What are the features of the molecular structure of solids. Question: what are the features of the molecular structure of gases, liquids and solids? urgently

The structure of gases, liquids and solids.

Basic Provisions of Molecular Kinetic Theory:

All substances are made up of molecules, and molecules are made up of atoms.

atoms and molecules are in constant motion,

There are attractive and repulsive forces between molecules.

AT gases the molecules move randomly, the distances between the molecules are large, the molecular forces are small, the gas occupies the entire volume provided to it.

AT liquids molecules are ordered only at small distances, and at large distances the order (symmetry) of the arrangement is violated - “short range order”. The forces of molecular attraction keep molecules close together. The movement of molecules is “jumps” from one stable position to another (usually within one layer. This movement explains the fluidity of a liquid. A liquid has no shape, but has volume.

Solids - substances that retain their shape, are divided into crystalline and amorphous. crystalline solid bodies have a crystal lattice, in the nodes of which there can be ions, molecules or atoms. They oscillate relative to stable equilibrium positions. Crystal lattices have a regular structure throughout the volume - a “long-range order” of location.

Amorphous bodies retain their shape, but do not have a crystal lattice and, as a result, do not have a pronounced melting point. They are called frozen liquids, since they, like liquids, have a “near” order of molecular arrangement.

Interaction forces of molecules

All molecules of a substance interact with each other by forces of attraction and repulsion. Proof of the interaction of molecules: the phenomenon of wetting, resistance to compression and stretching, low compressibility of solids and gases, etc. The reason for the interaction of molecules is the electromagnetic interaction of charged particles in matter. How to explain it? An atom consists of a positively charged nucleus and a negatively charged electron shell. The charge of the nucleus is equal to the total charge of all electrons, therefore, as a whole, the atom is electrically neutral. A molecule consisting of one or more atoms is also electrically neutral. Consider the interaction between molecules using the example of two immobile molecules. Gravitational and electromagnetic forces can exist between bodies in nature. Since the masses of molecules are extremely small, the negligible forces of gravitational interaction between molecules can be ignored. At very large distances, there is no electromagnetic interaction between molecules either. But, with a decrease in the distance between the molecules, the molecules begin to orient themselves so that their sides facing each other will have charges of different signs (in general, the molecules remain neutral), and attractive forces arise between the molecules. With an even greater decrease in the distance between the molecules, repulsive forces arise as a result of the interaction of negatively charged electron shells of the atoms of the molecules. As a result, the molecule is affected by the sum of the forces of attraction and repulsion. At large distances, the attractive force prevails (at a distance of 2-3 molecular diameters, attraction is maximum), at short distances, the repulsive force. There is such a distance between molecules at which the forces of attraction become equal to the forces of repulsion. This position of the molecules is called the position of stable equilibrium. Molecules located at a distance from each other and connected by electromagnetic forces have potential energy. In the position of stable equilibrium, the potential energy of molecules is minimal. In a substance, each molecule interacts simultaneously with many neighboring molecules, which also affects the value of the minimum potential energy of molecules. In addition, all the molecules of a substance are in continuous motion, i.e. have kinetic energy. Thus, the structure of a substance and its properties (solid, liquid and gaseous bodies) are determined by the ratio between the minimum potential energy of interaction of molecules and the kinetic energy of the thermal motion of molecules.

The structure and properties of solid, liquid and gaseous bodies

The structure of bodies is explained by the interaction of body particles and the nature of their thermal motion.

Solid

Solid bodies have permanent form and volume are practically incompressible. The minimum potential energy of interaction of molecules is greater than the kinetic energy of molecules. Strong interaction of particles. The thermal motion of molecules in a solid is expressed only by oscillations of particles (atoms, molecules) around the position of stable equilibrium.

Due to the large forces of attraction, molecules practically cannot change their position in a substance, which explains the invariance of the volume and shape of solids. Most solids have a spatially ordered arrangement of particles that form a regular crystal lattice. Particles of matter (atoms, molecules, ions) are located at the vertices - the nodes of the crystal lattice. The nodes of the crystal lattice coincide with the position of stable equilibrium of the particles. Such solids are called crystalline.

Liquid

Liquids have a certain volume, but do not have their own shape, they take the shape of the vessel in which they are located. The minimum potential energy of interaction of molecules is comparable to the kinetic energy of molecules. Weak particle interaction. The thermal motion of molecules in a liquid is expressed by oscillations around the position of stable equilibrium within the volume provided to the molecule by its neighbors. Molecules cannot move freely throughout the entire volume of a substance, but transitions of molecules to neighboring places are possible. This explains the fluidity of the liquid, the ability to change its shape.

In liquids, the molecules are quite strongly bound to each other by attractive forces, which explains the invariance of the volume of the liquid. In a liquid, the distance between molecules is approximately equal to the diameter of the molecule. With a decrease in the distance between molecules (compressing a liquid), the repulsive forces sharply increase, so liquids are incompressible. In terms of their structure and nature of thermal motion, liquids occupy an intermediate position between solids and gases. Although the difference between a liquid and a gas is much greater than between a liquid and a solid. For example, during melting or crystallization, the volume of a body changes many times less than during evaporation or condensation.

Gases do not have a constant volume and occupy the entire volume of the vessel in which they are located. The minimum potential energy of interaction of molecules is less than the kinetic energy of molecules. Particles of matter practically do not interact. Gases are characterized by a complete disorder in the arrangement and movement of molecules.

The distance between gas molecules is many times greater than the size of the molecules. Small forces of attraction cannot keep molecules near each other, so gases can expand indefinitely. Gases are easily compressed under the action of external pressure, because. the distances between molecules are large, and the interaction forces are negligible. The pressure of the gas on the walls of the vessel is created by the impacts of moving gas molecules.

Ordinary liquids are isotropic, structurally they are amorphous bodies. For internal structure liquids are characterized by a short-range order in the arrangement of molecules (an ordered arrangement of the nearest particles). The distances between molecules are small, the interaction forces are significant, which leads to low compressibility of liquids: a small decrease in the distance between molecules causes the appearance of large intermolecular repulsion forces.

Like solids, liquids are slightly compressible and have a high density; like gases, they take the shape of the vessel in which they are located. This nature of the properties of liquids is associated with the peculiarities of the thermal motion of their molecules. In gases, molecules move randomly, on short distances they move forward, and there is no order in the arrangement of particles. In crystalline bodies, particles oscillate around certain equilibrium positions - the nodes of the crystal lattice. According to the theory of Ya. I. Frenkel, the molecules of a liquid, like the particles of a solid body, oscillate around the equilibrium positions, but these equilibrium positions are not constant. After some time, called the time of "settled life", the molecule jumps to a new equilibrium position at a distance equal to the average distance between neighboring molecules.

Let us calculate the average distance between liquid molecules. You can mentally imagine the entire volume of liquid divided into small identical cubes with an edge of 8. Let, on average, there is one molecule in each cube. In this case, 5 can be considered as the average distance between liquid molecules. The volume of the liquid is V = δ 3 N, where N is the total number of liquid molecules. If n is the concentration of molecules (the number of molecules in 1 m 3), then N \u003d nV. From these equations we get

In order for a liquid molecule to jump from one equilibrium position to another, bonds with surrounding molecules must be broken and bonds with new neighbors formed. The process of breaking bonds requires the expenditure of energy E a (activation energy) released during the formation of new bonds. Such a transition of a molecule from one equilibrium position to another is a transition through a potential barrier of height E a. The molecule receives energy to overcome the potential barrier due to the energy of the thermal motion of neighboring molecules. The dependence of the relaxation time on the liquid temperature and activation energy is expressed by a formula following from the Boltzmann distribution (see § 2.4).

Where τ 0 is the average oscillation period of the molecule around the equilibrium position.

Knowing the average displacement of a molecule, equal to the distance between molecules δ, and the average time τ, we can determine the average speed of movement of molecules in a liquid:

This speed is small compared to average speed movement of molecules in a gas. So, for example, for water molecules it is 20 times less than for vapor molecules at the same temperature.

Surface tension

On the interfaces of a liquid and its saturated vapor, two immiscible liquids, a liquid and a solid, forces arise due to various intermolecular interactions of the adjacent media.

Each molecule located inside the liquid volume is uniformly surrounded by neighboring molecules and interacts with them, but the resultant of these forces is zero. Due to the inhomogeneity of the environment, a molecule located near the boundary of two media is affected by a force that is not compensated by other molecules of the liquid. Therefore, to move molecules from the volume to the surface layer, work must be done.

Surface tension (coefficient of surface tension) is determined by the ratio of the work expended on creating a certain surface of a liquid at a constant temperature to the area of this surface:

The condition for stable equilibrium of liquids is the minimum energy of the surface layer, therefore, in the absence of external forces or in a state of weightlessness, the liquid tends to have the Minimum surface area for a given volume and takes the form of a ball.

Surface tension can be determined not only energetically. The desire of the surface layer of the liquid to shrink means the presence of tangential forces in this layer - surface tension forces. If you choose a segment of length l on the surface of the liquid (Fig. 7.8), then you can conditionally depict these forces with arrows perpendicular to the segment.

DISTRIBUTION OF MOLECULES IN A POTENTIAL FIELD

GRAVITY FORCES (BOLTZMANN DISTRIBUTION)

When deriving the basic equation of the MKT of gases and the Maxwell distribution, it was assumed that external forces do not act on gas molecules, which means that the molecules are distributed uniformly over the volume. However, the molecules of any gas are always in the potential field of the Earth's gravity. Gravity, on the one hand, and the thermal motion of molecules, on the other hand, lead to a certain stationary state, in which the gas pressure decreases with increasing height.

Let's get the law of pressure change with height, assuming that over the entire height: the gravitational field is uniform (g = const); the temperature is the same (T = const); the masses of all molecules are the same.

Let the pressure p be at height h. Then at height h + dh the pressure is p + dp. Moreover, if dh >0, then dp< 0. (р + dp) – р = – r·g·dh. Из уравнения состояния Менделеева-Клапейрона, имеем:

Now or .

Let's integrate the right and left sides:

; .

Where, . (26)

This is the so-called barometric formula. It allows you to determine the pressure of the atmosphere as a function of altitude above sea level:

. (27)

Because pressure is directly proportional to the concentration of molecules, then you can get the law of change in the concentration of molecules with height, provided that the temperature does not change with height (T = const):

. (28)

Considering that M = m∙N A , and R = k∙N A from (27) we get:

Because mgh = U(h) is the potential energy of one molecule at height h, then

(30)

is the Boltzmann distribution.

NUMBER OF COLLISIONS AND AVERAGE FREE PATH OF IDEAL GAS MOLECULES.

As a result of chaotic motion, gas molecules continuously collide with each other. Between two successive collisions, the molecule travels a certain path λ, which is called the mean free path . In the general case, the length of this path is different, but since the number of collisions is very large, and the movement is random, then under constant external conditions we can talk about the average free path - . If the molecules of a given gas experience 1 second on average collisions, then

where is the arithmetic mean velocity of molecules.

We consider ideal gas molecules as spheres. Obviously, a collision will occur if two molecules approach up to a distance equal to two radii, i.e., the diameter of the molecules d. The minimum distance that the centers of two molecules approach during a collision is called the effective diameter of the molecules. This parameter depends on , and hence on the gas temperature.

To define, imagine a molecule with an effective diameter d, which moves with a speed among other molecules, which at the same time remain motionless. This molecule will collide with all molecules whose centers lie inside a "broken" cylinder of radius d. This means that is equal to the number of molecules in the volume of this cylinder

where n is the concentration of molecules, and V is the volume of the cylinder: . With this in mind -

. (32)

Taking into account the motion of other molecules increases the number of collisions by a factor. Finally, for z we get:

. (33)

Then (34)

Because p ~ n, then for different external conditions we have:

For air at n.o. (p \u003d 760 mm Hg; t 0 \u003d 0 0 С): z \u003d 10 9 s -1, a \u003d 5 ∙ 10 -8 m.

TRANSFER PHENOMENA

In thermodynamically nonequilibrium systems, i.e. in systems for which the values of macroparameters (T, p, ) are different at its different points, irreversible processes occur, which are called transport phenomena . As a result of such processes, energy is transferred from one local area of the system to another (the phenomenon of thermal conductivity), mass (the phenomenon of diffusion), momentum (internal friction), charge, etc. This leads to the alignment of the values of macroparameters by the volume of the system. It is clear that the transfer of any value is explained by the transition from place to place of a certain number of particles (molecules and atoms) as a result of their chaotic movement.

We obtain the general transport equation along an arbitrary direction. Let's direct the axis O along it X(Figure 3). Let us mentally single out an element of the plane with area ∆S, perpendicular to O X. Due to the randomness of the movement during the time ∆t through ∆S in the direction of O X move N particles:

(1)

Here n is the concentration of molecules (atoms), and is their arithmetic mean velocity. Passing through ∆S, each molecule transfers its inherent mass, charge, momentum, energy, or some other of its characteristics. Let us denote the value of the quantity carried by one molecule by the letter φ. Then during the time ∆t through the area ∆S in the direction of the O axis X quantity will be transferred physical quantity

(2).

Obviously, if the concentration on the right is also n, then the same number of particles will move from right to left. Those. the resulting carry in this case zero: ∆N = 0 and ∆Nφ = 0.

If the medium is inhomogeneous, i.e. either the concentration of particles or the values of φ for particles on the left and right are not the same, then transitions from regions where the value of (nφ) is larger to the region where it is smaller will be more likely. If we assume that (nφ) 1 > (nφ) 2, then the resulting transfer of the value of φ will be determined by the relation: . (3)

The minus sign in (3) reflects the fact that the value (nφ) decreases in the transfer direction.

Let us find out at what distance from ∆S on the left and right the values (nφ) should be taken. Because change physical characteristics molecules occurs only during collisions, and before the collision each of the molecules has traveled a distance equal to the free path, then we can assume that (nφ) molecules remain unchanged at a distance equal to the free path to the left and right of ∆S. Divide and multiply the right side of (3) by 2:

The distribution of quantities along any direction is determined by a characteristic called the gradient. A gradient is a change in magnitude with a distance equal to one length .

In this case, at the point with coordinate X 2 the value of the transferable value is (nφ) 2, and at the point X 1 – (nφ) 1 , then under the gradient of the value nφ, transferred along the O axis X, one should understand the relationship:

Then the gradient of nφ in the region ∆S.

. (5)

(5) is the general transfer equation.

Diffusion is the transfer of mass of matter . Provided that the masses of the molecules are the same (m 0 = const), the gas temperature is the same in volume (T = const) and the distribution of velocities is uniform over the volume ( = const), substituting the mass of the molecule in (5) instead of φ, we obtain:

Or . (6)

This is Fick's law. D = is the diffusion coefficient. [D] \u003d m 2 / s.

Thermal conductivity is the transfer of energy . Provided that the concentration of molecules over the entire volume of gas (n \u003d const), the masses of the molecules are the same (m 0 \u003d const), the distribution of velocities over the volume is uniform ( \u003d const), and the average kinetic energy of the translational motion of one molecule, we get the Fourier law:

, or . (7)

- coefficient of thermal conductivity. [χ] \u003d W / (m K) \u003d kg m / (s 3 K).

Viscosity is the transfer of momentum between parallel layers that move in an orderly manner at velocities u 1 and u 2. Provided that over the entire volume of the gas the concentration of molecules is n = const, the masses of the molecules are the same (m 0 = const), the distribution of velocities over the volume is uniform ( = const), and the momentum modulus of one molecule, associated with the speed of the ordered movement of the layers φ = p = m 0 u, for the momentum of the interaction force of the layers we have:

Or . ()

This is Newton's equation, which determines the magnitude of the force of internal friction (viscosity). is the transverse velocity gradient characterizing the rate of change of velocity in the direction X perpendicular to the movement of the rubbing layers. η – dynamic coefficient of viscosity . [η] = Pa s.

MOLECULAR FORCES

The forces of interaction between molecules, or, as they are also called, Van der Waals forces, are electrical in nature. These are the Coulomb forces of interaction of charged particles that make up atoms and molecules. They appear at distances commensurate with the size of the molecules themselves and decrease very quickly with increasing distance. At the same time, attractive forces (interaction of opposite charges) and repulsive forces (interaction of like charges) act simultaneously. Because real particles are not point, then the magnitude of these forces depends on the distance between them in different ways.

There are three types of van der Waals forces:

a) orientation - act between polar molecules:

where р is the electric dipole moment of the particles, r is the distance between them, k is the Boltzmann constant, Т is the thermodynamic temperature.

b) induction – describe the interaction of molecules, polarization

charges in which arises under the influence of electric fields of neighboring particles:

Here: р ind = ε 0 αЕ – acquired electric dipole moment of particles; α is the polarizability of molecules.

in) dispersion - determine the interaction of molecules, in which an asymmetric charge distribution occurs randomly, in the process of electrons moving along orbits, which leads to the formation of instantaneous dipoles:

In general, all three types of forces can act simultaneously:

F m \u003d F o + F and + F d.

Let us consider the dependence of intermolecular interaction forces on distance. The forces of attraction F pr are considered negative, and the forces of repulsion F from are considered positive. The sum of these forces gives the resultant - Fres = f(r). At some distance r 0 between the molecules |F pr | = |F from | and the resulting force F \u003d F pr + F from \u003d 0. If r< r 0 , то преобладают силы отталкивания. Если r >r 0 , then the forces of attraction prevail. However, at a distance of r > 10 -9 m, the van der Waals forces quickly tend to zero.

The system of interacting molecules is characterized by a certain reserve of potential energy, which depends on r in a complex way, E p = f(r):

r → ∞ – E p → 0 ;

r > r 0 and r → r 0 - E p → E p min, E p< 0 ;

r \u003d r 0 - E p \u003d E p min, E p< 0;

r< r 0 и уменьшается – Е п → ∞, Е п > 0.

The smallest potential energy of interaction is called the binding energy of molecules. It is equal to the work that must be done against the forces of attraction in order to separate molecules that are in equilibrium.

The ratio of the minimum potential energy (E p min) and the value of the doubled average energy of thermal motion per one degree of freedom is a criterion for the state of aggregation of a substance. If a:

a) E p min<< kT – газ;

b) E p min » kT – liquid;

c) E p min >> kT is a solid body.

Thus, any substance, depending on the temperature, can be in a gaseous, liquid or solid state of aggregation.

STRUCTURAL FEATURES OF GASES, LIQUIDS AND SOLID BODIES

R.N. Grabovsky. Physics course. 1980, pp. 168-174.

REAL GASES

The equations of the molecular kinetic theory quite well describe the behavior of real gases at a sufficiently high temperature and low pressure. This is understandable, because such a state of a real gas is closest to the model of an ideal gas, on the basis of which all the conclusions of the MKT are obtained. However, with increasing pressure and decreasing temperature, the average distance between molecules decreases and the forces of molecular interaction increase. For example, at n.o. the volume of molecules is 1/10000 of the volume occupied by the gas, and at a pressure of 500 atm (500 MPa) it will already be half of the total volume of gas. It is quite obvious that under these conditions the laws of the MKT cease to work, for example, PV ¹ const at T = const.

Thus, the task is to obtain such an equation of state for a real gas that would take into account the volume of molecules and their interaction.

All non-living matter consists of particles, the behavior of which may differ. The structure of gaseous, liquid and solid bodies has its own characteristics. Particles in solids are held together because they are very close to each other, which makes them very strong. In addition, they can keep a certain shape, since their smallest particles practically do not move, but only vibrate. Molecules in liquids are quite close to each other, but they can move freely, so they do not have their own shape. Particles in gases move very fast, and there is usually a lot of space around them, which suggests that they are easily compressed.

Properties and structure of solids

What is the structure and features of the structure of solids? They are made up of particles that are very close to each other. They cannot move and therefore their shape remains fixed. What are the properties of a solid body? It does not shrink, but if it is heated, its volume will increase with increasing temperature. This is because the particles begin to vibrate and move, which leads to a decrease in density.

One of the features of solids is that they have a fixed shape. When a solid is heated, the movement of the particles increases. Faster moving particles collide more violently, causing each particle to push its neighbors. Therefore, an increase in temperature usually leads to an increase in the strength of the body.

Crystal structure of solids

Intermolecular forces of interaction between adjacent molecules of a solid are strong enough to keep them in a fixed position. If these smallest particles are in a highly ordered configuration, then such structures are usually called crystalline. The internal ordering of particles (atoms, ions, molecules) of an element or compound is dealt with by a special science - crystallography.

The solid state is also of particular interest. By studying the behavior of particles, how they are arranged, chemists can explain and predict how certain types materials will behave under certain conditions. The smallest particles of a solid body are arranged in the form of a lattice. This is the so-called regular arrangement of particles, where various chemical bonds between them.

The band theory of the structure of a solid body considers it as a set of atoms, each of which, in turn, consists of a nucleus and electrons. In the crystal structure, the nuclei of atoms are located in the nodes of the crystal lattice, which is characterized by a certain spatial periodicity.

What is the structure of a liquid?

The structure of solids and liquids is similar in that the particles of which they are composed are at a close distance. The difference is that the molecules move freely, since the force of attraction between them is much weaker than in a solid.

What are the properties of a liquid? Firstly, it is fluidity, and secondly, the liquid will take the form of the container in which it is placed. If it is heated, the volume will increase. Due to the proximity of the particles to each other, the liquid cannot be compressed.

What is the structure and structure of gaseous bodies?

Gas particles are randomly arranged, they are so far apart that there can be no attractive force between them. What properties does a gas have and what is the structure of gaseous bodies? As a rule, the gas uniformly fills the entire space in which it was placed. It compresses easily. The speed of the particles of a gaseous body increases with increasing temperature. At the same time, there is also an increase in pressure.

The structure of gaseous, liquid and solid bodies is characterized by different distances between the smallest particles of these substances. The particles of a gas are much farther apart than in a solid or liquid state. In air, for example, the average distance between particles is about ten times the diameter of each particle. Thus, the volume of molecules occupies only about 0.1% of the total volume. The remaining 99.9% is empty space. In contrast, liquid particles fill about 70% of the total liquid volume.

Each gas particle moves freely along a straight path until it collides with another particle (gas, liquid or solid). The particles usually move fast enough that after two of them collide, they bounce off each other and continue on their way alone. These collisions change direction and speed. These properties of gas particles allow gases to expand to fill any shape or volume.

State change

The structure of gaseous, liquid and solid bodies can change if a certain external influence is exerted on them. They can even change into each other's states under certain conditions, such as during heating or cooling.

Evaporation. The structure and properties of liquid bodies allow them, under certain conditions, to pass into a completely different physical state. For example, if you accidentally spill gasoline while refueling a car, you can quickly smell its pungent smell. How does this happen? Particles move throughout the liquid, as a result, a certain part of them reaches the surface. Their directional motion can carry these molecules off the surface and into the space above the liquid, but the attraction will pull them back. On the other hand, if a particle is moving very fast, it can break away from others by a decent distance. Thus, with an increase in the speed of particles, which usually happens when heated, the process of evaporation occurs, that is, the transformation of liquid into gas.

Behavior of bodies in different physical states

The structure of gases, liquids, solids is mainly due to the fact that all these substances are composed of atoms, molecules or ions, but the behavior of these particles can be completely different. Gas particles are chaotically distant from each other, liquid molecules are close to each other, but they are not as rigidly structured as in a solid. Gas particles vibrate and move at high speeds. The atoms and molecules of a liquid vibrate, move, and slide past each other. Particles of a solid body can also vibrate, but motion as such is not characteristic of them.

Features of the internal structure

In order to understand the behavior of matter, one must first study its features. internal structure. What are the internal differences between granite, olive oil and helium in balloon? A simple model of the structure of matter will help answer this question.

A model is a simplified version of a real object or substance. For example, before actual construction begins, architects first construct a model construction project. Such a simplified model does not necessarily imply an exact description, but at the same time it can give a rough idea of what this or that structure will be like.

Simplified Models

In science, however, physical bodies are not always models. The last century has seen a significant increase in human understanding about the physical world. However, much of the accumulated knowledge and experience is based on extremely complex representations, for example in the form of mathematical, chemical and physical formulas.

In order to understand all this, you need to be quite well versed in these exact and complex sciences. Scientists have developed simplified models to visualize, explain, and predict physical phenomena. All this greatly simplifies the understanding of why some bodies have a constant shape and volume at a certain temperature, while others can change them, and so on.

All matter is made up of tiny particles. These particles are in constant motion. The volume of movement is related to temperature. An increased temperature indicates an increase in the speed of movement. The structure of gaseous, liquid and solid bodies differs in the freedom of movement of their particles, as well as in how strongly the particles are attracted to each other. Physical depend on his physical condition. water vapor, liquid water and ice have the same Chemical properties, but they physical properties differ significantly.

Features of the molecular structure of liquids

The liquid occupies an intermediate position in properties and structure between gases and solids. crystalline substances. Therefore, it has the properties of both gaseous and solid substances. In molecular kinetic theory, various aggregate states substances are associated with varying degrees ordering of molecules. For solids, the so-called long range order in the arrangement of particles, i.e. their orderly arrangement, repeating over long distances. In liquids, the so-called short range order in the arrangement of particles, i.e. their ordered arrangement, repeating at distances, is comparable with interatomic ones. At temperatures close to the crystallization temperature, the liquid structure is close to that of a solid. At high temperatures, close to the boiling point, the structure of the liquid corresponds to the gaseous state - almost all molecules participate in chaotic thermal motion.

Liquids, like solids, have a certain volume, and like gases, they take the shape of the vessel in which they are located. Gas molecules are practically not interconnected by the forces of intermolecular interaction, and in this case, the average energy of the thermal motion of gas molecules is much greater than the average potential energy due to the forces of attraction between them, so the gas molecules fly apart in different sides and the gas occupies the volume provided to it. In solid and liquid bodies, the forces of attraction between molecules are already significant and keep the molecules at a certain distance from each other. In this case, the average energy of the thermal motion of molecules is less than the average potential energy due to the forces of intermolecular interaction, and it is not enough to overcome the forces of attraction between molecules, so solids and liquids have a certain volume.

The pressure in liquids increases very sharply with increasing temperature and decreasing volume. The volumetric expansion of liquids is much less than that of vapors and gases, since the forces that bind molecules in a liquid are more significant; the same remark applies to thermal expansion.

The heat capacities of liquids usually increase with temperature (albeit slightly). The C p /C V ratio is practically equal to one.

The theory of fluid has not been fully developed to date. The development of a number of problems in the study of the complex properties of a liquid belongs to Ya.I. Frenkel (1894–1952). He explained the thermal motion in a liquid by the fact that each molecule oscillates for some time around a certain equilibrium position, after which it jumps to a new position, which is at a distance of the order of the interatomic distance from the initial one. Thus, the molecules of the liquid move quite slowly throughout the mass of the liquid. With increasing liquid temperature, the frequency oscillatory motion increases sharply, the mobility of molecules increases.

Based on the Frenkel model, it is possible to explain some distinctive features properties of the liquid. Thus, liquids, even near the critical temperature, have a much greater viscosity than gases, and the viscosity decreases with increasing temperature (rather than increases, as in gases). This is explained by a different nature of the momentum transfer process: it is transferred by molecules that jump from one equilibrium state to another, and these jumps become much more frequent with increasing temperature. Diffusion in liquids occurs only due to molecular jumps, and it occurs much more slowly than in gases. Thermal conductivity liquids is due to the exchange of kinetic energy between particles oscillating around their equilibrium positions with different amplitudes; sharp jumps of molecules do not play a noticeable role. The mechanism of heat conduction is similar to its mechanism in gases. characteristic feature liquid is its ability to have free surface(not limited by solid walls).

Several theories have been proposed for the molecular structure of liquids.

1. Zone model. At a given moment in time, a liquid can be considered as consisting of regions where the molecules are located in right order, forming a kind of microcrystal (zone). These areas are, as it were, separated by a substance in a gaseous state. Over time, these areas form in other places, and so on.

2. The theory of quasi-crystalline structure. Consider a crystal at absolute zero temperature (see Fig. 9.9.)

We select an arbitrary direction in it and plot the dependence of the probability P of finding a gas molecule at a certain distance from another molecule placed at the origin (Fig. 9.9. a), while the molecules are located at the nodes of the crystal lattice. At a higher temperature (Fig. 9.9, b) molecules oscillate around fixed equilibrium positions, near which they spend most of their time. The strict periodicity of the repetition of probability maxima in an ideal crystal extends arbitrarily far from the chosen particle; therefore, it is customary to say that a "long-range order" exists in a solid.

In the case of a liquid (Fig. 9.9, in) near each molecule, its neighbors are located more or less regularly, but far away this order is violated (short-range order). On the graph, the distances are measured in fractions of the radius of the molecule (r/r 0).