Diffusion in solids, liquids and gases: definition, conditions. Big encyclopedia of oil and gas

To understand what diffusion depends on, consider the factors that affect it.

Diffusion depends on temperature. The rate of diffusion will increase with increasing temperature, because as the temperature rises, the speed of movement of molecules will increase, that is, the molecules will mix faster. (You all know that sugar takes a very long time to dissolve in cold water)

And when adding external influence(a person stirs sugar in water) diffusion will proceed faster. Aggregate state of matter will also affect what diffusion depends on, namely, the rate of diffusion. Thermal diffusion depends on the type of molecules. For example, if the object is metal, then thermal diffusion proceeds faster, in contrast to if this object was made of synthetic material. Diffusion between solid materials proceeds very slowly.

So the diffusion rate depends on: temperature, concentration, external influences, state of aggregation of the substance

Diffusion is of great importance in nature and in human life.

Diffusion examples

To better understand what diffusion is, let's look at it with examples. Let's give examples of the diffusion process in gases together. Variants of manifestation of this phenomenon can be as follows:

Spreading the smell of flowers;

The spread of the smell of grilled chicken, which Antoshka likes so much;

Tears from cutting onions;

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

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

A simple and accessible to everyone example of diffusion of solids is to take two pieces of multi-colored plasticine and, kneading them in your hands, observe how the colors mix. And, accordingly, without external influence, if you simply press two pieces together, it will take months or even years for the two colors to mix at least a little, so to speak, to penetrate one into one.

Variants of manifestation of diffusion in liquids can be as follows:

Dissolving a drop of ink in water;

- "Linen faded" color of wet fabrics;

Salting vegetables and making jam

So, diffusion is the mixing of the molecules of a substance during their random thermal motion.

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 of this, just 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 yellow tablecloth. 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 hydrogen atoms (H 2) that have arisen in the course of some chemical reaction 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 a piece of ice with KMnO 4 crystals 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. In addition to the state of aggregation, the rate of interpenetration of particles is also affected by temperature.

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 rate of diffusion in solids, liquids in gases is affected by pressure and radiation (inductive or high-frequency). In addition, much depends on the presence of a catalytic substance, often it acts as a trigger mechanism 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 media, it can be noted that it has both positive and negative sides.

Gazizova Guzel

"Steps into Science - 2016"

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Municipal budgetary educational institution

"Arsk Secondary School No. 7" Arsky

Municipal district of the Republic of Tatarstan.

Republican scientific and practical conference

"Steps into Science - 2016"

Section: Physics and technical creativity

Research

Subject: Observation of diffusion in water and the effect of temperature on the rate of diffusion.

Position.

Gazizova Guzel Robertovna Zinnatullin Fidaris Faisalovich

7th grade student, physics teacher, 1st quarter. categories.

2016

1. Introduction Page 3

1. Research problem
2. Relevance of the topic and practical significance of the study
3. Object and subject of research
4. Goals and objectives
5. Research hypothesis

1. Main body of research work Page 5

1. Description of the place and conditions of observations and experiments
2. Research methodology, its validity
3. Main results of the experiment
4. Generalization and conclusions

1. Conclusion Page 6
2. References Page 7

Diffusion (Latin diffusio - spreading, spreading, scattering, interaction) is the process of mutual penetration of molecules or atoms of one substance between molecules or atoms of another, leading to spontaneous equalization of their concentrations throughout the occupied volume. In some situations, one of the substances already has an equal concentration and one speaks of the diffusion of one substance in another. In this case, the transfer of a substance occurs from an area with a high concentration to an area with a low concentration.

If water is carefully poured into a solution of copper sulphate, then a clear interface is formed between the two layers (copper sulphate is heavier than water). But in two days there will be a homogeneous liquid in the vessel. This happens completely randomly.

Another example is related to a solid body: if one end of the rod is heated, or electrically charged, heat (or, respectively, electric current) spreads from the hot (charged) part to the cold (uncharged) part. In the case of a metal rod, thermal diffusion develops rapidly, and the current flows almost instantly. If the rod is made of synthetic material, thermal diffusion is slow, and diffusion of electrically charged particles is very slow. Diffusion of molecules proceeds in general 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 covered with gold, then gold will diffuse into copper, but under normal conditions (room temperature and atmospheric pressure), the gold-bearing layer will reach a thickness of several micrometers only after several thousand years.

The first quantitative description of diffusion processes was given by the German physiologist A. Fick in 1855.

Diffusion takes place in gases, liquids and solids, and both particles of foreign substances in them and their own particles can diffuse.

Diffusion in human life

Studying the phenomenon of diffusion, I came to the conclusion that it is thanks to this phenomenon that a person lives. After all, as you know, the air we breathe consists of a mixture of gases: nitrogen, oxygen, carbon dioxide and water vapor. It is located in the troposphere - in the lower layer of the atmosphere. If there were no diffusion processes, then our atmosphere would simply stratify under the influence of gravity, which acts on all bodies located on the surface of the Earth or near it, including air molecules. At the bottom there would be a heavier layer of carbon dioxide, above it - oxygen, above - nitrogen and inert gases. But for normal life, we need oxygen, not carbon dioxide. Diffusion also occurs in the human body itself. Human respiration and digestion is based on diffusion. If we talk about respiration, then at each moment of time in the blood vessels braiding the alveoli, there is approximately 70 ml of blood, from which carbon dioxide diffuses into the alveoli, and oxygen diffuses in the opposite direction. The huge surface of the alveoli makes it possible to reduce the thickness of the layer of blood exchanging gases with the intraalveolar air to 1 micron, which makes it possible to saturate this amount of blood with oxygen and free it from excess carbon dioxide in less than 1 second.

This phenomenon also affects the human body - air oxygen penetrates into the blood capillaries of the lungs by diffusion through the walls of the alveoli, and then dissolving in them, it spreads throughout the body, enriching it with oxygen.

Diffusion is used in many technological processes: salting, sugar production (sugar beet shavings are washed with water, sugar molecules diffuse from the shavings into the solution), jam cooking, fabric dyeing, laundry, carburizing, welding and soldering of metals, including diffusion welding in vacuum (metals are welded that cannot be joined by other methods - steel with cast iron, silver with stainless steel, etc.) and diffusion metallization of products (surface saturation of steel products with aluminum, chromium, silicon), nitriding - saturation of the steel surface with nitrogen (steel becomes hard, wear-resistant), cementation - saturation of steel products with carbon, cyanidation - saturation of the steel surface with carbon and nitrogen.

As can be seen from the above examples, diffusion processes play a very important role in people's lives.

Problem: Why does diffusion proceed differently at different temperatures?

Relevance I see this study in the fact that the topic "Diffusion in liquid, solid and gaseous states" is vital not only for the course of physics. Knowledge of diffusion can be useful to me in everyday life. This information will help you prepare for the physics exam for the course of the primary and secondary schools. I really liked the topic, and I decided to study it deeper.

The object of my researchis the diffusion that occurs in water at different temperatures, andsubject of study– observations by setting up experiments in various temperature modes.

Objective:

1. Expand knowledge about diffusion, its dependence on various factors.
2. Explain the physical nature of the phenomenon of diffusion on the basis of the molecular structure of matter.
3. Find out the dependence of the diffusion rate on temperature in miscible liquids.
4. Confirm theoretical facts with experimental results.
5. Summarize the knowledge gained and develop recommendations.

Research objectives:

1. Investigate the rate of diffusion in water at different temperatures.
2. Prove that the evaporation of a liquid is the result of the movement of molecules

Hypothesis: At high temperatures, the molecules move faster and because of this, they mix faster.

The main part of the research work

For my research, I took two glasses. He poured warm water into one and cold water into the other. At the same time, he dropped a bag of tea into them. Warm water turned brown faster than cold water. It is known that in warm water the molecules move faster, since their speed depends on temperature. This means that tea molecules will quickly penetrate between water molecules. In cold water, the speed of molecules is slow, so the phenomenon of diffusion here proceeds more slowly. The phenomenon of the penetration of molecules of one substance between the molecules of another is called diffusion.

Then I poured the same amount of water into two glasses. I left one glass on the table in the room, and put the other in the refrigerator. Five hours later I compared the water levels. It turned out that in a glass from the refrigerator, the level practically did not change. In the second - the level has decreased markedly. This is due to the movement of molecules. And it is greater, the higher the temperature. At a higher speed, water molecules, approaching the surface, “jump out”. This movement of molecules is called evaporation. Experience has shown that evaporation proceeds faster at higher temperatures, since the faster the molecules move, the more molecules fly away from the liquid at the same time. In cold water, the speed is low, so they remain in the glass.

Conclusion:

Based on the experiment and observations of diffusion in water at different temperatures, I was convinced that temperature strongly affects the speed of molecules. This was evidenced by the varying degrees of evaporation. Thus, the hotter the substance, the greater the speed of the molecules. The colder it is, the slower the speed of the molecules. Therefore, diffusion in liquids will proceed faster at high temperature.

Literature:

1. A.V. Peryshkin. Physics grade 7. M.: Bustard, 2011.
2. Library "First of September". M .: "First of September", 2002.
3. Biophysics at physics lessons. From work experience. M., "Enlightenment", 1984.