What is thermal motion? What concepts are associated with it? Thermal movement. Temperature

In the world around us, there are various kinds of physical phenomena that are directly related to change in body temperature. Since childhood, we know that cold water, when heated, at first becomes barely warm and only after certain time hot.

With such words as “cold”, “hot”, “warm”, we define different degrees of “heating” of bodies, or, speaking in the language of physics, different temperatures of bodies. Temperature warm water slightly warmer than cold water. If we compare the temperature of summer and winter air, the difference in temperature is obvious.

Body temperature is measured with a thermometer and is expressed in degrees Celsius (°C).

As is known, diffusion at a higher temperature is faster. From this it follows that the speed of movement of molecules and temperature are deeply interconnected. If you increase the temperature, then the speed of movement of molecules will increase, if you decrease it, it will decrease.

Thus, we conclude: body temperature is directly related to the speed of movement of molecules.

Hot water consists of exactly the same molecules as cold water. The difference between them is only in the speed of movement of molecules.

Phenomena that are related to the heating or cooling of bodies, a change in temperature, are called thermal. These include heating or cooling air, melting metal, melting snow.

Molecules or atoms, which are the basis of all bodies, are in endless chaotic motion. The number of such molecules and atoms in the bodies around us is enormous. A volume equal to 1 cm³ of water contains approximately 3.34 x 10²² molecules. Any molecule has a very complex trajectory of motion. For example, gas particles moving at high speeds in different directions can collide both with each other and with the walls of the vessel. Thus, they change their speed and continue moving again.

Figure #1 shows the random movement of paint particles dissolved in water.

Thus, we make one more conclusion: the chaotic movement of the particles that make up bodies is called thermal motion.

Randomness is the most important feature of thermal motion. One of the most important evidence for the movement of molecules is diffusion and Brownian motion.(Brownian motion is the movement of the smallest solid particles in a liquid under the influence of molecular impacts. As observation shows, Brownian motion cannot stop).

In liquids, molecules can oscillate, rotate, and move relative to other molecules. If we take solids, then in them the molecules and atoms vibrate around some average positions.

Absolutely all molecules of the body participate in the thermal motion of molecules and atoms, which is why with a change in thermal motion, the state of the body itself, its various properties, also changes. Thus, if you increase the temperature of the ice, it begins to melt, while taking on a completely different form - the ice becomes a liquid. If, on the contrary, the temperature of, for example, mercury is lowered, then it will change its properties and turn from a liquid into a solid.

T body temperature directly depends on the average kinetic energy of the molecules. We draw an obvious conclusion: the higher the temperature of the body, the greater the average kinetic energy of its molecules. Conversely, as the body temperature decreases, the average kinetic energy of its molecules decreases.

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What do you think determines the rate of dissolution of sugar in water? You can do a simple experiment. Take two pieces of sugar and throw one into a glass of boiling water, the other into a glass of cold water.

You will see how sugar dissolves in boiling water several times faster than in cold water. The cause of dissolution is diffusion. This means that diffusion occurs faster at higher temperatures. Diffusion is caused by the movement of molecules. Therefore, we conclude that molecules move faster at higher temperatures. That is, the speed of their movement depends on temperature. That is why the random chaotic motion of the molecules that make up the body is called thermal motion.

Thermal motion of molecules

As the temperature rises, it increases thermal motion molecules, the properties of matter change. The solid melts, turning into a liquid, the liquid evaporates, turning into a gaseous state. Accordingly, if the temperature is lowered, then the average energy of the thermal motion of molecules will also decrease, and accordingly, the processes of changing the state of aggregation of bodies will occur in the opposite direction: water will condense into a liquid, the liquid will freeze, turning into a solid state. At the same time, we are always talking about the average values of temperature and molecular velocity, since there are always particles with larger and smaller values of these values.

Molecules in substances move, passing a certain distance, therefore, do some work. That is, we can talk about the kinetic energy of particles. As a result of their relative position there is also the potential energy of molecules. When in question about the kinetic and potential energy of bodies, then we are talking about the existence of the total mechanical energy of bodies. If the particles of the body have kinetic and potential energy, therefore, we can talk about the sum of these energies as an independent quantity.

Internal energy of the body

Consider an example. If we throw an elastic ball on the floor, then the kinetic energy of its movement is completely converted into potential energy at the moment it touches the floor, and then again goes into kinetic energy when it bounces. If we throw a heavy iron ball on a hard, inelastic surface, then the ball will land without bouncing. Its kinetic and potential energies after landing will be equal to zero. Where has the energy gone? Did she just disappear? If we examine the ball and the surface after the collision, we can see that the ball flattened a bit, a dent was left on the surface, and both of them warmed up slightly. That is, there was a change in the arrangement of the molecules of the bodies, and the temperature also increased. This means that the kinetic and potential energies of the particles of the body have changed. The energy of the body has not gone anywhere, it has passed into the internal energy of the body. Internal energy is called the kinetic and potential energy of all particles of the body. The collision of the bodies caused a change in the internal energy, it increased, and the mechanical energy decreased. This is what it consists

Topics of the USE codifier: thermal motion of atoms and molecules of matter, Brownian motion, diffusion, interaction of matter particles, experimental evidence of atomistic theory.

The great American physicist Richard Feynman, the author of the famous Feynman Lectures on Physics, wrote the following wonderful words:

– If, as a result of some global catastrophe, all the accumulated scientific knowledge would be destroyed and only one phrase would pass to the coming generations of living beings, then what statement, composed of least quantity words, would bring the most information? I think that is atomic hypothesis(you can call it not a hypothesis, but a fact, but this does not change anything): all bodies are made up of atoms of small bodies that are in constant motion, attract at a short distance, but repel if one of them is pressed closer to the other. That one sentence... contains an incredible amount of information about the world, you just have to apply a little imagination and a little consideration to it.

These words contain the essence of the molecular-kinetic theory (MKT) of the structure of matter. Namely, the main provisions of the MKT are the following three statements.

1. Any substance consists of the smallest particles of molecules and atoms. They are located discretely in space, that is, at certain distances from each other.
2. Atoms or molecules of a substance are in a state of random movement (this movement is called thermal movement), which never stops.
3. Atoms or molecules of a substance interact with each other by forces of attraction and repulsion, which depend on the distances between the particles.

These provisions are a generalization of numerous observations and experimental facts. Let's take a closer look at these provisions and give their experimental justification.

For example, is a water molecule consisting of two hydrogen atoms and one oxygen atom. Dividing it into atoms, we will no longer deal with a substance called "water". Further, by dividing the atoms and into component parts, we get a set of protons, neutrons and electrons and thereby lose the information that at first these were hydrogen and oxygen.

Atoms and molecules are called simply particles substances. What exactly is a particle - an atom or a molecule - in each specific case is not difficult to establish. If it's about chemical element, then the particle will be an atom; if considered complex substance, then its particle is a molecule consisting of several atoms.

Further, the first proposition of the MKT states that particles of matter do not fill space continuously. The particles are arranged discretely, that is, at separate points. Between the particles there are gaps, the size of which can vary within certain limits.

In favor of the first position of the MKT is the phenomenon thermal expansion tel. Namely, when heated, the distances between the particles of the substance increase, and the dimensions of the body increase. On cooling, on the contrary, the distances between the particles decrease, as a result of which the body contracts.

A striking confirmation of the first position of the MKT is also diffusion- mutual penetration of adjoining substances into each other.

For example, in fig. 1 shows the process of diffusion in a liquid. The particles of the solute are placed in a glass of water and are located first in the upper left part of the glass. Over time, the particles move (as they say, diffuse) from an area of high concentration to an area of low concentration. In the end, the concentration of particles becomes the same everywhere - the particles are evenly distributed throughout the entire volume of the liquid.

Rice. 1. Diffusion in a liquid

How to explain diffusion from the point of view of molecular-kinetic theory? Very simply: particles of one substance penetrate into the gaps between the particles of another substance. Diffusion goes the faster, the larger these gaps - therefore, gases are most easily mixed with each other (in which the distances between particles are many more sizes particles themselves).

Thermal motion of atoms and molecules

Recall once again the wording of the second provision of the MKT: the particles of matter perform random motion (also called thermal motion) that never stops.

Experimental confirmation of the second position of the MKT is again the phenomenon of diffusion, because the mutual penetration of particles is possible only with their continuous movement! But the most striking evidence of the eternal chaotic motion of particles of matter is Brownian motion. This is the name of the continuous erratic movement brownian particles- dust particles or grains (cm in size) suspended in a liquid or gas.

Brownian motion got its name in honor of the Scottish botanist Robert Brown, who saw through a microscope the continuous dance of pollen particles suspended in water. As proof that this movement takes forever, Brown found a piece of quartz with a cavity filled with water. Despite the fact that water got there many millions of years ago, the motes that got there continued their movement, which was no different from what was observed in other experiments.

The reason for Brownian motion is that a suspended particle experiences uncompensated impacts from liquid (gas) molecules, and due to the chaotic motion of molecules, the magnitude and direction of the resulting impact are absolutely unpredictable. Therefore, a Brownian particle describes complex zigzag trajectories (Fig. 2).

Rice. 2. Brownian motion

By the way, Brownian motion can also be considered as proof of the very fact of the existence of molecules, i.e., it can also serve as an experimental substantiation of the first position of the MKT.

Interaction of particles of matter

The third position of the MKT speaks of the interaction of particles of matter: atoms or molecules interact with each other by forces of attraction and repulsion, which depend on the distances between particles: as the distances increase, the forces of attraction begin to predominate, and as the distances decrease, the repulsive forces.

The validity of the third position of the MKT is evidenced by the elastic forces arising from the deformations of bodies. When a body is stretched, the distances between its particles increase, and the forces of attraction of particles to each other begin to prevail. When a body is compressed, the distances between particles decrease, and as a result, repulsive forces predominate. In both cases, the elastic force is directed in the direction opposite to the deformation.

Another confirmation of the existence of forces of intermolecular interaction is the presence of three aggregate states of matter.

In gases, the molecules are separated from each other by distances significantly exceeding the dimensions of the molecules themselves (in air under normal conditions, by about 1000 times). At such distances, the forces of interaction between molecules are practically absent, therefore gases occupy the entire volume provided to them and are easily compressed.

In liquids, the spaces between molecules are comparable to the size of the molecules. The forces of molecular attraction are very tangible and ensure the preservation of volume by liquids. But these forces are not strong enough for liquids to preserve their shape - liquids, like gases, take the form of a vessel.

In solids, the forces of attraction between particles are very strong: solid bodies retain not only volume, but also shape.

The transition of a substance from one state of aggregation to another is the result of a change in the magnitude of the forces of interaction between the particles of the substance. The particles themselves remain unchanged.

This lesson discusses the concept of thermal motion and such a physical quantity as temperature.

Thermal phenomena in human life are of great importance. We encounter them both during the weather forecast and during the boiling of ordinary water. Thermal phenomena are associated with such processes as the creation of new materials, the melting of metals, the combustion of fuel, the creation of new types of fuel for cars and aircraft, etc.

Temperature is one of the most important concepts associated with thermal phenomena, since often it is temperature that is the most important characteristic of the course of thermal processes.

Definition.thermal phenomena- these are phenomena associated with the heating or cooling of bodies, as well as with a change in their state of aggregation (Fig. 1).

Rice. 1. Ice melting, water heating and evaporation

All thermal phenomena are associated with temperature.

All bodies are characterized by the state of their thermal equilibrium. Main characteristic thermal equilibrium is temperature.

Definition.Temperature is a measure of the "warmth" of the body.

Since temperature is a physical quantity, it can and should be measured. An instrument used to measure temperature is called thermometer(from Greek. thermo- "warmly", metreo- “I measure”) (Fig. 2).

Rice. 2. Thermometer

The first thermometer (or rather, its analogue) was invented by Galileo Galilei (Fig. 3).

Rice. 3. Galileo Galilei (1564-1642)

The invention of Galileo, which he presented to his students at lectures at the university at the end of the 16th century (1597), was called thermoscope. The operation of any thermometer is based on the following principle: physical properties substances change with temperature.

Galileo's experience consisted of the following: he took a flask with a long stem and filled it with water. Then he took a glass of water and turned the flask upside down, placing it in a glass. Part of the water, of course, spilled out, but as a result, a certain level of water remained in the leg. If now the flask (which contains air) is heated, then the water level will drop, and if it is cooled, then, on the contrary, it will rise. This is due to the fact that when heated, substances (in particular, air) tend to expand, and when cooled, they narrow (which is why the rails are made discontinuous, and the wires between the poles sometimes sag a little).

Rice. 4. Experience of Galileo

This idea formed the basis of the first thermoscope (Fig. 5), which made it possible to estimate the change in temperature (it is impossible to accurately measure the temperature with such a thermoscope, since its readings will strongly depend on atmospheric pressure).

Rice. 5. Copy of Galileo's thermoscope

At the same time, the so-called degree scale was introduced. The very word degree in Latin means "step".

To date, three main scales have survived.

1. Celsius

The most widely used scale, which is known to everyone since childhood, is the Celsius scale.

Anders Celsius (Fig. 6) - Swedish astronomer, who proposed the following temperature scale: - boiling point of water; - freezing point of water. Nowadays, we are all used to the inverted Celsius scale.

Rice. 6 Andres Celsius (1701-1744)

Note: Celsius himself said that such a choice of the scale was caused by a simple fact: on the other hand, there would be no negative temperature in winter.

2. Fahrenheit scale

England, USA, France, Latin America and some other countries, the Fahrenheit scale is popular.

Gabriel Fahrenheit (Fig. 7) is a German researcher, engineer who first applied his own scale to glass making. The Fahrenheit scale is thinner: the dimension of the Fahrenheit scale is less than the degree of the Celsius scale.

Rice. 7 Gabriel Fahrenheit (1686-1736)

3. Réaumur scale

The technical scale was invented by the French researcher R.A. Réaumur (Fig. 8). According to this scale, it corresponds to the freezing point of water, but Réaumur chose a temperature of 80 degrees as the boiling point of water.

Rice. 8. René Antoine Réaumur (1683-1757)

In physics, the so-called absolute scale - Kelvin scale(Fig. 8). 1 degree Celsius is equal to 1 degree Kelvin, but the temperature in corresponds approximately (Fig. 9).

Rice. 9. William Thomson (Lord Kelvin) (1824-1907)

Rice. 10. Temperature scales

Recall that when the body temperature changes, its linear dimensions(when heated, the body expands, when cooled, it narrows). It has to do with the behavior of the molecules. When heated, the speed of movement of particles increases, respectively, they begin to interact more often and the volume increases (Fig. 11).

Rice. 11. Changing linear dimensions

From this we can conclude that temperature is associated with the movement of particles that make up bodies (this applies to solid, liquid, and gaseous bodies).

The movement of particles in gases (Fig. 12) is random (since molecules and atoms in gases practically do not interact).

Rice. 12. Movement of particles in gases

The movement of particles in liquids (Fig. 13) is "jumping", that is, the molecules lead " sedentary life", but are able to "jump" from one place to another. This determines the fluidity of liquids.

Rice. 13. Movement of particles in liquids

The motion of particles in solids (Fig. 14) is called oscillatory.

Rice. 14. Motion of particles in solids

Thus, all particles are in continuous motion. This movement of particles is called thermal motion(random, chaotic movement). This movement never stops (as long as the body has a temperature). The presence of thermal motion was confirmed in 1827 by the English botanist Robert Brown (Fig. 15), after whom this motion is called brownian motion.

Rice. 15. Robert Brown (1773-1858)

To date, it is known that the low temperature, which can be achieved is approximately . It is at this temperature that the movement of particles stops (however, the movement inside the particles themselves does not stop).

Galileo's experience was described earlier, and in conclusion we will consider another experience - the experience of the French scientist Guillaume Amonton (Fig. 15), who in 1702 invented the so-called gas thermometer. With minor changes, this thermometer has survived to this day.

Rice. 15. Guillaume Amonton (1663-1705)

Amonton experience

Rice. 16. Experience of Amonton

Take a flask with water and plug it with a stopper with a thin tube. If you now heat the water, then due to the expansion of the water, its level in the tube will increase. According to the level of water rise in the tube, it is possible to draw a conclusion about the change in temperature. Advantage Amonton thermometer is that it does not depend on atmospheric pressure.

In this lesson, we have considered such an important physical quantity, as temperature. We studied the methods of its measurement, characteristics and properties. In the next lesson, we will explore the concept internal energy .

Bibliography

Gendenstein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

Internet portal "class-fizika.narod.ru" ()
Internet portal "school.xvatit.com" ()
Internet portal "ponimai.su" ()

Homework

1. No. 1-4 (paragraph 1). Peryshkin A.V. Physics 8. - M.: Bustard, 2010.

2. Why can't Galileo's thermoscope be calibrated?

3. Iron nail heated on the stove:

How has the speed of iron molecules changed?

How will the speed of movement of molecules change if the nail is lowered into cold water?

How does this change the speed of water molecules?

How does the volume of the nail change during these experiments?

4. Balloon moved out of the room into the cold:

How will the volume of the ball change?

How will the speed of movement of air molecules inside the balloon change?

How will the speed of the molecules inside the ball change if it is returned to the room and, in addition, put to the battery?

IV Yakovlev | Materials on physics | MathUs.ru

Molecular physics and thermodynamics

This manual is devoted to the second section ¾Molecular physics. Thermodynamics¿ of the USE codifier in physics. It covers the following topics.

Thermal motion of atoms and molecules of matter. Brownian motion. Diffusion. Experimental evidence of atomistic theory. Interaction of particles of matter.

Models of the structure of gases, liquids and solids.

Ideal gas model. Relationship between pressure and average kinetic energy of thermal motion of ideal gas molecules. absolute temperature. Connection of gas temperature with average kinetic energy of its particles. Equation p = nkT . Mendeleev's equation of Clapeyron.

Isoprocesses: isothermal, isochoric, isobaric, adiabatic processes.

Saturated and unsaturated pairs. Air humidity.

Changes in the aggregate states of matter: evaporation and condensation, liquid boiling, melting and crystallization. Energy change in phase transitions.

Internal energy. Thermal balance. Heat transfer. Quantity of heat. Specific heat substances. Heat balance equation.

Work in thermodynamics. First law of thermodynamics.

Principles of operation of thermal machines. heat engine efficiency. The second law of thermodynamics. Problems of energy and environmental protection.

The manual also contains some additional material that is not included in USE codifier(but included in school curriculum!). This material allows you to better understand the topics covered.

1.2 . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.3 Liquids . . . . . . ten

	Basic formulas of molecular physics
	Temperature
		Thermodynamic system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
		Thermal equilibrium. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
		temperature scale. Absolute temperature . . . . . . . . . . . . . . . . . . .
	Ideal gas equation of state
		Average kinetic energy of gas particles. . . . . . . . . . . . . . . . . . . . . .

5.2 The basic equation of the MKT of an ideal gas. . . . . . . . . . . . . . . . . . . . . . . 16

5.3 Particle energy and gas temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . 16

6.1 Thermodynamic process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.2 Isothermal process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

6.3 Isothermal Process Graphs. . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6.4 Isobaric process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

6.5 Plots of the isobaric process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

	Isochoric process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	Isochoric Process Plots. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7 Saturated steam

7.1 Evaporation and condensation

7.2 dynamic balance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7.3 Saturated steam properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

8.1 Internal energy of a monatomic ideal gas. . . . . . . . . . . . . . . . . . 29

8.2 Status function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

8.3 Change in internal energy: doing work. . . . . . . . . . . . . . . . . . 30

8.4 Change in internal energy: heat transfer . . . . . . . . . . . . . . . . . . . . 30

8.5 Thermal conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

10 Phase transitions

10.1 Melting and crystallization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

10.2 Melting chart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

10.3 Specific heat of fusion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

10.4 Crystallization chart. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

10.5 Vaporization and condensation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

10.6 Boiling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

10.7 Boiling schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

10.8 Condensation curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11 First law of thermodynamics

11.1 The work of a gas in an isobaric process. . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

11.2 Gas work in an arbitrary process. . . . . . . . . . . . . . . . . . . . . . . . . . 45

11.3 Work done on gas. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

11.4 First law of thermodynamics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

11.5 Application of the first law of thermodynamics to isoprocesses. . . . . . . . . . . . . 46

11.6 adiabatic process. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

12.1 Heat engines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

12.2 Refrigeration machines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

13.1 Irreversibility of processes in nature. . . . . . . . . . . . . . . . . . . . . . . . . . . 54

13.2 Postulates of Clausius and Kelvin. . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

1 Key points molecular kinetic theory

The great American physicist Richard Feynman, the author of the famous course ¾Feynman Lectures on Physics¿, owns wonderful words:

If, as a result of some kind of global catastrophe, all the accumulated scientific knowledge would be destroyed and only one phrase would pass to the future generations of living beings, then what statement, composed of the smallest number of words, would bring the most information? I believe that this is an atomic hypothesis (you can call it not a hypothesis, but a fact, but this does not change anything): all bodies are composed of atoms of small bodies that are in constant motion, attract at a small distance, but repel if one of them press harder on the other. In this one sentence. . . contains an incredible amount of information about the world, you just have to put a little imagination and a little thought into it.

These words contain the essence of the molecular-kinetic theory (MKT) of the structure of matter. Namely, the main provisions of the MKT are the following three statements.

1. Any substance consists of the smallest particles of molecules and atoms. They are located discretely in space, that is, at certain distances from each other.

2. Atoms or molecules of matter are in a state of random motion 1 , which never terminates.

3. Atoms or molecules of a substance interact with each other by forces of attraction and repulsion, which depend on the distances between the particles.

These provisions are a generalization of numerous observations and experimental facts. Let's take a closer look at these provisions and give their experimental justification.

1.1 Atoms and molecules

Let's take a piece of paper and start dividing it into smaller and smaller parts. Will we get pieces of paper at each step, or will something new appear at some stage?

The first position of the MKT tells us that matter is not infinitely divisible. Sooner or later we will reach ¾ last frontier¿ the smallest particles of a given substance. These particles are atoms and molecules. They can also be divided into parts, but then the original substance will cease to exist.

An atom is the smallest particle of a given chemical element that retains all of its Chemical properties. There are not so many chemical elements; they are all summarized in the periodic table.

A molecule is the smallest particle of a given substance (not being a chemical element) that retains all of its chemical properties. A molecule is made up of two or more atoms of one or more chemical elements.

For example, H2O is a water molecule composed of two hydrogen atoms and one oxygen atom. By dividing it into atoms, we will no longer deal with a substance called ¾water¿. Further, by dividing the H and O atoms into their component parts, we get a set of protons, neutrons and electrons, and thereby lose the information that at first it was hydrogen and oxygen.

1 This movement is called thermal movement.

The size of an atom or molecule (consisting of a small number of atoms) is about 10 8 cm. This is such a small value that the atom cannot be seen with any optical microscope.

Atoms and molecules are called, for short, simply particles of matter. What exactly is a particle an atom or a molecule in each particular case is not difficult to establish. If we are talking about a chemical element, then an atom will be a particle; if a complex substance is considered, then its particle is a molecule consisting of several atoms.

Further, the first proposition of the MKT states that particles of matter do not fill space continuously. The particles are located discretely, that is, as if at separate points. Between the particles there are gaps, the size of which can vary within certain limits.

The phenomenon of thermal expansion of bodies testifies in favor of the first position of the MKT. Namely, when heated, the distances between the particles of the substance increase, and the dimensions of the body increase. On cooling, on the contrary, the distances between the particles decrease, as a result of which the body contracts.

Diffusion, the mutual penetration of contacting substances into each other, is also a striking confirmation of the first position of the MKT.

For example, in fig. 1 shows2 the process of diffusion in a liquid. The particles of the solute are placed in a glass of water and are located first in the upper left part of the glass. Over time, particles move (say, diffuse) from a region of high concentration to a region of low concentration. In the end, the concentration of particles becomes the same everywhere; the particles are evenly distributed throughout the volume of the liquid.

Rice. 1. Diffusion in a liquid

How to explain diffusion from the point of view of molecular-kinetic theory? Very simply: particles of one substance penetrate into the gaps between the particles of another substance. Diffusion goes the faster, the larger these gaps are; therefore, gases are most easily mixed with each other (in which the distances between particles are much larger than the sizes of the particles themselves).

1.2 Thermal motion of atoms and molecules

Recall once again the formulation of the second proposition of the MKT: the particles of matter perform random motion (also called thermal motion), which never stops.

Experimental confirmation of the second position of the MKT is again the phenomenon of diffusion, because the mutual penetration of particles is possible only with their continuous movement!

2 Image from en.wikipedia.org.

But the most striking proof of the eternal chaotic motion of particles of matter is Brownian motion. This is the name of the continuous random movement of Brownian particles of dust particles or grains (10 5 - 104 cm in size) suspended in a liquid or gas.

Rice. 2. Brownian motion

The size of Brownian particles is 1000–10000 times the size of an atom. On the one hand, a Brownian particle is small enough and still “feels” that a different number of molecules hit it in different directions; this difference in the number of impacts leads to noticeable displacements of the Brownian particle. On the other hand, Brownian particles are large enough to be seen with a microscope.

By the way, Brownian motion can also be considered as proof of the very fact of the existence of molecules, i.e., it can also serve as an experimental substantiation of the first position of the MKT.

1.3 Interaction of particles of matter

The third position of the MKT speaks of the interaction of particles of a substance: atoms or molecules interact with each other by forces of attraction and repulsion, which depend on the distances between the particles: as the distances increase, the forces of attraction begin to prevail, with a decrease in the repulsive force.

3 Image from the site nv-magadan.narod.ru.

Another confirmation of the existence of forces of intermolecular interaction is the presence of three aggregate states of matter.

AT In gases, the molecules are separated from each other by distances significantly exceeding the dimensions of the molecules themselves (in air under normal conditions, by about 1000 times). At such distances, the forces of interaction between molecules are practically absent, therefore gases occupy the entire volume provided to them and are easily compressed.

AT In liquids, the spaces between molecules are comparable to the size of the molecules. The forces of molecular attraction are very tangible and ensure the preservation of volume by liquids. But these forces are not strong enough for liquids to retain their form, and liquids, like gases, take the form of a vessel.

AT In solids, the forces of attraction between particles are very strong: solids retain not only volume, but also shape.