The distance between molecules in the solid state. Movement of molecules in solids. Molecular physics made easy

Arrangement of molecules in solids. In solids, the distances between molecules are equal to the sizes of the molecules, so solids retain their shape. Molecules are arranged in a certain order, called a crystal lattice, so under normal conditions solids retain their volume.

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Thermal phenomena

“Diffusion in nature” - Widely used in the food industry for canning vegetables and fruits. When making steel. An example of diffusion is the mixing of gases or liquids. What is diffusion? Diffusion in respiration. The phenomenon of diffusion has important manifestations in nature and is used in science and industry.

“Changes in the aggregate states of matter” - Aggregate transformations of matter. Specific heat of vaporization. Boiling temperature. Boiling. Temperature graph of changes in the aggregate states of water. Melting and crystallization temperature. Vaporization conditions. Aggregate transformations. Vaporization. Calculation of the amount of heat. Melting and solidification process.

“3 states of matter” - Solve the crossword puzzle. Crystallization. Arrangement of molecules in solids. Examples of processes. States. Substance. Properties of gases. Vaporization. Questions for the crossword. Properties of liquids. Arrangement of molecules in liquids. Ice. Properties of solids. Condensation. The nature of the movement and interaction of particles.

“Diffusion of substances” - Fragrant leaves. Dark color. Proverbs. Thales of Miletus. Heraclitus. Let's solve problems. Scientists of Ancient Greece. Diffusion in technology and nature. Tasks for biology lovers. Diffusion. The phenomenon of diffusion. Democritus Observations. Diffusion in gases.

“Thermal phenomena during dissolution” - D.I. Mendeleev. Briefing. Dissolution of potassium permanganate in water. Exothermic process. Check your desk neighbor. We wish you success in further knowledge of the laws of physics and chemistry. Diffusion rate. What is called thermal motion. Mutual penetration of molecules. The meaning of solutions. Practical problems.

“Interaction of molecules” - Is it possible to connect two pieces of an iron nail? Attraction holds particles together. Option I Natural mixtures do not include: a) clay; b) cement; c) soil. Gaseous substances. Option II The artificial mixture is: a) clay; b) cement; c) soil. The distance between gas molecules is greater than the size of the molecules themselves.

There are a total of 23 presentations in the topic

Essay
In the discipline "Concepts of modern natural science"
Topic: “Model ideas about the structure of liquids, gases and crystals”

Completed by: student gr. 10E1 A. Antoshkina
Checked by: Associate Professor G. V. Surovitskaya

Penza 2010

Content
Introduction
Chapter 1. Liquid
1.1.The concept of liquid

1.3. Liquid properties
Chapter 2. Gas
2.1.The concept of gas
2.2 Molecular movement
2.3.Gas properties
Chapter 3. Crystals
3.1.The concept of crystals
3.2.types of crystal lattices
3.3. Properties of crystals, shape and system
Conclusion
Bibliography

Introduction
According to the sensations that various substances (bodies of substances) cause in the human senses, they can all be divided into three main groups: gaseous, liquid and crystalline (solid).
Gases do not have their own surface and their own volume. They completely occupy the vessel in which they are located. Gases have an unlimited ability to expand with increasing temperature and decreasing pressure. The distances between molecules in gases are many times larger than the sizes of the molecules themselves, and the interactions between them, the so-called intermolecular interactions, are weak, and the molecules in the gas move almost independently of each other. The arrangement of particles in a gas is almost completely random (chaotic).
Crystals, like all solids, have a surface that separates them from other solids and a volume corresponding to it, which do not change (or rather, change very slightly) in a gravitational field. The distances between particles in crystals are much smaller than in gases, and intermolecular or interatomic (if the crystal is built from atoms of one element) interactions are much stronger than in gases and liquids. The particles in a crystal are distributed in some fairly strict regular order, forming a crystal lattice. The particles that make up the crystal lattice are relatively firmly fixed in place. A distinctive feature of crystals is that their properties are not the same in different directions. This phenomenon is called anisotropy of properties.
Liquids combine many of the properties of gaseous and crystalline states. They have a surface and volume, which are affected by changes in the position of the vessel with liquid in the gravitational field. A liquid in a gravitational field occupies the lower part of the vessel in which it is located. Molecules in a liquid substance are connected to each other by much stronger intermolecular forces than in a gas. The order in the arrangement of particles in liquid substances is also much higher than in gases. In some liquids, such as water, individual very small volumes have an order close to that in crystals.
In the report, I tried to reveal the essence of each state of matter: liquid, gaseous and crystalline. She described the properties of substances, the arrangement of molecules and crystal lattices. Now let’s take a closer look at each substance, presenting it in a model manner.

Chapter 1. Liquid
1.1 Concept of liquid
Each of us can easily recall many substances that he considers liquids. However, it is not so easy to give an exact definition of this state of matter. A liquid occupies an intermediate position between a crystalline solid, characterized by complete orderliness in the arrangement of the particles that form it (ions, atoms, molecules) and a gas, the molecules of which are in a state of chaotic (disorderly) movement.
The shape of liquid bodies can be determined entirely or partly by the fact that their surface behaves like an elastic membrane. So, water can collect in drops. But a liquid is capable of flowing even under its stationary surface, and this also means that the form (internal parts of the liquid body) is not preserved.
Liquid molecules do not have a definite position, but at the same time they do not have complete freedom of movement. There is an attraction between them, strong enough to keep them close. A substance in a liquid state exists in a certain temperature range, below which it turns into a solid state (crystallization occurs or transformation into a solid-state amorphous state - glass), above which it turns into a gaseous state (evaporation occurs). The boundaries of this interval depend on pressure. As a rule, a substance in a liquid state has only one modification. (The most important exceptions are quantum liquids and liquid crystals.) Therefore, in most cases, a liquid is not only a state of aggregation, but also a thermodynamic phase (liquid phase). All liquids are usually divided into pure liquids and mixtures. Some mixtures of liquids are of great importance for life: blood, sea water, etc. Liquids can act as solvents.
1.2. Arrangement of molecules in a liquid
The molecules of a substance in a liquid state are located almost close to each other. Unlike solid crystalline bodies, in which molecules form ordered structures throughout the entire volume of the crystal and can perform thermal vibrations around fixed centers, liquid molecules have greater freedom. Each molecule of a liquid, just like in a solid, is “sandwiched” on all sides by neighboring molecules and undergoes thermal vibrations around a certain equilibrium position. However, from time to time any molecule may move to a nearby vacant site. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely located molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called short-range order (Fig. 1).

Fig.1. an example of short-range order of liquid molecules and long-range order of molecules of a crystalline substance: 1.1 – water; 1. – ice.

Rice. 2. water vapor (1) and water (2). Water molecules are enlarged approximately 5·107 times.
Figure 2 illustrates the difference between a gaseous substance and a liquid using water as an example. The water molecule H2O consists of one oxygen atom and two hydrogen atoms located at an angle of 104°. The average distance between steam molecules is tens of times greater than the average distance between water molecules. Unlike Fig. 1, where water molecules are depicted as balls, Fig. 2 gives an idea of ​​the structure of the water molecule. Due to the dense packing of molecules, the compressibility of liquids, i.e., the change in volume with a change in pressure, is very small; it is tens and hundreds of thousands of times less than in gases.

1.3. Liquid properties
Fluidity. The main property of liquids is fluidity. If an external force is applied to a section of a liquid that is in equilibrium, then a flow of liquid particles arises in the direction in which this force is applied: the liquid flows. Thus, under the influence of unbalanced external forces, the liquid does not retain its shape and relative arrangement of parts, and therefore takes the shape of the vessel in which it is located. Unlike plastic solids, a liquid does not have a yield limit: it is enough to apply an arbitrarily small external force for the liquid to flow.
Volume conservation. One of the characteristic properties of a liquid is that it has a certain volume (under constant external conditions). Liquids are extremely difficult to compress mechanically because, unlike gases, there is very little free space between the molecules. The pressure exerted on a liquid enclosed in a vessel is transmitted without change to each point in the volume of this liquid (Pascal’s law is also valid for gases). This feature, along with very low compressibility, is used in hydraulic machines. Liquids generally increase in volume (expand) when heated and decrease in volume (contract) when cooled. However, there are exceptions, for example, water contracts when heated, at normal pressure and at temperatures from 0 °C to approximately 4 °C.
Viscosity. In addition, liquids (like gases) are characterized by viscosity. It is defined as the ability to resist the movement of one part relative to another - that is, as internal friction. When adjacent layers of liquid move relative to each other, collisions of molecules inevitably occur in addition to that caused by thermal motion. Forces arise that inhibit orderly movement. In this case, the kinetic energy of ordered movement turns into thermal energy - the energy of chaotic movement of molecules. The liquid in the vessel, set in motion and left to itself, will gradually stop, but its temperature will increase.
Free surface formation and surface tension. Due to the conservation of volume, the liquid is able to form a free surface. Such a surface is the interface between the phases of a given substance: on one side there is a liquid phase, on the other there is a gaseous phase (steam), and, possibly, other gases, for example, air. If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the interface area - surface tension forces. The interface behaves like an elastic membrane that tends to contract. Surface tension can be explained by the attraction between liquid molecules. Each molecule attracts other molecules, strives to “surround” itself with them, and therefore leave the surface. Accordingly, the surface tends to decrease. Therefore, soap bubbles and bubbles tend to take a spherical shape when boiling: for a given volume, a sphere has the minimum surface area. If only surface tension forces act on a liquid, it will necessarily take a spherical shape - for example, water drops in zero gravity. Small objects with a density greater than that of the liquid are able to “float” on the surface of the liquid, since the force of gravity is less than the force that prevents the increase in surface area. (See Surface tension.)
Evaporation and condensation. Evaporation is the gradual transition of a substance from a liquid to the gaseous phase (steam). During thermal movement, some molecules leave the liquid through its surface and become vapor. At the same time, some molecules pass back from vapor to liquid. If more molecules leave a liquid than enter, then evaporation occurs. Condensation is a reverse process, the transition of a substance from a gaseous state to a liquid one. In this case, more molecules pass into the liquid from the vapor than into the vapor from the liquid. Evaporation and condensation are nonequilibrium processes; they occur until local equilibrium is established (if established), and the liquid can completely evaporate, or come into equilibrium with its vapor, when as many molecules leave the liquid as return.
Boiling is the process of vaporization within a liquid. At a sufficiently high temperature, the vapor pressure becomes higher than the pressure inside the liquid, and vapor bubbles begin to form there, which (under the conditions of gravity) float to the top.
Wetting is a surface phenomenon that occurs when a liquid comes into contact with a solid surface in the presence of steam, that is, at the interfaces of three phases. Wetting characterizes the “sticking” of a liquid to a surface and spreading over it (or, conversely, repulsion and not spreading). There are three cases: no wetting, limited wetting and complete wetting.
Miscibility is the ability of liquids to dissolve in each other. An example of miscible liquids: water and ethyl alcohol, an example of immiscible liquids: water and liquid oil.
Diffusion. When there are two mixed liquids in a vessel, the molecules, as a result of thermal movement, begin to gradually pass through the interface, and thus the liquids gradually mix. This phenomenon is called diffusion (also occurs in substances in other states of aggregation).
Overheating and hypothermia. A liquid can be heated above its boiling point so that no boiling occurs. This requires uniform heating, without significant temperature changes within the volume and without mechanical influences such as vibration. If you throw something into a superheated liquid, it will instantly boil. Superheated water is easily obtained in the microwave. Subcooling is the cooling of a liquid below its freezing point without turning into a solid state of aggregation. As with overheating, supercooling requires the absence of vibration and significant temperature changes.
Coexistence with other phases. Formally speaking, for the equilibrium coexistence of a liquid phase with other phases of the same substance - gaseous or crystalline - strictly defined conditions are required. So, at a given pressure, a strictly defined temperature is needed. However, in nature and in technology everywhere, liquid coexists with steam, or also with a solid state of aggregation - for example, water with steam and often with ice (if we consider steam as a separate phase present along with air). This is due to the following reasons:
- Non-equilibrium state. It takes time for a liquid to evaporate; until the liquid has completely evaporated, it coexists with steam. In nature, water evaporates constantly, as does the reverse process - condensation.
- Closed volume. The liquid in a closed vessel begins to evaporate, but since the volume is limited, the vapor pressure increases, it becomes saturated even before the liquid has completely evaporated, if its quantity was large enough. When the saturation state is reached, the amount of evaporated liquid is equal to the amount of condensed liquid, the system comes into equilibrium. Thus, in a limited volume, the conditions necessary for the equilibrium coexistence of liquid and vapor can be established.
- The presence of an atmosphere in conditions of earth's gravity. A liquid is affected by atmospheric pressure (air and steam), while for steam almost only its partial pressure must be taken into account. Therefore, liquid and vapor above its surface correspond to different points on the phase diagram, in the region of existence of the liquid phase and in the region of existence of the gaseous phase, respectively. This does not cancel evaporation, but evaporation requires time during which both phases coexist. Without this condition, the liquids would boil and evaporate very quickly.

Chapter 2. Gas
2.1. Gas concept
GAS is one of the aggregate states of a substance in which its constituent particles (atoms, molecules) are located at considerable distances from each other and are in free movement. Unlike a liquid and a solid, where molecules are at close distances and are connected to each other by significant forces of attraction and repulsion, the interaction of molecules in a gas manifests itself only during short moments of their approach (collision). In this case, there is a sharp change in the magnitude and direction of the speed of movement of the colliding particles.
The name “gas” comes from the Greek word “chaos” and was introduced by Van Helmont at the beginning of the 17th century; it well reflects the true nature of the movement of particles in a gas, which is characterized by complete disorder and chaos. Unlike, for example, liquids, gases do not form a free surface and uniformly fill the entire volume available to them. The gaseous state, if we include ionized gases, is the most common state of matter in the Universe (planetary atmospheres, stars, nebulae, interstellar matter, etc.).
2.2. Molecular movement
The movement of molecules in gases is random: the velocities of molecules do not have any preferred direction, but are distributed chaotically in all directions. Due to collisions of molecules with each other, their speeds change all the time both in direction and in magnitude. Therefore, the speeds of molecules can differ greatly from each other. At any moment in a gas there are molecules moving extremely quickly and molecules moving relatively slowly. However, the number of molecules moving much slower or much faster than the others is small. Most molecules move at speeds that differ relatively little from some average speed, depending on the type of molecules and body temperature. In what follows, when speaking about the speed of molecules, we will mean their average speed. We will address the issue of measuring and calculating the average speed of molecules later. In many discussions regarding the movement of gas molecules, the concept of mean free path plays an important role. The mean free path is the average distance traveled by molecules between two successive collisions. As the gas density decreases, the mean free path increases. At atmospheric pressure and 0°C, the average free path of air molecules is approximately 10-8-10-7 m (Fig. 371).

Rice. 371. This is approximately the path of an air molecule at normal pressure (increased a million times)
In very rarefied gases (for example, inside hollow light bulbs), the average free path reaches several centimeters and even tens of centimeters. Here the molecules move from wall to wall with almost no collisions. In solids, molecules vibrate around average positions. In liquids, molecules also vibrate around average positions. However, from time to time each molecule jumps to a new average position, separated from the previous one by several intermolecular distances.
2.3. Gas properties
In the gas state, the energy of interaction between particles is much less than their kinetic energy: EMMB<< Екин.
Therefore, gas molecules (atoms) are not held together, but move freely in a volume significantly larger than the volume of the particles themselves. Intermolecular interaction forces appear when molecules come close enough to each other. Weak intermolecular interaction determines the low density of the gas, the desire for limitless expansion, and the ability to exert pressure on the walls of the vessel, which impede this desire. Gas molecules are in random, chaotic motion, and there is no order in the gas regarding the arrangement of molecules. The state of a gas is characterized by: temperature - T, pressure - p and volume - V. At low pressures and high temperatures, all typical gases behave approximately the same. But already at ordinary and, especially, low temperatures and high pressures, the individuality of gases begins to appear. An increase in external pressure and a decrease in temperature brings gas particles closer together, so intermolecular interaction begins to manifest itself to a greater extent. For such gases it is no longer possible to apply the Mendeleev-Clapeyron equation: but the van der Waals equation should be used:
where a and b are constant terms that take into account the presence of attractive forces between molecules and the intrinsic volume of molecules, respectively.
When gases are compressed, when their density increases significantly, the IMF forces become more and more noticeable, which leads to the creation of conditions for the formation of various associates from molecules. Associates are relatively unstable groups of molecules. From the nature of the components of the IMF it follows that the universal interaction forces increase with increasing atomic sizes, the polarizability sharply increases, therefore, the heavier the same type of particles (atoms or molecules) of a substance, the usually higher the degree of their association at a given temperature, the lower temperatures such a substance transforms from gas to liquid.

Chapter 3. Crystals
3.1.The concept of crystals
The world of crystals is a world no less beautiful, diverse, developing, and often no less mysterious than the world of living nature. The importance of crystals for geological sciences lies in the fact that the vast majority of the earth's crust is in a crystalline state. In the classification of such fundamental objects of geology as minerals and rocks, the concept of a crystal is primary, elementary, similar to an atom in the periodic table of elements or a molecule in the chemical classification of substances. According to the aphoristic statement of the famous mineralogist, professor of the St. Petersburg Mining Institute D.P. Grigoriev, “a mineral is a crystal.” It is clear that the properties of minerals and rocks are closely related to the general properties of the crystalline state.
The word "crystal" is Greek (????????????), its original meaning is "ice". However, already in ancient times, this term was transferred to transparent natural polyhedra of other substances (quartz, calcite, etc.), since it was believed that this was also ice, which for some reason became stable at high temperatures. In Russian, this word has two forms: “crystal” itself, meaning a naturally occurring multifaceted body, and “crystal” - a special type of glass with a high refractive index, as well as transparent colorless quartz (“rock crystal”). Most European languages ​​use the same word for both of these concepts (compare the English "Crystal Palace" - "Crystal Palace" in London and "Crystal Growth" - an international journal on crystal growth).
Humanity became acquainted with crystals in ancient times. This is due, first of all, to their ability to self-cut, which is often realized in nature, that is, to spontaneously take the form of amazingly perfect polyhedra. Even a modern person, when encountering natural crystals for the first time, most often does not believe that these polyhedrons are not the work of a skilled craftsman. The shape of crystals has long been given magical significance, as evidenced by some archaeological finds. Mentions of “crystal” (apparently, we are still talking about “crystal”) are repeatedly found in the Bible (see, for example: Revelation of John, 21, 11; 32, 1, etc.). Among mathematicians, there is a reasoned opinion that the prototypes of the five regular polyhedra (Platonic solids) were natural crystals. Many Archimedean (semi-regular) polyhedra also have exact or very close analogues in the world of crystals. And in the applied art of antiquity, crystalline polyhedra were sometimes used as role models, and those that were obviously not considered by the science of that time. For example, the State Hermitage contains a string of beads, the shape of which accurately reproduces the characteristic shape of crystals of the beautiful semi-precious mineral garnet. These beads are made of gold (presumably Middle Eastern work of the 1st-5th centuries AD). Thus, crystals have long had a noticeable impact on the main areas of human interest: emotional (religion, art), ideological (religion), intellectual (science, art).
3.2. Main types of crystal lattices
In solids, atoms can be arranged in space in two ways: 1) Random arrangement of atoms, when they do not occupy a specific place relative to each other. Such bodies are called amorphous. 2) An ordered arrangement of atoms, when atoms occupy well-defined places in space. Such substances are called crystalline.
The atoms oscillate relative to their average position with a frequency of about 1013 Hz. The amplitude of these oscillations is proportional to temperature. Thanks to the ordered arrangement of atoms in space, their centers can be connected by imaginary straight lines. The set of such intersecting lines represents a spatial lattice, which is called a crystal lattice.
The outer electron orbits of the atoms touch, so the packing density of the atoms in the crystal lattice is very high. Crystalline solids consist of crystalline grains - crystallites. In neighboring grains, the crystal lattices are rotated relative to each other at a certain angle. In crystallites, short- and long-range orders are observed. This means the presence of an ordered arrangement and stability of both its nearest neighbors surrounding a given atom (short-range order) and atoms located at significant distances from it, up to the grain boundaries (long-range order).

a) b)
Rice. 1.1. Arrangement of atoms in crystalline (a) and amorphous (b) matter
Due to diffusion, individual atoms can leave their places in the nodes of the crystal lattice, but the order of the crystal structure as a whole is not disrupted.
All metals are crystalline bodies that have a certain type of crystal lattice, consisting of low-mobility positively charged ions, between which free electrons (the so-called electron gas) move. This type of structure is called a metallic bond. The type of lattice is determined by the shape of an elementary geometric body, the repeated repetition of which along three spatial axes forms the lattice of a given crystalline body.

A) B)

C) D)
Rice. 1.2. The main types of metal crystal lattices:
A) cubic (1 atom per cell)
B) body-centered cubic (bcc) (2 atoms per cell)
etc.................

Molecular physics made easy!

Molecular interaction forces

All molecules of a substance interact with each other through forces of attraction and repulsion.
Evidence of the interaction of molecules: the phenomenon of wetting, resistance to compression and tension, low compressibility of solids and gases, etc.
The reason for the interaction of molecules is the electromagnetic interactions of charged particles in a substance.

How to explain this?

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 the electrons, so the atom as a whole is electrically neutral.
A molecule consisting of one or more atoms is also electrically neutral.

Let's consider the interaction between molecules using the example of two stationary molecules.

Gravitational and electromagnetic forces can exist between bodies in nature.
Since the masses of molecules are extremely small, negligible forces of gravitational interaction between molecules can be ignored.

At very large distances there is also no electromagnetic interaction between molecules.

But, as the distance between molecules decreases, the molecules begin to orient themselves in such a way 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 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 acted upon by the sum of the forces of attraction and repulsion. At large distances, the force of attraction predominates (at a distance of 2-3 diameters of the molecule, attraction is maximum), at short distances the force of repulsion prevails.

There is a distance between molecules at which the attractive forces become equal to the repulsive forces. 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 a stable equilibrium position, the potential energy of the 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 the molecules.

In addition, all 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 relationship between the minimum potential energy of interaction of molecules and the reserve of kinetic energy of thermal motion of molecules.

Structure and properties of solid, liquid and gaseous bodies

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

Solid

Solids have a constant shape and volume and are practically incompressible.
The minimum potential energy of interaction of molecules is greater than the kinetic energy of molecules.
Strong particle interaction.

The thermal motion of molecules in a solid is expressed only by vibrations of particles (atoms, molecules) around a stable equilibrium position.

Due to the large forces of attraction, molecules practically cannot change their position in matter, this explains the invariability 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 - 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 between molecules is comparable to the kinetic energy of molecules.
Weak particle interaction.
The thermal motion of molecules in a liquid is expressed by vibrations around a stable equilibrium position 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 and the ability to change its shape.

In liquids, molecules are quite firmly bound to each other by forces of attraction, 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. When the distance between molecules decreases (compression of the liquid), the repulsive forces increase sharply, so liquids are incompressible.

In terms of their structure and the nature of thermal movement, 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 between molecules is less than the kinetic energy of molecules.
Particles of matter practically do not interact.
Gases are characterized by complete disorder in the arrangement and movement of molecules.

In gases, the distance between molecules and atoms is usually much greater than the size of the molecules, and the attractive forces are very small. Therefore, gases do not have their own shape and constant volume. Gases are easily compressed because repulsive forces over large distances are also small. Gases have the property of expanding indefinitely, filling the entire volume provided to them. Gas molecules move at very high speeds, collide with each other, and bounce off each other in different directions. Numerous impacts of molecules on the walls of the vessel create gas pressure.

Movement of molecules in liquids

In liquids, molecules not only oscillate around the equilibrium position, but also make jumps from one equilibrium position to the next. These jumps occur periodically. The time interval between such jumps is called average time of settled life(or average relaxation time) and is designated by the letter ?. In other words, relaxation time is the time of oscillations around one specific equilibrium position. At room temperature this time averages 10 -11 s. The time of one oscillation is 10 -12 ... 10 -13 s.

The time of sedentary life decreases with increasing temperature. The distance between the molecules of a liquid is smaller than the size of the molecules, the particles are located close to each other, and the intermolecular attraction is strong. However, the arrangement of liquid molecules is not strictly ordered throughout the volume.

Liquids, like solids, retain their volume, but do not have their own shape. Therefore, they take the shape of the vessel in which they are located. The liquid has the following properties: fluidity. Thanks to this property, the liquid does not resist changing shape, is slightly compressed, and its physical properties are the same in all directions inside the liquid (isotropy of liquids). The nature of molecular motion in liquids was first established by the Soviet physicist Yakov Ilyich Frenkel (1894 - 1952).

Movement of molecules in solids

The molecules and atoms of a solid are arranged in a certain order and form crystal lattice. Such solids are called crystalline. Atoms perform vibrational movements around the equilibrium position, and the attraction between them is very strong. Therefore, solids under normal conditions retain their volume and have their own shape.

The liquid occupies an intermediate position in properties and structure between gases and solid crystalline substances. Therefore, it has the properties of both gaseous and solid substances. In molecular kinetic theory, different states of aggregation of a substance are associated with different degrees of molecular order. For solids, the so-called long range order in the arrangement of particles, i.e. their ordered arrangement, repeating over large distances. In liquids there is a so-called close order in the arrangement of particles, i.e. their ordered arrangement, repeating over distances, is comparable to interatomic ones. At temperatures close to the crystallization temperature, the structure of the liquid is close to 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 container in which they are located. Gas molecules are practically not connected to each other by the forces of intermolecular interaction, and in this case, the average energy of thermal motion of gas molecules is much greater than the average potential energy caused by the forces of attraction between them, so the gas molecules fly apart in different directions and the gas occupies the volume provided to it. In solids and liquids, 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 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, therefore 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 connecting the molecules in the liquid are more significant; the same remark applies to thermal expansion.

The heat capacities of liquids usually increase with temperature (albeit only slightly). The ratio Ср/СV is practically equal to unity.

The theory of liquids has not yet been fully developed. The development of a number of problems in the study of complex properties of liquids belongs to Ya.I. Frenkel (1894–1952). He explained thermal motion in a liquid by the fact that each molecule oscillates for some time around a certain equilibrium position, after which it abruptly moves to a new position, separated from the original one at a distance of the order of interatomic. Thus, the molecules of the liquid move rather slowly throughout the entire mass of the liquid. As the temperature of the liquid increases, the frequency of vibrational motion increases sharply and 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 much greater viscosity than gases, and the viscosity decreases with increasing temperature (and does not increase, as for gases). This is explained by the different nature of the momentum transfer process: it is transmitted by molecules making a jump from one equilibrium state to another, and these jumps become significantly 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 caused by the exchange of kinetic energy between particles oscillating around their equilibrium positions with different amplitudes; sudden jumps of molecules do not play a noticeable role. The mechanism of thermal conductivity is similar to its mechanism in gases. A characteristic feature of a liquid is its ability to have free surface(not limited by solid walls).