Lesson No. 2/5 2

Topic No. 26: “Model of the structure of liquid. Saturated and unsaturated pairs. Air humidity."

1 Liquid structure model

Liquid one of states of matter. The main property of a liquid, which distinguishes it from other states of aggregation, is the ability to change its shape indefinitely under the influence of tangential mechanical stresses, even arbitrarily small, while practically maintaining its volume.

Fig.1

The liquid state is usually considered intermediate between solid and gas : a gas retains neither volume nor shape, but a solid retains both.

Molecules liquids 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 range temperatures , below which it turns intosolid state(crystallization occurs or transformation into a solid amorphous state glass), above into gaseous (evaporation occurs). The boundaries of this interval depend on pressure

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 perform the function solvents

The main property of liquids is fluidity. If you apply to a section of liquid that is in equilibrium external force , 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, liquids do not haveyield strength: it is enough to apply an arbitrarily small external force to make the liquid flow.

One of the characteristic properties of a liquid is that it has a certain volume ( under constant external conditions). Liquid is extremely difficult to compress mechanically because, unlike gas , 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 shrinks when heated, at normal pressure and at temperatures from 0 °C to approximately 4 °C.

In addition, liquids (like gases) are characterized 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 bythermal movement. Forces arise that inhibit orderly movement. In this case, the kinetic energy of ordered motion transforms into thermal energy of chaotic motion of molecules.

The liquid in the vessel, set in motion and left to its own devices, will gradually stop, but its temperature will increase.In a vapor, like a gas, one can almost ignore the adhesion forces and consider the movement as the free flight of molecules and their collision with each other and with surrounding bodies (walls and liquid covering the bottom of the vessel). In a liquid, molecules, as in a solid, interact strongly, holding each other. However, while in a solid body each molecule retains a certain equilibrium position inside the body indefinitely and its movement is reduced to oscillation around this equilibrium position, the nature of movement in a liquid is different. Liquid molecules move much more freely than solid molecules, although not as freely as gas molecules. Each molecule in a liquid moves here and there for some time, without moving away, however, from its neighbors. This movement resembles the vibration of a solid molecule around its equilibrium position. However, from time to time, a liquid molecule escapes from its environment and moves to another place, ending up in a new environment, where it again performs a motion similar to vibration for some time.

Thus, the movement of liquid molecules is something like a mixture of movements in a solid and in a gas: “oscillatory” movement in one place is replaced by a “free” transition from one place to another. In accordance with this, the structure of a liquid is something between the structure of a solid and the structure of a gas. The higher the temperature, i.e., the greater the kinetic energy of liquid molecules, the greater the role played by “free” movement: the shorter the intervals of the “vibrational” state of the molecule and the more often “free” transitions, i.e., the more the liquid becomes like a gas. At a sufficiently high temperature characteristic of each liquid (the so-called critical temperature), the properties of the liquid do not differ from the properties of a highly compressed gas.

2 Saturated and unsaturated pairs and their properties

There are always vapors of this liquid above the free surface of a liquid. If the vessel with the liquid is not closed, then the concentration of vapor particles at a constant temperature can vary within wide limits, down and up.

Evaporation process in a confined space(closed container with liquid)can occur at a given temperature only up to a certain limit. This is explained by the fact that condensation of steam occurs simultaneously with the evaporation of the liquid. First, the number of molecules flying out of the liquid in 1 s is greater than the number of molecules returning back, and the density, and therefore the vapor pressure, increases. This leads to an increase in the rate of condensation. After some time, dynamic equilibrium occurs, in which the vapor density above the liquid becomes constant.

Vapor that is in a state of dynamic equilibrium with its liquid is called saturated vapor. Vapor that is not in a state of dynamic equilibrium with its liquid is called unsaturated.

Experience shows that unsaturated pairs obey all gas laws , and the more accurately, the further they are from saturation. Saturated vapors are characterized by the following properties:

1. density and pressure of saturated steam at a given temperature these are the maximum density and pressure that steam can have at a given temperature;
2. The density and pressure of saturated vapor depend on the type of substance. The lower the specific heat of vaporization of a liquid, the faster it evaporates and the greater the pressure and density of its vapor;
3. the pressure and density of saturated steam are uniquely determined by its temperature (do not depend on how the steam reached this temperature: during heating or cooling);
4. pressure and vapor density increase rapidly with increasing temperature (Fig. 1, a, b).

Experience shows that when a liquid is heated, the liquid level in a closed vessel decreases. Consequently, the mass and density of the vapor increase. A stronger increase in the pressure of saturated vapor compared to an ideal gas (Gay-Lussac’s law is not applicable to saturated vapor) is explained by the fact that here the pressure increases not only due to an increase in the average kinetic energy of the molecules (as in an ideal gas), but also due to increasing the concentration of molecules;

1. at constant temperature, the pressure and density of saturated vapor do not depend on volume. For comparison, Figure 2 shows the isotherms of an ideal gas (a) and saturated vapor (b).

Rice. 2

Experience shows that during isothermal expansion the liquid level in the vessel decreases, and during compression it increases, i.e. the number of vapor molecules changes so that the vapor density remains constant.

3 Humidity

Air containing water vapor is called wet . To characterize the water vapor content in the air, a number of quantities are introduced: absolute humidity, water vapor pressure and relative humidity.

Absolute humidityρ air is a quantity numerically equal to the mass of water vapor contained in 1 m 3 air (i.e. the density of water vapor in the air under given conditions).

Water vapor pressure p is the partial pressure of water vapor contained in the air. The SI units of absolute humidity and elasticity are respectively kilogram per cubic meter (kg/m 3) and pascal (Pa).

If only absolute humidity or water vapor pressure is known, it is still impossible to judge how dry or humid the air is. To determine the degree of air humidity, you need to know whether water vapor is close or far from saturation.

Relative humidity air φ is the ratio of absolute humidity to density expressed as a percentageρ 0 saturated steam at a given temperature (or the ratio of water vapor pressure to pressure p 0 saturated steam at a given temperature):

The lower the relative humidity, the further the steam is from saturation, the more intense evaporation occurs. Saturated steam pressure p 0 at a given temperature table value. Water vapor pressure (and therefore absolute humidity) is determined by the dew point.

When isobarically cooled to a temperature tp the steam becomes saturated and its state is represented by a dot IN . Temperature tp , at which water vapor becomes saturated is called dew point . When cooling below the dew point, vapor condensation begins: fog appears, dew falls, and windows fog up.

4 Air humidity measurement

Measuring instruments are used to measure air humidity hygrometers. There are several types of hygrometers, but the main ones are: hair and psychrometric.

Since it is difficult to directly measure the pressure of water vapor in the air, relative humidity is measuredindirectly.

Operating principlehair hygrometerbased on the property of defatted hair (human or animal)change your lengthdepending on the humidity of the air in which it is located.

Hair stretched over a metal frame. The change in hair length is transmitted to the arrow moving along the scale. In winter, a hair hygrometer is the main instrument for measuring outdoor air humidity.

A more accurate hygrometer is a psychrometric hygrometer psychrometer
(in other Greek “psychros” means cold).
It is known that the relative humidity of the air depends evaporation rate.
The lower the air humidity, the easier it is for moisture to evaporate.

The psychrometer has two thermometers . One is ordinary, they call it dry It measures the ambient air temperature. The bulb of another thermometer is wrapped in a fabric wick and placed in a container of water. The second thermometer does not show the air temperature, but the temperature of the wet wick, hence the name moisturized thermometer. The lower the air humidity, the more intense moisture evaporates from the wick, the greater the amount of heat per unit time is removed from the moistened thermometer, the lower its readings, therefore, the greater the difference between the readings of the dry and moistened thermometers.

The dew point is determined using hygrometers. The condensation hygrometer is a metal box A , front wall TO which is well polished (Fig. 2) An easily evaporating liquid ether is poured inside the box and a thermometer is inserted. Passing air through the box using a rubber bulb G , cause strong evaporation of ether and rapid cooling of the box. The thermometer measures the temperature at which dew droplets appear on the polished surface of the wall. TO . The pressure in the area adjacent to the wall can be considered constant, since this area communicates with the atmosphere and the decrease in pressure due to cooling is compensated by an increase in vapor concentration. The appearance of dew indicates that the water vapor has become saturated. Knowing the air temperature and dew point, you can find the partial pressure of water vapor and relative humidity.

Rice. 2

5 Problems to solve independently

Problem 1

It's cold autumn rain outside. In what case will laundry hanging in the kitchen dry faster: when the window is open or when it is closed? Why?

Problem 2

Air humidity is 78%, and the dry bulb reading is 12 °C. What temperature does the wet bulb thermometer show?(Answer: 10 °C.)

Problem 3

The difference in the readings of dry and wet thermometers is 4 °C. Relative humidity 60%. What are the dry and wet bulb readings?(Answer: t c -l9 °С, t m ​​= 10 °С.)

All nonliving matter is made up of particles that may behave differently. The structure of gaseous, liquid and solid bodies has its own characteristics. The particles in solids are held together by being very close together, which makes them very strong. In addition, they can maintain 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 quickly and there is usually a lot of space around them, which means they can be easily compressed.

Properties and structure of solids

What is the structure and structural features of solids? They consist of particles that are located very close to each other. They cannot move and therefore their shape remains fixed. What are the properties of a solid? It does not compress, but if it is heated, its volume will increase with increasing temperature. This happens because the particles begin to vibrate and move, causing the density to decrease.

One of the characteristics of solids is that they have a constant shape. When a solid heats up, 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 results in an increase in the strength of the body.

Crystal structure of solids

The intermolecular forces of interaction between neighboring 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. Questions of the internal order of particles (atoms, ions, molecules) of an element or compound are dealt with by a special science - crystallography.

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

The band theory of the structure of a solid body considers it as a collection of atoms, each of which, in turn, consists of a nucleus and electrons. In the crystalline 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 located at close range. The difference is that the molecules move freely, since the force of attraction between them is much weaker than in a solid body.

What properties does the liquid have? The first is fluidity, and the second is that the liquid will take the shape of the container in which it is placed. If you heat it up, the volume will increase. Due to the close proximity of the particles to each other, the liquid cannot be compressed.

What is the structure and structure of gaseous bodies?

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

The structure of gaseous, liquid and solid bodies is characterized by different distances between the smallest particles of these substances. Gas particles are much further apart than solid or liquid particles. In air, for example, the average distance between particles is approximately 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 quite quickly, and 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 they are exposed to a certain external influence. They can even transform 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 transform into a completely different physical state. For example, if you accidentally spill gasoline while refueling your car, you can quickly notice its pungent odor. How does this happen? Particles move throughout the liquid, eventually reaching the surface. Their directed motion can carry these molecules beyond the surface into the space above the liquid, but gravity will pull them back. On the other hand, if a particle moves very quickly, it can become separated from others by a considerable distance. Thus, with an increase in the speed of particles, which usually occurs when heated, the process of evaporation occurs, that is, the conversion of liquid into gas.

Behavior of bodies in different physical states

The structure of gases, liquids, and solids is mainly due to the fact that all these substances consist of atoms, molecules or ions, but the behavior of these particles can be completely different. Gas particles are randomly spaced 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 movement as such is not characteristic of them.

Features of the internal structure

In order to understand the behavior of matter, you must first study the features of its internal structure. What are the internal differences between granite, olive oil and helium in a 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 of the construction project. Such a simplified model does not necessarily imply an exact description, but at the same time it can give an approximate idea of ​​what a particular structure will be like.

Simplified models

In science, however, models are not always physical bodies. 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 concepts, such as 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 amount of movement is related to temperature. An increased temperature indicates an increase in movement speed. The structure of gaseous, liquid and solid bodies is distinguished by the freedom of movement of their particles, as well as by 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 their physical properties are significantly different.

Models of the structure of gases, liquids and solids

All substances can exist in three states of aggregation.

Gas– a state of aggregation in which a substance does not have a definite volume and shape. In gases, particles of a substance are removed at distances significantly exceeding the particle size. The attractive forces between particles are small and cannot hold them near each other. The potential energy of particle interaction is considered equal to zero, that is, it is much less than the kinetic energy of particle motion. The particles scatter chaotically, occupying the entire volume of the vessel in which the gas is located. The trajectories of gas particles are broken lines (from one impact to another, the particle moves uniformly and rectilinearly). Gases are easily compressed.

Liquid- a state of aggregation in which a substance has a certain volume, but does not retain its shape. In liquids, the distances between particles are comparable to the particle sizes, therefore the interaction forces between particles in liquids are large. The potential energy of particle interaction is comparable to their kinetic energy. But this is not enough for an ordered arrangement of particles. In liquids, only the mutual orientation of neighboring particles is observed. Particles of liquids perform chaotic oscillations around certain equilibrium positions and after some time change places with their neighbors. These jumps explain the fluidity of liquids.

Solid– a state of aggregation in which a substance has a certain volume and retains its shape. In solids, the distances between particles are comparable to the particle sizes, but smaller than in liquids, so the interaction forces between particles are enormous, which allows the substance to maintain its shape. The potential energy of interaction of particles is greater than their kinetic energy, therefore in solids there is an ordered arrangement of particles, called a crystal lattice. Particles of solids undergo chaotic oscillations around the equilibrium position (crystal lattice node) and very rarely change places with their neighbors. Crystals have a characteristic property - anisotropy - the dependence of physical properties on the choice of direction in the crystal.

Structure of gases, liquids and solids. Features of the structure of solutions. The concept of a “reactive field”
The theory of the structure of liquids: comparison with the structure of gases and solids Structure (structure) of liquids. The structure of liquids is currently the subject of close study by physical chemists. For research in this direction, the most modern methods are used, including spectral (IR, NMR, light scattering of various wavelengths), X-ray scattering, quantum mechanical and statistical calculation methods, etc. The theory of liquids is much less developed than that of gases, since the properties of liquids depend on the geometry and polarity of mutually closely located molecules. In addition, the lack of a specific structure of liquids makes their formalized description difficult - in most textbooks much less space is devoted to liquids than to gases and crystalline solids. What are the features of each of the three aggregate states of matter: solid, liquid and gas. (table)
1) Solid: the body retains volume and shape
2) Liquids retain volume, but easily change shape.
3) Gas has neither shape nor volume.

These states of the same substance differ not in the sort of molecules (it is the same), but in how the molecules are located and move.
1) In gases, the distance between molecules is much greater than the size of the molecules themselves
2) The molecules of the liquid do not disperse over long distances and the liquid under normal conditions retains its volume.
3) Particles of solids are arranged in a certain order. Each particle moves around a certain point in the crystal lattice, like a clock pendulum, that is, it oscillates.
When the temperature decreases, liquids solidify, and when they rise above the boiling point, they turn into a gaseous state. This fact alone indicates that liquids occupy an intermediate position between gases and solids, differing from both. However, the liquid has similarities with each of these states.
There is a temperature at which the boundary between gas and liquid completely disappears. This is the so-called critical point. For each gas there is a known temperature above which it cannot be liquid at any pressure; at this critical temperature the boundary (meniscus) between the liquid and its saturated vapor disappears. The existence of a critical temperature (“absolute boiling point”) was established by D.I. Mendeleev in 1860. The second property that unites liquids and gases is isotropy. That is, at first glance it can be assumed that liquids are closer to gases than to crystals. Just like gases, liquids are isotropic, i.e. their properties are the same in all directions. Crystals, on the contrary, are anisotropic: the refractive index, compressibility, strength and many other properties of crystals in different directions turn out to be different. Solid crystalline substances have an ordered structure with repeating elements, which allows them to be studied by X-ray diffraction (X-ray diffraction method, used since 1912).

What do liquids and gases have in common?
A) Isotropy. The properties of liquids, like gases, are the same in all directions, i.e. are isotropic, unlike crystals, which are anisotropic.
B) Liquids, like gases, do not have a specific shape and take the shape of a container (low viscosity and high fluidity).
Molecules of both liquids and gases move fairly freely, colliding with each other. Previously, it was believed that within the volume occupied by a liquid, any distance exceeding the sum of their radii was considered equally probable, i.e. the tendency towards an ordered arrangement of molecules was denied. Thus, liquids and gases were to a certain extent opposed to crystals.
As research progressed, an increasing number of facts indicated the presence of similarities between the structure of liquids and solids. For example, the values ​​of heat capacities and compressibility coefficients, especially near the melting point, practically coincide with each other, while these values ​​for liquid and gas differ sharply.
Already from this example we can conclude that the picture of thermal motion in liquids at a temperature close to the solidification temperature resembles thermal motion in solids, and not in gases. Along with this, one can note such significant differences between the gaseous and liquid states of matter. In gases, molecules are distributed throughout space completely chaotically, i.e. the latter is considered an example of structureless education. The liquid still has a certain structure. This is experimentally confirmed by X-ray diffraction, which shows at least one clear maximum. The structure of a liquid is the way its molecules are distributed in space. The table illustrates the similarities and differences between gas and liquid states.
Gas phase Liquid phase
1. The distance between molecules l is usually (for low pressures) much larger than the radius of the molecule r: l  r ; Almost the entire volume V occupied by gas is free volume. In the liquid phase, on the contrary, l 2. The average kinetic energy of particles, equal to 3/2kT, is greater than the potential energy U of their intermolecular interaction. The potential energy of interaction of molecules is greater than the average kinetic energy of their movement: U3/2 kT
3. Particles collide during their translational motion, the collision frequency factor depends on the mass of the particles, their size and temperature. Each particle undergoes oscillatory motion in a cage created by the molecules surrounding it. The vibration amplitude a depends on the free volume, a  (Vf/ L)1/3
4. Diffusion of particles occurs as a result of their translational motion, diffusion coefficient D  0.1 - 1 cm2/s (p  105 Pa) and depends on gas pressure
(D  p-1) Diffusion occurs as a result of a particle jumping from one cell to another with activation energy ED,
D  e-ED/RT in non-viscous liquids
D  0.3 - 3 cm2/day.
5. The particle rotates freely, the rotation frequency r is determined only by the moments of inertia of the particle and temperature, the rotation frequency r T1/2 The rotation is inhibited by the walls of the cell, the rotation of the particle is accompanied by overcoming the potential barrier Er, which depends on the forces of intermolecular interaction, vr  e- Er/RT
However, the liquid state is close to the solid state in a number of important indicators (quasicrystallinity). The accumulation of experimental facts indicated that liquids and crystals have much in common. Physicochemical studies of individual liquids have shown that almost all of them possess some elements of a crystalline structure.
Firstly, intermolecular distances in a liquid are close to those in a solid. This is proven by the fact that when the latter melts, the volume of the substance changes slightly (usually it increases by no more than 10%). Secondly, the energy of intermolecular interaction in a liquid and in a solid differs slightly. This follows from the fact that the heat of fusion is much less than the heat of evaporation. For example, for water Hpl = 6 kJ/mol, and Hsp = 45 kJ/mol; for benzene Hpl = 11 kJ/mol, and Hsp = 48 kJ/mol.
Thirdly, the heat capacity of a substance changes very little during melting, i.e. it is close for both of these states. It follows that the nature of the motion of particles in a liquid is close to that in a solid. Fourthly, a liquid, like a solid, can withstand large tensile forces without breaking.
The difference between a liquid and a solid is fluidity: a solid retains its shape, a liquid easily changes it even under the influence of a small force. These properties arise from such structural features of the liquid as strong intermolecular interaction, short-range order in the arrangement of molecules and the ability of molecules to change their position relatively quickly. When a liquid is heated from the freezing point to the boiling point, its properties gradually change; with heating, its similarities with a gas gradually increase.
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, since liquids have such physical properties that in some respects they resemble solids and in others they resemble gases. The similarities between liquids and solids are most pronounced in glassy materials. Their transition from solid to liquid with increasing temperature occurs gradually, and not as a pronounced melting point, they simply become softer and softer, so it is impossible to indicate in which temperature range they should be called solids and in which liquids. We can only say that the viscosity of a glassy substance in a liquid state is less than in a solid state. Solid glasses are therefore often called supercooled liquids. Apparently, the most characteristic property of liquids, which distinguishes them from solids, is low viscosity, i.e. high turnover. Thanks to it, they take the shape of the vessel into which they are poured. At the molecular level, high fluidity means relatively greater freedom of fluid particles. In this respect, liquids resemble gases, although the forces of intermolecular interaction between liquids are greater, the molecules are located closer together and are more limited in their movement.
This can be approached differently - from the point of view of the idea of ​​long-range and short-range order. Long-range order exists in crystalline solids, the atoms of which are arranged in a strictly ordered manner, forming three-dimensional structures that can be obtained by repeating the unit cell many times. There is no long-range order in liquids and glass. This, however, does not mean that they are not ordered at all. The number of nearest neighbors for all atoms is almost the same, but the arrangement of atoms as they move away from any selected position becomes more and more chaotic. Thus, order exists only at short distances, hence the name: short-range order. An adequate mathematical description of the structure of a liquid can only be given with the help of statistical physics. For example, if a liquid consists of identical spherical molecules, then its structure can be described by the radial distribution function g(r), which gives the probability of detecting any molecule at a distance r from the given one chosen as a reference point. This function can be found experimentally by studying the diffraction of x-rays or neutrons, and with the advent of high-speed computers, it began to be calculated by computer simulation, based on existing data on the nature of the forces acting between molecules, or on assumptions about these forces, as well as on Newton's laws of mechanics . By comparing radial distribution functions obtained theoretically and experimentally, it is possible to verify the correctness of assumptions about the nature of intermolecular forces.
In organic substances, the molecules of which have an elongated shape, in one temperature range or another, regions of the liquid phase with long-range orientational order are sometimes found, which manifests itself in a tendency to parallel alignment of the long axes of the molecules. In this case, orientational ordering can be accompanied by coordination ordering of the centers of molecules. Liquid phases of this type are usually called liquid crystals. The liquid crystalline state is intermediate between crystalline and liquid. Liquid crystals possess both fluidity and anisotropy (optical, electrical, magnetic). Sometimes this state is called mesomorphic (mesophase) - due to the absence of long-range order. The upper limit of existence is the clearing temperature (isotropic liquid). Thermotropic (mesogenic) FAs exist above a certain temperature. Typical ones are cyanobiphenyls. Lyotropic - when dissolved, for example, aqueous solutions of soaps, polypeptides, lipids, DNA. The study of liquid crystals (mesophase - melting in two stages - cloudy melt, then transparent, transition from the crystalline phase to the liquid through an intermediate form with anisotropic optical properties) is important for technology purposes - liquid crystal display.
Molecules in a gas move chaotically (randomly). In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves. Molecules in gas move at high speeds (hundreds of m/s). When they collide, they bounce off each other like absolutely elastic balls, changing the magnitude and direction of the velocities. At large distances between molecules, the attractive forces are small and are not able to hold gas molecules near each other. Therefore, gases can expand without limit. Gases are easily compressed, the average distance between molecules decreases, but still remains larger than their size. Gases retain neither shape nor volume; their volume and shape coincide with the volume and shape of the vessel they fill. Numerous impacts of molecules on the walls of the vessel create gas pressure.
Atoms and molecules of solids vibrate around certain equilibrium positions. Therefore, solids retain both volume and shape. If you mentally connect the centers of equilibrium positions of atoms or ions of a solid, you will get a crystal lattice.
The molecules of the liquid are located almost close to each other. Therefore, liquids are very difficult to compress and retain their volume. Molecules of a liquid vibrate around an equilibrium position. From time to time, a molecule makes transitions from one stationary state to another, usually in the direction of the action of an external force. The time of the settled state of a molecule is short and decreases with increasing temperature, and the time of transition of the molecule to a new settled state is even shorter. Therefore, liquids are fluid, do not retain their shape and take the shape of the vessel into which they are poured.

Kinetic theory of liquids Developed by Ya. I. Frenkel, the kinetic theory of liquids considers a liquid as a dynamic system of particles, partly reminiscent of a crystalline state. At temperatures close to the melting point, thermal motion in a liquid is reduced mainly to harmonic vibrations of particles around certain average equilibrium positions. In contrast to the crystalline state, these equilibrium positions of molecules in a liquid are temporary in nature for each molecule. After oscillating around one equilibrium position for some time t, the molecule jumps to a new position located nearby. Such a jump occurs with the expenditure of energy U, therefore the “settled life” time t depends on temperature as follows: t = t0 eU/RT, where t0 is the period of one oscillation around the equilibrium position. For water at room temperature t » 10-10 s, t0 = 1.4 x 10-12 s, i.e. one molecule, having completed about 100 vibrations, jumps to a new position, where it continues to oscillate. From data on the scattering of X-rays and neutrons, it is possible to calculate the particle distribution density function  depending on the distance r from one particle chosen as the center. In the presence of long-range order in a crystalline solid, the function (r) has a number of clear maxima and minima. In a liquid, due to the high mobility of particles, only short-range order is maintained. This clearly follows from the X-ray diffraction patterns of liquids: the function (r) for a liquid has a clear first maximum, a blurry second one, and then (r) = const. The kinetic theory describes melting as follows. In the crystal lattice of a solid, there are always small amounts of vacancies (holes) that slowly wander around the crystal. The closer the temperature is to the melting point, the higher the concentration of “holes”, and the faster they move through the sample. At the melting point, the process of formation of “holes” acquires an avalanche-like cooperative character, the system of particles becomes dynamic, long-range order disappears, and fluidity appears. The decisive role in melting is played by the formation of free volume in the liquid, which makes the system fluid. The most important difference between a liquid and a solid crystalline body is that there is a free volume in the liquid, a significant part of which has the form of fluctuations (“holes”), the wandering of which through the liquid gives it such a characteristic quality as fluidity. The number of such “holes”, their volume and mobility depend on temperature. At low temperatures, a liquid, if it has not turned into a crystalline body, becomes an amorphous solid with very low fluidity due to a decrease in volume and mobility of “holes”. Along with the kinetic theory, the statistical theory of liquids has been successfully developing in recent decades.

Structure of ice and water. The most important and common liquid under normal conditions is water. This is the most common molecule on Earth! It is an excellent solvent. For example, all biological fluids contain water. Water dissolves many inorganic (salts, acids, bases) and organic substances (alcohols, sugars, carboxylic acids, amines). What is the structure of this liquid? We will again have to return to the issue that we considered in the first lecture, namely, to such a specific intermolecular interaction as the hydrogen bond. Water, both in liquid and crystalline form, exhibits anomalous properties precisely because of the presence of many hydrogen bonds. What are these anomalous properties: high boiling point, high melting point and high enthalpy of vaporization. Let's look first at the graph, then at the table, and then at the diagram of a hydrogen bond between two water molecules. In fact, each water molecule coordinates 4 other water molecules around itself: two due to oxygen, as a donor of two lone electron pairs to two protonated hydrogens, and two due to protonated hydrogens, coordinated with the oxygens of other water molecules. In the previous lecture, I showed you a slide with graphs of the melting point, boiling point and enthalpy of vaporization of group VI hydrides depending on the period. These dependences have a clear anomaly for oxygen hydride. All these parameters for water are noticeably higher than those predicted from the almost linear dependence for the following hydrides of sulfur, selenium and tellurium. We explained this by the existence of a hydrogen bond between protonated hydrogen and the electron density acceptor - oxygen. Hydrogen bonding is most successfully studied using vibrational infrared spectroscopy. The free OH group has a characteristic vibrational energy that causes the O-H bond to alternately lengthen and shorten, giving rise to a characteristic band in the infrared absorption spectrum of the molecule. However, if the OH group is involved in a hydrogen bond, the hydrogen atom becomes bound by atoms on both sides and thus its vibration is “damped” and the frequency decreases. The following table shows that increasing the strength and “concentration” of the hydrogen bond leads to a decrease in the absorption frequency. In the above figure, curve 1 corresponds to the maximum of the infrared absorption spectrum of O-H groups in ice (where all H-bonds are connected); curve 2 corresponds to the maximum of the infrared absorption spectrum of O-H groups of individual H2O molecules dissolved in CCl4 (where there are no H bonds - the solution of H2O in CCl4 is too dilute); and curve 3 corresponds to the absorption spectrum of liquid water. If in liquid water there were two types of O-H groups - those that form hydrogen bonds and those that do not - and some O-H groups in water would vibrate in the same way (with the same frequency) as in ice (where they form H- bonds), and others - as in the environment of CCl4 (where they do not form H-bonds). Then the spectrum of water would have two maxima, corresponding to two states of O-H groups, their two characteristic vibration frequencies: with the frequency at which the group vibrates, it absorbs light. But the “two-maximum” picture is not observed! Instead, on curve 3 we see one, very blurred maximum, extending from the maximum of curve 1 to the maximum of curve 2. This means that all O-H groups in liquid water form hydrogen bonds - but all these bonds have a different energy, “loose” (have different energy), and in different ways. This shows that the picture in which some of the hydrogen bonds in water are broken and some are preserved is, strictly speaking, incorrect. However, it is so simple and convenient for describing the thermodynamic properties of water that it is widely used - and we will also turn to it. But we must keep in mind that it is not entirely accurate.
Thus, IR spectroscopy is a powerful method for studying hydrogen bonding, and much information about the structure of liquids and solids associated with it has been obtained using this spectral method. As a result, for liquid water the ice-like model (O.Ya. Samoilov’s model) is one of the most generally accepted. According to this model, liquid water has an ice-like tetrahedral framework disturbed by thermal motion (evidence and consequence of thermal motion - Brownian motion, which was first observed by the English botanist Robert Brown in 1827 on pollen under a microscope) (each water molecule in an ice crystal is connected by hydrogen bonds with a reduced energy compared to that in ice - “loose” hydrogen bonds) with four water molecules surrounding it), the voids of this frame are partially filled with water molecules, and the water molecules located in the voids and in the nodes of the ice-like frame are energetically unequal.

Unlike water, in an ice crystal, at the nodes of the crystal lattice there are water molecules of equal energy and they can only perform vibrational movements. In such a crystal there is both short- and long-range order. In liquid water (as for a polar liquid), some elements of the crystal structure are preserved (and even in the gas phase, liquid molecules are ordered into small, unstable clusters), but there is no long-range order. Thus, the structure of a liquid differs from the structure of a gas in the presence of short-range order, but differs from the structure of a crystal in the absence of long-range order. This is most convincingly demonstrated by the study of X-ray scattering. The three neighbors of each molecule in liquid water are located in one layer and are at a greater distance from it (0.294 nm) than the fourth molecule from the adjacent layer (0.276 nm). Each water molecule in the ice-like framework forms one mirror-symmetric (strong) and three centrally symmetric (less strong) bonds. The first refers to the bonds between water molecules of a given layer and neighboring layers, the rest - to the bonds between water molecules of the same layer. Therefore, a quarter of all connections are mirror-symmetric, and three-quarters are centrally symmetric. Ideas about the tetrahedral environment of water molecules have led to the conclusion that its structure is highly delicacy and the presence of voids in it, the dimensions of which are equal to or greater than the dimensions of water molecules.

Elements of the structure of liquid water. a - elementary water tetrahedron (open circles - oxygen atoms, black halves - possible positions of protons on the hydrogen bond); b - mirror-symmetric arrangement of tetrahedra; c - centrally symmetrical arrangement; d - location of oxygen centers in the structure of ordinary ice. Water is characterized by significant forces of intermolecular interaction due to hydrogen bonds, which form a spatial network. As we said in the previous lecture, a hydrogen bond is caused by the ability of a hydrogen atom connected to an electronegative element to form an additional bond with an electronegative atom of another molecule. The hydrogen bond is relatively strong and amounts to several 20-30 kilojoules per mole. In terms of strength, it occupies an intermediate place between the van der Waals energy and the energy of a typical ionic bond. In a water molecule, the energy of the H-O chemical bond is 456 kJ/mol, and the energy of the H…O hydrogen bond is 21 kJ/mol.

Hydrogen compounds
Molecular weight Temperature,  C
Freezing Boiling
H2Te 130 -51 -4
H2Se 81 -64 -42
H2S 34 -82 -61
H2O 18 0! +100!

Ice structure. Normal ice. Dotted line - H-bonds. In the openwork structure of the ice, small cavities are visible, surrounded by H2O molecules.
Thus, the structure of ice is an openwork structure of water molecules connected to each other only by hydrogen bonds. The arrangement of water molecules in the ice structure determines the presence of wide channels in the structure. As ice melts, water molecules “fall” into these channels, which explains the increase in the density of water compared to the density of ice. Ice crystals occur in the form of regular hexagonal plates, tabular formations, and intergrowths of complex shapes. The structure of normal ice is dictated by hydrogen H bonds: it is good for the geometry of these bonds (the O-H faces directly at the O), but not so good for the tight Vander Waals contact of the H2O molecules. Therefore, the structure of ice is openwork; in it, H2O molecules envelop microscopic (smaller than an H2O molecule in size) pores. The lacy structure of ice leads to two well-known effects: (1) ice is less dense than water, it floats in it; and (2) under strong pressure - for example, the blade of a skate melts the ice. Most of the hydrogen bonds that exist in ice are also preserved in liquid water. This follows from the small heat of melting of ice (80 cal/g) compared to the heat of boiling of water (600 cal/g at 0°C). One could say that in liquid water only 80/(600+80) = 12% of the H-bonds existing in ice are broken. However, this picture - that some of the hydrogen bonds in water are broken, and some are preserved - is not entirely accurate: rather, all the hydrogen bonds in water are becoming loose. This is well illustrated by the following experimental data.

Structure of solutions. From specific examples for water, let's move on to other liquids. Different liquids differ from each other in the sizes of their molecules and the nature of intermolecular interactions. Thus, in each specific liquid there is a certain pseudocrystalline structure, characterized by short-range order and, to some extent, reminiscent of the structure obtained when a liquid freezes and turns into a solid. When another substance is dissolved, i.e. When a solution is formed, the nature of intermolecular interactions changes and a new structure appears with a different arrangement of particles than in a pure solvent. This structure depends on the composition of the solution and is specific to each specific solution. The formation of liquid solutions is usually accompanied by a solvation process, i.e. alignment of solvent molecules around solute molecules due to the action of intermolecular forces. There are short-range and long-range solvation, i.e. Primary and secondary solvation shells are formed around the molecules (particles) of the dissolved substance. In the primary solvation shell, there are solvent molecules in close proximity, which move together with the solute molecules. The number of solvent molecules located in the primary solvation shell is called the solvation coordination number, which depends on both the nature of the solvent and the nature of the solute. The secondary solvation shell includes solvent molecules that are located at significantly greater distances and affect the processes occurring in the solution due to interaction with the primary solvation shell.
When considering the stability of solvates, a distinction is made between kinetic and thermodynamic stability.
In aqueous solutions, the quantitative characteristics of kinetic hydration (O.Ya. Samoilov) are the values ​​i/ and Ei=Ei-E, where i and  are the average residence time of water molecules in the equilibrium position near the i-th ion and in pure water , and Ei and E are the activation energy of exchange and the activation energy of the self-diffusion process in water. These quantities are related to each other by an approximate relationship:
i/  exp(Ei/RT) In this case,
if EI  0, i/  1 (the exchange of water molecules closest to the ion occurs less frequently (slower) than the exchange between molecules in pure water) – positive hydration
if EI  0, i/  1 (the exchange of water molecules closest to the ion occurs more often (faster) than the exchange between molecules in pure water) – negative hydration

So, for the lithium ion EI = 1.7 kJ/mol, and for the cesium ion Ei= - 1.4 kJ/mol, i.e. a small “hard” lithium ion holds water molecules more strongly than a large and “diffuse” cesium ion having the same charge. The thermodynamic stability of the resulting solvates is determined by the change in the Gibbs energy during solvation (solvG) = (solvH) - T(solvS). The more negative this value is, the more stable the solvate. This is mainly determined by negative values ​​of the enthalpy of solvation.
The concept of solutions and theories of solutions. True solutions are obtained spontaneously when two or more substances come into contact, due to the destruction of bonds between particles of one type and the formation of bonds of another type, and the distribution of the substance throughout the volume due to diffusion. Solutions according to their properties are divided into ideal and real, solutions of electrolytes and non-electrolytes, diluted and concentrated, unsaturated, saturated and supersaturated. The properties of rasters depend on the nature and magnitude of the IMF. These interactions can be of a physical nature (van der Waals forces) and a complex physicochemical nature (hydrogen bond, ion-molecular, charge transfer complexes, etc.). The process of solution formation is characterized by the simultaneous manifestation of attractive and repulsive forces between interacting particles. In the absence of repulsive forces, particles would merge (stick together) and liquids could be compressed indefinitely; in the absence of attractive forces, liquids or solids could not be obtained. In the previous lecture we looked at the physical and chemical theories of solutions.
However, the creation of a unified theory of solutions encounters significant difficulties and at present it has not yet been created, although research is being carried out using the most modern methods of quantum mechanics, statistical thermodynamics and physics, crystal chemistry, X-ray diffraction analysis, optical methods, and NMR methods. Reactive field. Continuing our consideration of the forces of intermolecular interaction, let us consider the concept of a “reactive field,” which is important for understanding the structure and structure of condensed matter and real gases, in particular the liquid state, and therefore the entire physical chemistry of liquid solutions.
The reactive field occurs in mixtures of polar and nonpolar molecules, for example, for mixtures of hydrocarbons and naphthenic acids. Polar molecules influence a field of a certain symmetry (the symmetry of the field is determined by the symmetry of vacant molecular orbitals) and intensity H on non-polar molecules. The latter are polarized due to charge separation, which leads to the appearance (induction) of a dipole. A molecule with an induced dipole, in turn, affects a polar molecule, changing its electromagnetic field, i.e. excites a reactive (response) field. The emergence of a reactive field leads to an increase in the interaction energy of particles, which is expressed in the creation of strong solvation shells of polar molecules in a mixture of polar and non-polar molecules.
The reactive field energy is calculated using the following formula: where:
sign “-” - determines the attraction of molecules
S – static electrical permittivity
infinite – dielectric constant due to the electronic and atomic polarizability of molecules
NA - Avogadro's number
VM – volume occupied by 1 mole of a polar substance in an isotropic liquid v = dipole moment
ER - energy of 1 mole of polar substance in solution
The "reactive field" concept will allow us to better understand the structure of pure liquids and solutions. The quantum chemical approach to the study of the reactive field was developed in the works of M. V. Bazilevsky and his colleagues at the Scientific Research Institute of Physics and Chemistry named after. L. Ya. Karpova Thus, the problem of the liquid state awaits its young researchers. The cards are in your hands.

All objects and things that surround us every day are composed of various substances. At the same time, we are accustomed to consider only something solid as objects and things - for example, a table, chair, cup, pen, book, and so on.

Three states of matter

But we don’t consider water from the tap or steam coming from hot tea to be objects and things. But all this is also part of the physical world, it’s just that liquids and gases are in a different state of matter. So, There are three states of matter: solid, liquid and gaseous. And any substance can be in each of these states in turn. If we take an ice cube out of the freezer and heat it, it will melt and turn into water. If we leave the burner on, the water will heat up to 100 degrees Celsius and soon turn into steam. Thus, we observed the same substance, that is, the same set of molecules, in turn in different states of matter. But if the molecules remain the same, what then changes? Why is ice hard and retains its shape, water easily takes the shape of a cup, and steam completely scatters in different directions? It's all about the molecular structure.

Molecular structure of solids such that the molecules are located very close to each other (the distance between the molecules is much less than the size of the molecules themselves), and it is very difficult to move the molecules in this arrangement. Therefore, solids retain volume and keep their shape. Molecular structure of liquid characterized by the fact that the distance between the molecules is approximately equal to the size of the molecules themselves, that is, the molecules are no longer as close as in solids. This means that they are easier to move relative to each other (which is why liquids take on different shapes so easily), but the attractive force of the molecules is still sufficient to prevent the molecules from flying apart and maintaining their volume. And here molecular structure of gas, on the contrary, does not allow the gas to either maintain volume or maintain shape. The reason is that the distance between gas molecules is much greater than the size of the molecules themselves, and even the slightest force can destroy this shaky system.

The reason for the transition of a substance to another state

Now let’s find out what is the reason for the transition of a substance from one state to another. For example, why does ice become water when heated? The answer is simple: The thermal energy of the burner is converted into the internal energy of ice molecules. Having received this energy, the ice molecules begin to vibrate faster and faster and, in the end, become out of control of neighboring molecules. If we turn off the heating device, then the water will remain water, but if we leave it on, then the water will turn into steam for a reason already known there.

Due to the fact that solids retain volume and shape, they are the ones we associate with the world around us. But if we look closely, we will find that gases and liquids also occupy an important part of the physical world. For example, the air around us consists of a mixture of gases, the main of which, nitrogen, can also be a liquid - but for this it must be cooled to a temperature of almost minus 200 degrees Celsius. But the main element of an ordinary paw - a tungsten filament - can be melted, that is, turned into liquid, on the contrary, only at a temperature of 3422 degrees Celsius.