines the rate of change in density in the direction of the axis X ;

D- diffusion coefficient, which is calculated from molecular kinetic theory using the formula

(1.3)

where is the average speed of thermal movement of molecules;

Average free path of molecules.

The minus sign indicates that mass transfer occurs in the direction of decreasing density.

Equation (1.2) is called the diffusion equation or Fick's law.

II. 3. Diffusion rate.

When a particle moves in a substance, it constantly collides with its molecules. This is one of the reasons why, under normal conditions, diffusion is slower than normal movement. What does the rate of diffusion depend on?

Firstly, on the average distance between particle collisions, i.e. free path length. The longer this length, the faster the particle penetrates the substance.

Secondly, pressure affects speed. The denser the packing of particles in a substance, the more difficult it is for an alien particle to penetrate such a packing.

Thirdly, the molecular weight of the substance has a major role on the diffusion rate. The larger the target, the more likely it is to hit, and after a collision the speed always slows down.

And fourthly, temperature. As the temperature rises, the vibrations of the particles increase, and the speed of the molecules increases. However, the speed of diffusion is a thousand times slower than the speed of free movement.

All types of diffusion obey the same laws and are described by the diffusion coefficient D, which is a scalar quantity and is determined from Fick’s first law.

For one-dimensional diffusion ,

where J is the flux density of atoms or defects of the substance,
D - diffusion coefficient,
N is the concentration of atoms or defects of a substance.

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

If in a mixture of gases one molecule is four times heavier than another, then such a molecule moves twice as slow as its movement in a pure gas. Accordingly, its diffusion rate is also lower. This difference in the rate of diffusion of light and heavy molecules is used to separate substances with different molecular weights. An example is the separation of isotopes. If a gas containing two isotopes is passed through a porous membrane, the lighter isotopes pass through the membrane faster than the heavier ones. For better separation, the process is carried out in several stages. This process was widely used to separate uranium isotopes (separation of 235U, which fissile under neutron irradiation, from the bulk 238U). Since this separation method requires a lot of energy, other, more economical separation methods have been developed. For example, the use of thermal diffusion in a gas environment 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 in the cold region over time.

Conclusion. Diffuse changes are affected by:

· molecular weight of the substance (the higher the molecular weight, the lower the speed);

· the average distance between particle collisions (the longer the path length, the greater the speed);

· pressure (the larger the particle packing, the more difficult it is to break through),

· temperature (as temperature increases, speed increases).

II.4. Diffusion in inanimate nature.

Did you know that our whole life is built on a strange paradox of nature? Everyone knows that the air we breathe consists of gases of different densities: nitrogen N2, oxygen O2, carbon dioxide CO2 and a small amount of other impurities. And these gases must be arranged in layers, according to the force of gravity: the heaviest, CO 2, is at the very surface of the earth, above it is O 2, and even higher is N 2. But this doesn't happen. We are surrounded by a homogeneous mixture of gases. Why doesn't the flame go out? After all, the oxygen surrounding it quickly burns out? Here, as in the first case, the alignment mechanism operates. Diffusion prevents imbalance in nature!

Why is the sea salty? We know that rivers break through the thickness of rocks and minerals and wash salts into the sea. How does salt and water mix? This can be explained with a simple experiment:

DESCRIPTION OF EXPERIENCE: Pour an aqueous solution of copper sulfate into a glass vessel. Carefully pour clean water over the solution. We observe the boundary between liquids.

Question: What will happen to these liquids over time, and what will we observe?

Over time, the boundary between the contacting liquids will begin to blur. A vessel with liquids can be placed in a closet and day after day you can observe how spontaneous mixing of liquids occurs. Eventually, a homogeneous pale blue liquid, almost colorless in the light, forms in the vessel.

Copper sulfate particles are heavier than water, but due to diffusion they slowly rise upward. The reason is the structure of the liquid. Liquid particles are packed into compact groups - pseudonuclei. They are separated from each other by voids - holes. Nuclei are not stable; their particles do not remain in equilibrium for long. As soon as energy is imparted to the particle, the particle breaks away from the nucleus and falls into the void. From there it easily jumps to another core, etc.

Molecules of a foreign substance begin their journey through the liquid from holes. On the way, they collide with nuclei, knock out particles from them, and take their place. Moving from one free place to another, they slowly mix with liquid particles. We already know that the diffusion rate is low. Therefore, under normal conditions, this experiment lasted 18 days, with heating - 2-3 minutes.

Conclusion: In the flame of the Sun, the life and death of distant luminous stars, in the air we breathe, changes in weather, in almost all physical phenomena we see the manifestation of almighty diffusion!

II.5. Diffusion in living nature.

Diffusion processes have now been well studied, their physical and chemical laws have been established, and they are quite applicable to the movement of molecules in a living organism. Diffusion in living organisms is inextricably linked with the plasma membrane of the cell. Therefore, it is necessary to find out how it is structured and how the features of its structure are related to the transport of substances in the cell.

The plasma membrane (plasmalemma, cell membrane), a surface, peripheral structure surrounding the protoplasm of plant and animal cells, serves not only as a mechanical barrier, but, most importantly, limits the free two-way flow of low- and high-molecular substances into and out of the cell. Moreover, the plasmalemma acts as a structure that “recognizes” various chemical substances and regulates the selective transport of these substances into the cell

The outer surface of the plasma membrane is covered with a loose fibrous layer of substance 3-4 nm thick - the glycocalyx. It consists of branching chains of complex carbohydrates, membrane integral proteins, between which cell-secreted compounds of proteins with sugars and proteins with fats can be located. Some cellular enzymes involved in the extracellular breakdown of substances (extracellular digestion, for example, in the intestinal epithelium) are also found here.

Since the interior of the lipid layer is hydrophobic, it represents a virtually impenetrable barrier to most polar molecules. Due to the presence of this barrier, leakage of cell contents is prevented, but because of this, the cell was forced to create special mechanisms to transport water-soluble substances across the membrane.

The plasma membrane, like other lipoprotein cell membranes, is semi-permeable. Water and gases dissolved in it have the maximum penetrating ability. Ion transport can occur along a concentration gradient, i.e. passively, without energy consumption. In this case, some membrane transport proteins form molecular complexes, channels through which ions pass through the membrane by simple diffusion. In other cases, special membrane transport proteins selectively bind to one or another ion and transport it across the membrane. This type of transport is called active transport and is carried out using protein ion pumps. For example, spending 1 ATP molecule, the K-Na pump system pumps out 3 Na ions from the cell in one cycle and pumps 2 K ions against the concentration gradient. In combination with active ion transport, various sugars, nucleotides and amino acids penetrate the plasmalemma. Macromolecules, such as proteins, do not pass through the membrane. They, as well as larger particles of the substance, are transported into the cell through endocytosis. During endocytosis, a certain area of ​​the plasmalemma captures, envelops extracellular material, and encloses it in a membrane vacuole. This vacuole - an endosome - merges in the cytoplasm with the primary lysosome and digestion of the captured material occurs. Endocytosis is formally divided into phagocytosis (uptake of large particles by the cell) and pinocytosis (uptake of solutions). The plasma membrane also takes part in the removal of substances from the cell using exocytosis, a process reverse to endocytosis.

The diffusion of ions in aqueous solutions is especially important for living organisms. The role of diffusion in respiration, photosynthesis, and transpiration of plants is no less important; in the transfer of air oxygen through the walls of the alveoli of the lungs and its entry into the blood of humans and animals. Diffusion of molecular ions across membranes is accomplished by electrical potential within the cell. Possessing selective permeability, membranes play the role of customs when moving goods across the border: some substances are allowed through, others are retained, and others are generally “expelled” from the cell. The role of membranes in cell life is very important. A dying cell loses control over the ability to regulate the concentration of substances through the membrane. The first sign of a dying cell is the beginning of changes in the permeability and malfunction of its outer membrane.

In addition to conventional transport - the kinetic process of transferring particles of a substance under the influence of gradients of electrical or chemical potential, temperature or pressure - active transport also takes place in cellular processes - the movement of molecules and ions against the concentration gradient of substances. This diffusion mechanism is called osmosis. (Osmosis was first observed by A. Nolle in 1748, but research into this phenomenon began a century later.) This process is carried out due to different osmotic pressure in an aqueous solution on different sides of a biological membrane. Water often passes freely through osmosis through a membrane, but this membrane can be impermeable to substances dissolved in water. It is curious that water flows against the diffusion of this substance, but obeying the general law of concentration gradient (in this case, water).

Therefore, water tends from a more dilute solution, where its concentration is higher, to a more concentrated solution of a substance, in which the water concentration is lower. Not being able to directly absorb and pump out water, the cell does this through osmosis, changing the concentration of dissolved substances in it. Osmosis equalizes the concentration of the solution on both sides of the membrane. The tense state of the cell membrane, which is called turgor pressure, depends on the osmotic pressure of solutions of substances on both sides of the cell membrane and the elasticity of the cell membrane, which is called turgor pressure (turgor - from the Latin turgere - to be swollen, filled). Typically, the elasticity of animal cell membranes (excluding some coelenterates) is low; they lack high turgor pressure and retain integrity only in isotonic solutions or those that differ little from isotonic ones (the difference between internal and external pressure is less than 0.5-1.0 am). In living plant cells, the internal pressure is always greater than the external pressure, however, rupture of the cell membrane does not occur in them due to the presence of a cellulose cell wall. The difference between internal and external pressures in plants (for example, in halophyte plants - salt-loving mushrooms) reaches 50-100 am. But even so, the safety margin of the plant cell is 60-70%. In most plants, the relative elongation of the cell membrane due to turgor does not exceed 5-10%, and turgor pressure lies in the range of 5-10 am. Thanks to turgor, plant tissues have elasticity and structural strength. (Experiments No. 3, No. 4 confirm this). All processes of autolysis (self-destruction), withering and aging are accompanied by a drop in turgor pressure.

When considering diffusion in living nature, one cannot fail to mention absorption. Absorption is the process of entry of various substances from the environment through cell membranes into cells, and through them into the internal environment of the body. In plants, this is the process of absorption of water with substances dissolved in it by roots and leaves through osmosis and diffusion; in invertebrates - from the environment or cavity fluid. In primitive organisms, absorption occurs through pinocytosis and phagocytosis. In vertebrates, absorption can occur both from the cavity organs - lungs, uterus, bladder, and from the surface of the skin, from the wound surface, etc. Volatile gases and vapors are absorbed by the skin.

The greatest physiological significance is absorption in the gastrointestinal tract, which occurs mainly in the small intestine. For efficient transfer of substances, the large surface area of ​​the intestine and the constantly high blood flow in the mucous membrane are of particular importance, due to which a high concentration gradient of absorbed compounds is maintained. In humans, mesenteric blood flow during meals is about 400 ml/min, and at the height of digestion - up to 750 ml/min, with the main share (up to 80%) being blood flow in the mucous membrane of the digestive organs. Due to the presence of structures that increase the surface of the mucous membrane - circular folds, villi, microvilli, the total area of ​​the absorption surface of the human intestine reaches 200 m2.

Water and salt solutions can diffuse on both sides of the intestinal wall, in both the small and large intestines. Their absorption occurs mainly in the upper parts of the small intestine. The transport of Na+ ions in the small intestine is of great importance, due to which electrical and osmotic gradients are mainly created. Absorption of Na+ ions occurs through both active and passive mechanisms.

If the cell did not have systems for regulating osmotic pressure, then the concentration of solutes inside it would be greater than their external concentrations. Then the concentration of water in the cell would be less than its concentration outside. As a result, there would be a constant flow of water into the cell and its rupture. Fortunately, animal cells and bacteria control the osmotic pressure in their cells by actively pumping out inorganic ions such as Na. Therefore, their total concentration inside the cell is lower than outside. For example, amphibians spend a significant part of their time in water, and the salt content in their blood and lymph is higher than in fresh water. Amphibian organisms continuously absorb water through their skin. Therefore, they produce a lot of urine. A frog, for example, if its cloaca is bandaged, swells like a balloon. And, conversely, if an amphibian gets into salty sea water, it becomes dehydrated and dies very quickly. Therefore, the seas and oceans are an insurmountable obstacle for amphibians. Plant cells have rigid walls that protect them from swelling. Many protozoa avoid bursting from the water entering the cell with the help of special mechanisms that regularly throw out the incoming water.

Thus, the cell is an open thermodynamic system, exchanging matter and energy with the environment, but maintaining a certain constancy of the internal environment. These two properties of a self-regulating system - openness and constancy - are fulfilled simultaneously, and metabolism (metabolism) is responsible for the constancy of the cell. Metabolism is the regulator that contributes to the preservation of the system; it ensures an appropriate response to environmental influences. Therefore, a necessary condition for metabolism is the irritability of a living system at all levels, which at the same time acts as a factor in the systematicity and integrity of the system.

Membranes can change their permeability under the influence of chemical and physical factors, including as a result of depolarization of the membrane when an electrical impulse passes through the neuronal system and influences it.

A neuron is a piece of nerve fiber. If a stimulus acts at one end of it, an electrical impulse occurs. Its value is about 0.01 V for human muscle cells, and it propagates at a speed of about 4 m/s. When the impulse reaches a synapse - a connection between neurons, which can be considered as a kind of relay that transmits a signal from one neuron to another, the electrical impulse is converted into a chemical impulse through the release of neurotransmitters - specific intermediary substances. When molecules of such an intermediary enter the gap between neurons, the neurotransmitter reaches the end of the gap by diffusion and excites the next neuron.

However, a neuron reacts only if there are special molecules on its surface - receptors that can only bind a given transmitter and not react to another. This occurs not only on the membrane, but also in any organ, such as a muscle, causing it to contract. Signals-impulses through synapses can inhibit or enhance the transmission of others, and therefore neurons perform logical functions (“and”, “or”), which to a certain extent served as the basis for N. Wiener to believe that computational processes in the brain of a living organism and in computers follow essentially the same pattern. Then the information approach allows us to describe inanimate and living nature in a unified way.

The very process of the signal influencing the membrane consists of changing its high electrical resistance, since the potential difference on it is also of the order of 0.01 V. A decrease in resistance leads to an increase in the electric current pulse and the excitation is transmitted further in the form of a nerve impulse, thereby changing the possibility of passing through membrane of certain ions. Thus, information in the body can be transmitted in combination by chemical and physical mechanisms, and this ensures the reliability and diversity of channels for its transmission and processing in a living system.

The processes of normal respiration of a living organism, which requires oxygen O2 obtained as a result of photosynthesis, are closely related to the processes of normal respiration of a living organism, when ATP molecules are formed in the mitochondria of a cell, providing it with the necessary energy. The mechanisms of these processes are also based on the laws of diffusion. Essentially, these are the material and energy components that are necessary for a living organism. Photosynthesis is the process of storing solar energy by forming new bonds in the molecules of synthesized substances. The starting materials for photosynthesis are water H 2 O and carbon dioxide CO 2. From these simple inorganic compounds, more complex, energy-rich nutrients are formed. Molecular oxygen O2 is formed as a by-product, but very important for us. An example is a reaction that occurs due to the absorption of light quanta and the presence of the chlorophyll pigment contained in chloroplasts.

The result is one molecule of sugar C 6 H 12 O 6 and six molecules of oxygen O 2. The process proceeds in stages, first at the stage of photolysis, hydrogen and oxygen are formed by splitting water, and then hydrogen, combining with carbon dioxide, forms a carbohydrate - sugar C 6 H 12 O 6. Essentially, photosynthesis is the conversion of the radiant energy of the Sun into the energy of chemical bonds of emerging organic substances. Thus, photosynthesis, which produces oxygen O 2 in the light, is the biological process that provides living organisms with free energy. The process of normal respiration as a metabolic process in the body associated with the consumption of oxygen is the reverse of the process of photosynthesis. Both of these processes can follow the following chain:

Solar energy (photosynthesis)

nutrients + (respiration)

Energy of chemical bonds.

The end products of respiration serve as starting materials for photosynthesis. Thus, the processes of photosynthesis and respiration participate in the cycle of substances on Earth. Part of the solar radiation is absorbed by plants and some organisms, which, as we already know, are autotrophs, i.e. self-feeding (food for them is sunlight). As a result of the process of photosynthesis, autotrophs bind atmospheric carbon dioxide and water, forming up to 150 billion tons of organic substances, assimilating up to 300 billion tons of CO 2, and releasing about 200 billion tons of free oxygen O 2 annually.

The resulting organic substances are used as food by humans and herbivores, which, in turn, feed on other heterotrophs. Plant and animal remains are then decomposed into simple inorganic substances, which can again participate in the form of CO 2 and H 2 O in photosynthesis. Part of the resulting energy, including that stored in the form of fossil energy fuel, is used for consumption by living organisms, while part is uselessly dissipated into the environment. Therefore, the process of photosynthesis, due to the ability to provide it with the necessary energy and oxygen, is at a certain stage of the development of the Earth's biosphere a catalyst for the evolution of living things.

Diffusion processes underlie metabolism in the cell, which means that with their help these processes are carried out at the organ level. This is how absorption processes take place in the root hairs of plants, the intestines of animals and humans; gas exchange in plant stomata, lungs and tissues of humans and animals, excretory processes.

Biologists have been studying the structure and study of cells for more than 150 years, starting with Schleiden, Schwann, Purime and Virchow, who in 1855 established the mechanism of cell growth by dividing them. It was found that each organism develops from a single cell, which begins to divide and as a result of this, many cells are formed that are noticeably different from each other. But since the development of the organism initially began from the division of the first cell, then at one stage of our life cycle we retain similarities with a very distant unicellular ancestor, and one can jokingly say that we are more likely to have descended from an amoeba than from a monkey.

Organs are formed from cells, and the cell system acquires qualities that its constituent elements do not have, i.e. individual cells. These differences are due to the set of proteins synthesized by a given cell. There are muscle cells, nerve cells, blood cells (erythrocytes), epithelial cells and others, depending on their functionality. Cell differentiation occurs gradually during the development of the organism. In the process of cell division, their life and death, continuous replacement of cells occurs throughout the life of the organism.

Not a single molecule in our body remains unchanged for more than a few weeks or months. During this time, molecules are synthesized, fulfill their role in the life of the cell, are destroyed and replaced by other, more or less identical molecules. The most amazing thing is that living organisms as a whole are much more constant than the molecules that make them up, and the structure of the cells and the entire body consisting of these cells remains unchanged in this non-stop cycle, despite the replacement of individual components.

Moreover, this is not a replacement of individual parts of the car, but, as S. Rose figuratively compares, the body with a brick building, “from which a crazy mason continuously takes out one brick after another night and day and inserts new ones in their place. At the same time, the external appearance of the building remains the same, but the material is constantly replaced.” We are born with some neurons and cells, and die with others. An example is the consciousness, understanding and perception of a child and an old person. All cells contain complete genetic information for the construction of all proteins of a given organism. Storage and transmission of hereditary information is carried out using the cell nucleus.

Conclusion: The role of plasma membrane permeability in cell life cannot be exaggerated. Most of the processes associated with providing the cell with energy, obtaining products and ridding it of decay products are based on the laws of diffusion through this semi-permeable living barrier.

Osmosis- in essence, the simple diffusion of water from places with a higher concentration of water to places with a lower concentration of water.

Passive transport– this is the transfer of substances from places with a high electrochemical potential to places with a lower value. The transfer of small water-soluble molecules is carried out using special transport proteins. These are special transmembrane proteins, each of which is responsible for the transport of specific molecules or groups of related molecules.

It is often necessary to ensure the transport of molecules across a membrane against their electrochemical gradient. This process is called active transport and is carried out by carrier proteins, whose activity requires energy. If you connect a carrier protein with an energy source, you can get a mechanism that ensures the active transport of substances across the membrane.

II.6. Application of diffusion.

Man has been using diffusion phenomena since ancient times. This process involves cooking and heating the home. We encounter diffusion during heat treatment of metals (welding, soldering, cutting, coating, etc.); applying a thin layer of metals to the surface of metal products to increase the chemical resistance, strength, hardness of parts and devices, or for protective and decorative purposes (galvanizing, chrome plating, nickel plating).

The natural flammable gas we use at home for cooking has neither color nor odor. Therefore, it would be difficult to immediately notice a gas leak. And when there is a leak, the gas spreads throughout the room due to diffusion. Meanwhile, at a certain ratio of gas to air in a closed room, a mixture is formed that can explode, for example, from a lit match. The gas can also cause poisoning.

To make the flow of gas into a room noticeable, at distribution stations the flammable gas is pre-mixed with special substances that have a strong unpleasant odor that is easily perceived by humans even at very low concentrations. This precaution allows you to quickly notice the accumulation of gas in the room if a leak occurs.

In modern industry, vacuum forming is used, a method for manufacturing products from sheet thermoplastics. A product of the required configuration is obtained due to the pressure difference resulting from the vacuum in the mold cavity over which the sheet is fixed. It is used, for example, in the production of containers, refrigerator parts, and instrument housings. Due to diffusion in this way, it is possible to weld something that is impossible to weld on its own (metal with glass, glass and ceramics, metals and ceramics, and much more).

Due to the diffusion of various isotopes of uranium through porous membranes, fuel for nuclear reactors is treated. Sometimes nuclear fuel is called nuclear fuel.

Absorption (resorption) of substances when introduced into the subcutaneous tissue, into muscles or when applied to the mucous membranes of the eye, nose, or skin of the ear canal occurs mainly due to diffusion. This is the basis for the use of many medicinal substances, and absorption in the muscles occurs faster than in the skin.

Popular wisdom says: “mow your hair while there is dew.” Tell me, what does diffusion and morning mowing have to do with it? The explanation is very simple. During morning dew, grasses have increased turgor pressure, stomata are open, and the stems are elastic, which makes them easier to mow (grass mowed with closed stomata dries worse).

In horticulture, when budding and grafting plants, callus is formed on sections due to diffusion (from the Latin Callus - callus) - wound tissue in the form of an influx in places of damage and promotes their healing, ensures the fusion of the scion with the rootstock.

Callus is used to obtain isolated tissue culture (explantation). This is a method of long-term preservation and cultivation in special nutrient media of cells, tissues, small organs or their parts isolated from the human body, animals and plants. Based on methods of growing a culture of microorganisms that provide asepsis, nutrition, gas exchange and removal of metabolic products of cultivated objects. One of the advantages of the tissue culture method is the ability to observe the vital activity of cells using a microscope. To do this, plant tissue is grown on nutrient media containing auxins and cytokinins. Callus usually consists of poorly differentiated homogeneous cells of educational tissue, but when growing conditions change, especially the content of phytohormones in the nutrient medium, the formation of phloem, xylem and other tissues is possible in it, as well as the development of various organs and the whole plant.

II.7. Design of individual experiments.

Using scientific literature, I tried to repeat the experiments that were most interesting to me. I depicted the diffusion mechanism and the results of these experiments in the presentation in the form of animation models.

EXPERIENCE 1. Take two test tubes: one half filled with water, the other half filled with sand. Pour the water into a test tube with sand. The volume of a mixture of water and sand in a test tube is less than the sum of the volumes of water and sand.

EXPERIENCE 2. Fill a long glass tube halfway with water, and then pour colored alcohol on top. Mark the general level of liquids in the tube with a rubber ring. After mixing water and alcohol, the volume of the mixture decreases.

(Experiments 1 and 2 prove that there are gaps between the particles of matter; during diffusion, they are filled with particles of the alien substance.)

EXPERIENCE 3. We bring a cotton wool moistened with ammonia into contact with a cotton wool moistened with the indicator phenolphthalein. We observe the coloring of the fleeces in a crimson color.

Now a cotton wool moistened with ammonia is placed on the bottom of a glass vessel, and one moistened with phenolphthalein. Attach it to the lid and cover the glass vessel with this lid. After some time, the cotton wool soaked in phenolphthalein begins to color.

As a result of interaction with ammonia, phenolphthalein turns crimson, which is what we observed when the cotton wool came into contact. But why then in the second case, cotton wool soaked in phenolphthalein. It is also painted, because now the fleeces are not brought into contact? Answer: continuous chaotic movement of particles of substances.

EXPERIENCE 4. Place a narrow strip of filter paper soaked in a mixture of starch paste and phenolphthalein indicator solution along the wall inside a tall cylindrical vessel. Place iodine crystals at the bottom of the vessel. Close the vessel tightly with a lid from which cotton wool soaked in ammonia solution is suspended.

Due to the interaction of iodine with starch, a blue-violet color rises up the strip of paper. At the same time, a crimson color spreads downwards - evidence of the movement of ammonia molecules. After a few minutes, the boundaries of the colored areas of the paper will meet, and then the blue and crimson colors mix, that is, diffusion occurs.[10]

EXPERIENCE 5.(spend it together) Take a watch with a second hand, a tape measure, a bottle of eau de toilette and stand in different corners of the room. One notes the time and opens the bottle. Another notes the time when he smells the eau de toilette. By measuring the distance between the experimenters, we find the diffusion rate. For accuracy, the experiment is repeated 3–4 times, and the average speed value is found. If the distance between experimenters is 5 meters, then the smell is felt after 12 minutes. That is, the diffusion speed in this case is 2.4 m/min.

EXPERIENCE 6. DETERMINATION OF PLASMA VISCOSITY BY PLASMOLYSIS METHOD (according to P.A. Genkel).

Advance speed convex plasmolysis in plant cells when they are treated with a hypertanic solution, it depends on the viscosity of the cytoplasm; the lower the viscosity of the cytoplasm, the sooner concave plasmolysis turns into convex. The viscosity of the cytoplasm depends on the degree of dispersion of colloidal particles and their hydration, on the water content in the cell, on the age of the cells and other factors.

Progress. Make a thin section of the epidermis from an aloe leaf, or tear off the epidermis from the soft scales of an onion. The prepared sections are tinted in a watch glass for 10 minutes in a neutral red solution at a concentration of 1:5000. Then the sections of the object are placed on a glass slide in a drop of low concentration sucrose and covered with one coverslip. Under a microscope, the state of plasmolysis is noted. First, concave plasmolysis is observed in the cells. Subsequently, this shape is either preserved or, with varying speed, transforms into a convex shape. It is important to note the time of transition from concave to convex plasmolysis. The period of time during which concave plasmolysis transforms into convex plasmolysis is an indicator of the degree of protoplasm viscosity. The longer the transition time to convex plasmolysis, the greater the plasma viscosity. Plasmolysis in onion cells begins faster than in aloe skin. This means that the cytoplasm of aloe cells is more viscous.

EXPERIENCE 7. PLASMOLYSIS. DEPLASMOLYSIS. PENETRATION OF SUBSTANCES INTO THE VACUOLE [2]

Some organic substances penetrate quite quickly into the vacuole. In cells, when they are kept in solutions of such substances, plasmolysis is relatively quickly lost and deplasmolysis occurs.

Deplasmolysis is the restoration of turgor in cells(i.e. the opposite phenomenon to plasmolysis).

Progress. Sections of the upper epidermis of colored onion scales (concave side) are placed in a drop of I M solution of plant fertilizer urea or glycerol directly on a glass slide and covered with a coverslip. After 15-30 minutes, objects are examined under a microscope. Plasmolyzed cells are clearly visible. Leave the sections in a drop of solution for another 30-40 minutes. Then they look again under a microscope and observe deplasmolysis - restoration of turgor.

Conclusion : Plants cannot clearly control the amount of chemicals entering and exiting cells.

III. Conclusion.

The laws of diffusion govern the processes of physical and chemical movements of elements in the earth’s interior and in the Universe, as well as the vital processes of cells and tissues of living organisms. Diffusion plays an important role in various fields of science and technology, in processes occurring in living and inanimate nature. Diffusion influences the course of many chemical reactions, as well as many physicochemical processes and phenomena: membrane, evaporation, condensation, crystallization, dissolution, swelling, combustion, catalytic, chromatographic, luminescent, electrical and optical in semiconductors, neutron moderation in nuclear reactors etc. Diffusion is of great importance in the formation of a double electrical layer at phase boundaries, diffusion and electrophoresis, in photographic processes for quickly obtaining images, etc. Diffusion serves as the basis for many common technical operations: sintering of powders, chemical-thermal treatment of metals, metallization and welding of materials, tanning leather and fur, dyeing fibers, moving gases using diffusion pumps. The role of diffusion has increased significantly due to the need to create materials with predetermined properties for developing fields of technology (nuclear energy, astronautics, radiation and plasma-chemical processes, etc.). Knowledge of the laws governing diffusion makes it possible to prevent unwanted changes in products that occur under the influence of high loads and temperatures, radiation and much, much more...

What would the world be like without diffusion? Stop the thermal movement of particles - and everything around will become dead!

In my work, I summarized the material collected on the topic of the abstract and prepared a presentation made in the Power Point editor for its defense. This presentation, in my opinion, can diversify the lesson material on this topic. Some of the experiments described in the literature were repeated and slightly modified by me. The most interesting examples of diffusion are presented on the presentation slides in animated models.

IV. Used Books:

1. Antonov V.F., Chernysh A.M., Pasechnik V.I., et al. Biophysics.

M., Arktos-Vika-press, 1996

2. Afanasyev Yu.I., Yurina N.A., Kotovsky E.F. and others. Histology.

M. Medicine, 1999.

3. Alberts B., Bray D., Lewis J. et al. Molecular biology of the cell.

In 3 volumes. Volume 1. M., Mir, 1994.

4. Great Encyclopedia of Cyril and Methodius 2006

5. Varikash V.M. and others. Physics in living nature. Minsk, 1984.

6. Demyankov E.N. Problems in biology. M. Vlados, 2004.

7. Nikolaev N.I. Diffusion in membranes. M. Chemistry, 1980, p. 76

8. Peryshkin A.V. Physics. 7. M. Bustard, 2004.

9. Physical encyclopedic dictionary, M., 1983, p. 174-175, 652, 754

10. Shablovsky V. Entertaining physics. St. Petersburg, “trigon” 1997, p.416

11.xttp//bio. fizten/ru./

12.xttp//markiv. narod.ru./

13. “http://ru.wikipedia.org/wiki/%D0%94%D0%B8%D1%84%D1%84%D1%83%D0%B7%D0%B8%D1%8F” Categories: Phenomena at the atomic level | Thermodynamic phenomena | Transfer phenomena | Diffusion

In the school physics course (approximately in the seventh grade), schoolchildren learn that diffusion is a process that represents the mutual penetration of particles of one substance between particles of another substance, resulting in an equalization of concentrations throughout the occupied volume. This is a rather difficult definition to understand. To understand what simple diffusion is, the law of diffusion, its equation, it is necessary to study in detail the materials on these issues. However, if a general idea is enough for a person, then the data below will help to gain basic knowledge.

Physical phenomenon - what is it

Due to the fact that many people are confused or do not know at all what a physical phenomenon is and how it differs from a chemical one, as well as what type of phenomena diffusion refers to, it is necessary to understand what a physical phenomenon is. So, as everyone knows, physics is an independent science belonging to the field of natural science, which studies the general natural laws about the structure and movement of matter, and also studies matter itself. Accordingly, a physical phenomenon is a phenomenon as a result of which new substances are not formed, but only a change in the structure of the substance occurs. The difference between a physical phenomenon and a chemical one is precisely that new substances are not produced as a result. Thus, diffusion is a physical phenomenon.

Definition of the term diffusion

As you know, there can be many formulations of a particular concept, but the general meaning should not change. And the phenomenon of diffusion is no exception. The generalized definition is as follows: diffusion is a physical phenomenon that represents the mutual penetration of particles (molecules, atoms) of two or more substances until uniform distribution throughout the entire volume occupied by these substances. As a result of diffusion, no new substances are formed, which is why it is precisely a physical phenomenon. Simple diffusion is called diffusion, as a result of which particles move from an area of ​​​​highest concentration to an area of ​​lower concentration, which is caused by thermal (chaotic, Brownian) movement of particles. In other words, diffusion is the process of mixing particles of different substances, and the particles are distributed evenly throughout the entire volume. This is a very simplified definition, but the most understandable.

Types of diffusion

Diffusion can be recorded both when observing gaseous and liquid substances, as well as solid ones. Therefore, it includes several types:

• Quantum diffusion is the process of diffusion of particles or point defects (local disturbances in the crystal lattice of a substance), which occurs in solids. Local disturbances are disturbances at a specific point in the crystal lattice.

• Colloidal - diffusion occurring throughout the entire volume of the colloidal system. A colloidal system is a medium in which particles, bubbles, drops of another medium, different in state of aggregation and composition from the first, are distributed. Such systems, as well as the processes occurring in them, are studied in detail in the course of colloidal chemistry.
• Convective - transfer of microparticles of one substance by macroparticles of the medium. A special branch of physics, called hydrodynamics, deals with the study of the motion of continuous media. From there you can gain knowledge about flow states.
• Turbulent diffusion is the process of transfer of one substance into another, caused by the turbulent movement of the second substance (typical of gases and liquids).

The statement is confirmed that diffusion can occur both in gases and liquids, and in solids.

What is Fick's law?

The German scientist, physicist Fick, derived a law showing the dependence of the particle flux density through a unit area on the change in the concentration of a substance per unit length. This law is the law of diffusion. The law can be formulated as follows: the particle flow, which is directed along the axis, is proportional to the derivative of the number of particles with respect to the variable plotted along the axis relative to which the direction of the particle flow is determined. In other words, the flow of particles moving in the direction of the axis is proportional to the derivative of the number of particles with respect to the variable, which is plotted along the same axis as the flow. Fick's law allows us to describe the process of transfer of matter in time and space.

Diffusion equation

When there are flows in a substance, a redistribution of the substance itself in space occurs. In this regard, there are several equations that describe this redistribution process from a macroscopic point of view. The diffusion equation is differential. It follows from the general equation of matter transfer, which is also called the continuity equation. In the presence of diffusion, Fick's law is used, which is described above. The equation looks like this:

dn/dt=(d/dx)*(D*(dn/dx)+q.

Diffusion methods

The diffusion method, or more precisely the method of its implementation in solid materials, has been widely used recently. This is due to the advantages of the method, one of which is the simplicity of the equipment used and the process itself. The essence of the diffusion method from solid sources is the deposition of films doped with one or more elements onto semiconductors. There are several other methods of performing diffusion, in addition to the solid source method:

• in a closed volume (ampoule method). Minimal toxicity is an advantage of the method, but its high cost, due to the disposability of the ampoule, is a significant drawback;
• in an open volume (thermal diffusion). The possibility of using many elements is excluded due to high temperatures, as well as lateral diffusion are the big disadvantages of this method;
• in a partially closed volume (box method). This is an intermediate method between the two described above.

In order to learn more about the methods and features of diffusion, it is necessary to study additional literature devoted specifically to these issues.

Diffusion rate

Diffusion is one of the simplest phenomena that are studied as part of a physics course. This process can be represented at the everyday everyday level.

Diffusion is a physical process of mutual penetration of atoms and molecules of one substance between the same structural elements of another substance. The result of this process is the equalization of the concentration level in the penetrating compounds. Diffusion or mixing can be seen every morning in your own kitchen when preparing tea, coffee or other drinks that contain several basic components.

A similar process was first able to be scientifically described by Adolf Fick in the mid-19th century. He gave it the original name, which is translated from Latin as interaction or distribution.

The rate of diffusion depends on several factors:

• body temperature;
• state of aggregation of the substance under study.

In various gases, where there are very large distances between molecules, the rate of diffusion will be greatest. In liquids, where the distance between molecules is noticeably smaller, the speed also decreases. The lowest diffusion rate is observed in solids, since molecular bonds exhibit strict order. Atoms and molecules themselves perform slight vibrational movements in one place. The rate of diffusion increases with increasing ambient temperature.

Fick's law

Note 1

The rate of diffusion is usually measured by the amount of substance that is transferred per unit of time. All interactions must occur through the cross-sectional area of ​​the solution.

The basic formula for diffusion rate is:

$\frac(dm)(dt)=-DC\frac(dC)(dx)$, where:

• $D$ is the proportionality coefficient,
• $S$ is the surface area, and the “-” sign means that diffusion proceeds from an area of ​​higher concentration to a lower one.

This formula was presented in the form of a mathematical description by Fick.

According to it, the rate of diffusion is directly proportional to the concentration gradient and the area through which the diffusion process occurs. The proportionality coefficient determines the diffusion of a substance.

The famous physicist Albert Einstein derived equations for the diffusion coefficient:

$D=RT/NA \cdot 1/6\pi\etaŋr$, where:

• $R$ is the universal gas constant,
• $T$ is absolute temperature,
• $r$ is the radius of diffusing particles,
• $D$ - diffusion coefficient,
• $ŋ$ is the viscosity of the medium.

From these equations it follows that the rate of diffusion will increase:

• when the temperature rises;
• with increasing concentration gradient.

The rate of diffusion decreases:

• with increasing solvent viscosity;
• with increasing size of diffusing particles.

If the molar mass increases, then the diffusion coefficient decreases. In this case, the diffusion rate also decreases.

Acceleration of diffusion

There are various conditions that help accelerate diffusion. The rate of diffusion depends on the state of aggregation of the substance under study. The high density of the material slows down the chemical reaction. The rate of interaction of molecules is affected by temperature. A quantitative characteristic of the diffusion rate is the coefficient. In the SI measurement system, it is denoted by the Latin capital letter D. It is measured in square centimeters or meters per second of time.

Definition 1

The diffusion coefficient is equal to the amount of a substance that is distributed among another substance through a certain unit of surface. The interaction must be carried out over a unit of time. To effectively solve the problem, it is necessary to achieve a condition when the difference in densities on both surfaces is equal to unity.

Also, the rate of diffusion in solids and liquids in gases is affected by pressure and radiation. Radiation can be of different types, including induction and high-frequency. Diffusion begins when exposed to a certain catalyst substance. They often act as a trigger for the emergence of a stable process of particle dispersion.

Using the Arrhenius equation, the dependence of the coefficient on temperature is described. It looks like this:

$D = D0exp(-E/TR)$, where:

• $T$ is the absolute temperature, which is measured in Kelvin,
• $E$ is the minimum energy required for diffusion.

The formula allows us to understand more about the characteristics of the entire diffusion process and determines the rate of the reaction.

Special diffusion methods

Today it is practically impossible to use conventional methods to determine the molecular weight of proteins. They are usually based on the measurement:

• vapor pressure;
• increasing the boiling point;
• lowering the freezing point of solutions.

To effectively solve the problem, special methods are used that are developed for studying substances with a high molecular structure. They involve determining the diffusion rate or viscosity of solutions.

The method for determining the orientation and shape of pores from the diffusion rate is based on the study of dialysis rates. Free diffusion should occur in the membrane at this moment.

Various radioisotopes can also be used to determine the rate of sodium diffusion. This special method is used to solve problems in the field of mineralogy and geology.

The diffusion method is actively used, which is based on determining the diffusion of macromolecules in solution. It was developed for polymer materials. According to the method, the diffusion coefficient is determined, and then the mass-average molecular weight is determined from these data.

Currently, there are no direct methods for determining the rate of hydrogen diffusion in a catalyst. For this, the so-called second activation pathway is used.

To determine speed, it is customary to use special devices. They differ in appearance from the assigned practical and scientific tasks.