The structure of gaseous, liquid and solid bodies. Structure of liquid and gas Molecular structure of liquid

The molecular kinetic theory makes it possible to understand why a substance can be in gaseous, liquid and solid states.

Gas. In gases, the distance between atoms or molecules is, on average, many times greater than the size of the molecules themselves (Fig. 10). For example, at atmospheric pressure, the volume of a vessel is tens of thousands of times greater than the volume of gas molecules in the vessel.

Gases are easily compressed, since when a gas is compressed, only the average distance between molecules decreases, but the molecules do not “squeeze” each other (Fig. 11).

Molecules with huge speeds - hundreds of meters per second - move in space. Colliding, they bounce off each other in different directions like billiard balls.
Weak forces of attraction of gas molecules are not able to keep them near each other. Therefore, gases can expand indefinitely. They retain neither shape nor volume.
Numerous impacts of molecules on the walls of the vessel create gas pressure.

Liquids. In liquids, the molecules are located almost close to each other (Fig. 12). Therefore, a molecule in a liquid behaves differently than in a gas. Clamped, as in a cell, by other molecules, it performs a “run in place” (oscillates around the equilibrium position, colliding with neighboring molecules). Only from time to time does it make a "jump", breaking through the "bars of the cage", but then it falls into a new "cage" formed by new neighbors. The “sedentary life” time of a water molecule, i.e., the time of oscillations around one specific equilibrium position, at room temperature is on average 10–11 s. The time of one oscillation is much shorter (10–12 – 10–13 s). As the temperature rises, the “sedentary life” of molecules decreases. The nature of molecular motion in liquids, first established by the Soviet physicist Ya. I. Frenkel, makes it possible to understand the basic properties of liquids.

Frenkel Yakov Ilyich (1894 - 1952) - an outstanding Soviet theoretical physicist who made a significant contribution to various fields of physics. Ya. I. Frenkel is the author of the modern theory of the liquid state of matter. He laid the foundations of the theory of ferromagnetism. The works of Ya. I. Frenkel on atmospheric electricity and the origin of the Earth's magnetic field are widely known. The first quantitative theory of fission of uranium nuclei was created by Ya. I. Frenkel.

Liquid molecules are located directly next to each other. Therefore, when you try to change the volume of the liquid even by a small amount, the deformation of the molecules themselves begins (Fig. 13). And this requires a lot of power. This explains the low compressibility of liquids.

Liquids, as you know, are fluid, that is, they do not retain their shape. This is explained as follows. If the liquid does not flow, then the jumps of molecules from one "sedentary" position to another occur with the same frequency but in all directions (Fig. 12). The external force does not noticeably change the number of molecular jumps per second, but the jumps of molecules from one "sedentary" position to another occur predominantly in the direction of the external force (Fig. 14). That is why the liquid flows and takes the form of a vessel.
Solids. Atoms or molecules of solids, unlike liquids, oscillate around certain equilibrium positions. True, sometimes molecules change their equilibrium position, but this happens extremely rarely. That is why solids retain not only volume, but also shape.

There is another important difference between liquids and solids. A liquid can be compared to a crowd, the individual members of which are uneasily pushing in place, and a solid body is like a slender cohort, the members of which, although they do not stand at attention (due to thermal motion), maintain certain intervals on average between themselves. If we connect the centers of equilibrium positions of atoms or ions of a solid body, then we get the correct spatial lattice, called crystalline. Figures 15 and 16 show the crystal lattices of table salt and diamond. The internal order in the arrangement of atoms of crystals leads to geometrically correct external forms. Figure 17 shows Yakut diamonds.

A qualitative explanation of the basic properties of matter on the basis of molecular kinetic theory, as you have seen, is not particularly difficult. However, the theory that establishes quantitative relationships between experimentally measured quantities (pressure, temperature, etc.) and the properties of the molecules themselves, their number and speed of movement, is very complex. We confine ourselves to consideration of the theory of gases.

1. Provide evidence for the existence of thermal motion of molecules. 2. Why is Brownian motion noticeable only for particles of small mass? 3. What is the nature of molecular forces? 4. How do the forces of interaction between molecules depend on the distance between them? 5. Why do two lead bars with smooth, clean cuts stick together when pressed against each other? 6. What is the difference between the thermal motion of the molecules of gases, liquids and solids?

Basic physical characteristics of liquids and gases.

LECTURE 3

The subject of the study of fluid and gas mechanics is a physical body, in which the relative position of its elements changes by a significant amount when sufficiently small forces of the corresponding direction are applied. Thus, the main property of a liquid body (or simply liquid) is fluidity. The property of fluidity is possessed by both drop liquids (actually liquids, such as, for example, water, gasoline, industrial oils), and gases (air, nitrogen, hydrogen, carbon dioxide). A significant difference in the behavior of liquids and gases, explained from the point of view of the molecular structure, will be determined by the presence of a free surface in a dropping liquid adjoining the gas, the presence of surface tension, the possibility of a phase transition, etc.

All material bodies, regardless of their state of aggregation: solid, liquid or gaseous, have an internal molecular (atomic) structure with a characteristic internal thermal, microscopic the movement of molecules. Depending on the quantitative relationship between the kinetic energy of molecular motion and the potential energy of intermolecular force interaction, various molecular structures and varieties of internal motion of molecules arise.

AT solids is of primary importance molecular interaction energy molecules, as a result of which, under the action of adhesion forces, the molecules are arranged in regular crystal lattices with positions of stable equilibrium at the nodes of this lattice. Thermal motions in a solid are vibrations of molecules relative to lattice nodes with a frequency of about 10 12 Hz and an amplitude proportional to the distance between lattice nodes.

In contrast to a solid body, gases there are no cohesive forces between molecules. Gas molecules make random motions, and their interaction is reduced only to collisions. In the intervals between collisions, the interaction between molecules can be neglected, which corresponds to the smallness of the potential energy of the force interaction of molecules compared to the kinetic energy of their chaotic motion. The average distance between two successive collisions of molecules determines free path length. The average speed of the thermal motion of molecules is comparable to the speed of propagation of small perturbations (the speed of sound) in a given state of the gas.

liquid bodies in terms of their molecular structure and thermal motion, molecules occupy an intermediate state between solid and gaseous bodies. According to existing views around some, central, molecules are grouped by neighboring molecules that perform small vibrations with a frequency close to the frequency of vibrations of molecules in the lattice of a solid body and an amplitude of the order of the average distance between molecules. The central molecule either (when the liquid is at rest) remains immobile or migrates at a speed that coincides in value and direction with the average velocity of the macroscopic movement of the liquid. In a liquid, the potential energy of interaction of molecules comparable in order with the kinetic energy of their thermal motion. The proof of the presence of vibrations of molecules in liquids is the "Brownian motion" of the smallest solid particles introduced into the liquid. The oscillations of these particles are easily observed in the field of a microscope and can be considered as the result of the collision of solid particles with liquid molecules. The presence of intermolecular interaction in liquids determines the existence of the surface tension of the liquid at its boundary with any other medium, which forces it to take a form in which its surface is minimal. Small volumes of liquid are usually in the form of a globular droplet. Because of this, fluids in hydraulics are called drip.

It should be noted that the boundary between solid and liquid bodies is not always clearly defined. Thus, when large forces are applied to a dropping liquid (for example, a liquid jet), with a short interaction time, the latter acquires properties close to those of a brittle solid. A jet of liquid at high pressures in front of the hole has properties close to those of a solid body. So, at pressures greater than 10 8 Pa, a water jet cuts a steel plate; at a pressure of about 5 10 7 Pa - cuts granite, at pressures of 1.5 10 7 - 2 10 7 Pa - destroys coal. Pressure (1.5 - 2)·10 6 Pa is sufficient for the destruction of various soils.

Under certain conditions, the boundary between liquid and gaseous bodies may also be absent. Gases fill the entire volume provided to them, their density can vary over a wide range depending on the applied forces. Liquids, filling a vessel of a larger volume than the volume of the liquid, form a free surface - the interface between liquid and gas. Under normal conditions, the volume of a liquid depends little on the forces applied to it. Near the critical state, the difference between liquid and gas becomes hardly noticeable. Recently, the concept of a fluid state has appeared, when particles of a liquid with sizes of several nanometers are sufficiently uniformly mixed with their vapor. In this case, there is no visual difference between liquid and vapor.

Steam differs from gas in that its state when moving is close to saturation. Therefore, under certain conditions, it can partially condense and form a two-phase medium. With rapid expansion, the condensation process is delayed, and then, when a certain supercooling is reached, it proceeds like an avalanche. In this case, the laws of vapor flow can differ significantly from the laws of the flow of liquids and gases.

The properties of solids, liquids and gases are due to their different molecular structure . However, the main hypothesis of fluid and gas mechanics is the continuum hypothesis, according to which the fluid is represented as a continuously distributed substance (continuum) that fills space without voids.

Due to the weak bonds between the molecules of liquids and gases (which is why they are fluid), a concentrated force cannot be applied to their surfaces, but only a distributed load. The directed movement of a liquid is composed of the movement of a huge number of molecules moving randomly in all directions relative to each other. In fluid and gas mechanics, which studies their directional motion, the distribution of all fluid characteristics in the space under consideration is assumed to be continuous. The molecular structure is taken into account only in the mathematical description of the physical characteristics of a liquid or gas, which was done when considering transport processes in gases.

The model of a continuous medium is very useful in studying its motion, as it allows the use of a well-developed mathematical apparatus of continuous functions.

Quantitatively, the limits of applicability of the mathematical apparatus of continuum mechanics for gas are set by the value of the Knudsen criterion - the ratio of the mean free path of gas molecules l to the characteristic size of the flow L

If a Kn< 0.01, then the gas flow can be considered as a continuous medium flow. When a continuous medium flows around a solid surface, its molecules stick to it (Prandtl's hypothesis of sticking), and therefore the velocity of the liquid on the surface of solids is always equal to the velocity of this surface, and the temperature of the liquid on the wall is equal to the temperature of the wall.

If a Kn> 0.01, then the motion of a rarefied gas is considered using the mathematical apparatus of molecular kinetic theory.

In mechanical engineering, the continuum hypothesis may not hold true when calculating the flow of a liquid or gas in narrow gaps. The molecules have dimensions of the order of 10 -10 m; at gaps of the order of 10 -9 m, typical for nanotechnology, significant deviations of the calculated data obtained using the usual equations of fluid dynamics can be observed

The liquid state, occupying an intermediate position between gases and crystals, combines some of the features of both of these states. In particular, for liquids, as well as for crystalline bodies, the presence of a certain volume is characteristic, and at the same time, a liquid, like a gas, takes the form of the vessel in which it is located. Further, the crystalline state is characterized by an ordered arrangement of particles (atoms or molecules); in gases, in this sense, complete chaos reigns. According to radiographic studies, in relation to the nature of the arrangement of the particles of the liquid, they also occupy an intermediate position. The so-called short-range order is observed in the arrangement of liquid particles. This means that with respect to any particle, the location of its nearest neighbors is ordered. However, as one moves away from a given particle, the arrangement of other particles with respect to it becomes less and less ordered, and rather quickly the order in the arrangement of particles completely disappears. In crystals, there is a long-range order: the ordered arrangement of particles with respect to any particle is observed within a significant volume.

The presence of short-range order in liquids is the reason why the structure of liquids is called quasi-crystalline (crystal-like).

Due to the lack of long-range order, liquids, with few exceptions, do not show the anisotropy that is characteristic of crystals with their regular arrangement of particles. In liquids with elongated molecules, the same orientation of molecules is observed within a significant volume, which determines the anisotropy of optical and some other properties. Such liquids are called liquid crystals. They have ordered only the orientation of the molecules, while the mutual arrangement of the molecules, as in ordinary liquids, does not show long-range order.

The intermediate position of liquids is due to the fact that the liquid state is particularly complex in its properties. Therefore, his theory is much less developed than the theory of crystalline and gaseous states. Until now, there is no completely complete and generally accepted theory of liquids. Significant contributions to the development of a number of problems in the theory of the liquid state belong to the Soviet scientist Ya. I. Frenkel.

According to. Frenkel, thermal motion in liquids has the following character. Each molecule oscillates around a certain equilibrium position for some time. From time to time, the molecule changes its place of equilibrium, jumping to a new position, separated from the previous one by a distance of the order of the size of the molecules themselves. Thus, the molecules only move slowly inside the liquid, staying part of the time near certain places. According to the figurative expression of Ya. I. Frenkel, molecules wander throughout the entire volume of the liquid, leading a nomadic lifestyle, in which short-term journeys are replaced by relatively long periods of settled life. The durations of these stops are very different and randomly alternate with each other, but the average duration of oscillations around the same equilibrium position turns out to be a certain value for each liquid, which decreases sharply with increasing temperature. In this regard, with increasing temperature, the mobility of molecules increases greatly, which in turn entails a decrease in the viscosity of liquids.

There are solids that in many respects are closer to liquids than to crystals. Such bodies, called amorphous, do not exhibit anisotropy. In the arrangement of their particles, as in liquids, there is only short-range order. The transition from an amorphous solid to a liquid upon heating occurs continuously, while the transition from a crystal to a liquid occurs abruptly (more on this will be discussed in § 125). All this gives grounds to consider amorphous solids as supercooled liquids, the particles of which, due to the greatly increased viscosity, have limited mobility.

Glass is a typical example of an amorphous solid. Amorphous bodies also include resins, bitumen, etc.

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LIQUID THEORY. 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 similarity between liquids and solids is most pronounced in glassy materials. Their transition from solid to liquid with increasing temperature occurs gradually, they just become softer and softer, so it is impossible to specify 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 the liquid state is less than in the solid state. Solid glass is therefore often referred to as a supercooled liquid.

Apparently, the most characteristic property of liquids, which distinguishes them from solids, is their low viscosity (high fluidity). Thanks to her, they take the shape of the vessel in which they are poured. At the molecular level, high fluidity means a relatively large freedom of fluid particles. In this, liquids resemble gases, although the forces of intermolecular interaction of liquids are greater, the molecules are closer and more limited in their movement.

What has been said can be approached in another way - 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 repeated repetition of the unit cell. An example of a two-dimensional long-range order is shown in fig. one, a. There is no long-range order in liquid and glass. This, however, does not mean that they are not ordered at all. The liquid is characterized by a pattern similar to that shown in Fig. one, b. 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 small 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 the reference point. Experimentally, this function can be found 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 the available data on the nature of the forces acting between molecules, or on assumptions about these forces, as well as on the laws of Newtonian mechanics . Comparing the radial distribution functions obtained theoretically and experimentally, one can verify the correctness of the assumptions about the nature of intermolecular forces.

In organic substances, the molecules of which have an elongated shape, in one or another temperature range, regions of the liquid phase with a 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 molecular centers. Liquid phases of this type are commonly referred to as liquid crystals; computer simulation is also very useful for understanding their structural properties.

In gases, there is no order in the arrangement of molecules. Thus, liquids occupy an intermediate position between crystalline solids and gases, i.e. between completely ordered and completely disordered molecular systems. That is why the theory of liquids is so complicated. Below, we will consider the relationship between solids, liquids, and gases, as well as between the various properties of liquids, using simple molecular models.

Liquid, gas and intermolecular forces.

1 cm 3 of gas at a temperature of 0 ° C and normal pressure contains approximately 2.7 × 10 19 molecules, so that the average distance between them is about 30 × 10 -8 cm, or 30 Å. Since the diameter of the molecules themselves is only a few angstroms, it is logical to assume that the interaction between gas molecules is always negligibly small, except for the moments of their collisions. Thus, we arrive at a model of a gas, in which the molecules are represented as balls moving independently of each other, colliding with each other and with the walls of the vessel in which the gas is enclosed. At a temperature of 0 ° C, the speed of the molecules is several hundred meters per second, and their collisions with the walls of the vessel create a noticeable pressure. A more detailed consideration of this model gives the relationship between pressure P, volume V and thermodynamic temperature T (T= °C + 273)

(1)PV/T= const (for a given amount of gas).

This relationship - the so-called ideal gas equation of state - is a generalized record of the laws of Boyle - Mariotte, Gay-Lussac and Charles, and the behavior of most gases is described by him with good accuracy. Equation (1) would always hold if the gas remained a gas, regardless of a decrease in temperature or an increase in pressure. However, it is well known that all gases can be liquefied if compressed or cooled sufficiently. For every gas there is a so-called critical temperature T c, below which it can always be liquefied by increasing the pressure; higher T c gas cannot be liquefied under any circumstances. This means that the model of independently moving molecules under conditions where the temperature is higher T c, is only approximate, and below T c at high pressures and densities, it is generally incorrect. The existence of a liquid state below T c suggests that attractive forces act between the molecules, since otherwise it is generally impossible to understand why they remain close to each other. However, in addition to attraction, molecules also experience mutual repulsion - we are convinced of this when we try to reduce the volume of a liquid (or solid). Attractive forces act at greater distances than repulsive forces, but both are electrostatic in nature.

If we introduce corrections for the cohesion of molecules and their volume into the ideal gas model, then we obtain an equation, generally speaking, different from (1). One of these equations, derived by J. van der Waals, has the form

(2)(P + a/V 2) (V - b)/T= const.

Here a and b are constants characteristic of the given gas. This equation also predicts the existence of a critical temperature T c and qualitatively describes the observed transition between the gaseous and liquid phases.

Let us consider some practical consequences of equation (2). On fig. 2 is a plot of gas pressure versus volume. Let some amount of gas occupy the volume V 1 at temperature T 1 and pressure P one . As the volume decreases, the pressure increases and the state of the gas changes: from the point A he goes to the point B. Here, the gas begins to condense, and a further decrease in volume no longer leads to a change in pressure. When moving along a straight line BC the amount of liquid increases until at the point C the gas will not be completely liquefied. The constant pressure corresponding to this process is called the saturation vapor pressure at a given temperature. T one . At all points of the segment BC there is an equilibrium (thermodynamic) between liquid and gas. This means that the number of molecules evaporating from the surface of a liquid in 1 s is exactly equal to the number of molecules condensing from vapor to liquid. To further reduce the volume, it is necessary to create a very high pressure in order to overcome the forces of mutual repulsion of the liquid molecules. This situation corresponds to the vertical line CD. Curve ABCD is called an isotherm because all its points have the same temperature. If the same experiment is carried out at a higher temperature, then, in accordance with the van der Waals equation, we will obtain an isotherm with the same course, only the segment BC will become shorter. Finally, at the critical temperature T c this segment is generally contracted to a point with coordinates T c and P c. At this point, liquid and gas are indistinguishable. At temperatures above T c, the van der Waals equation (2) transforms into equation (1) (the curve corresponding to the temperature T 2 in fig. 2). The values of critical temperatures and their corresponding pressures are given in the following table:

Surface tension.

As we have seen, taking into account intermolecular forces makes it possible to correctly explain the process of gas condensation. Let us now try to describe some of the physical properties of liquids, taking these forces into account.

Imagine a drop of mercury. We can flatten it slightly with our finger, but as soon as we remove the finger, the drop will again gather into a ball. She behaves as if she were wrapped in an elastic film. This is the manifestation of the surface tension effect. Its nature will become clear if we turn to Fig. 3. Here A and B- two molecules of liquid, the first in the volume, the second on the surface. In both cases, they are affected by attractive forces from other molecules, but only those that are inside a sphere with a diameter of several angstroms, since these forces quickly decrease with distance. For a molecule A such a sphere lies completely inside the fluid, so the resultant of all forces is zero. Molecule B, located on the surface, will be drawn into the liquid, since only attractive forces from the molecules located in the lower hemisphere act on it. The same forces, perpendicular to the surface and directed inside the liquid, act on all molecules near the surface; they create surface tension.

Surface tension S quantitatively defined as the force acting per unit length of a line on the surface of a liquid. Consider a soap film stretched over a vertical frame of two thin wires TUV and PQ(Fig. 4). wire PQ not fixed and can move freely. It will move downward under the action of gravity until the latter is balanced by the force due to surface tension. Since the film has two surfaces, the force acting on the wire is 2 SL, where L- the length of the wire section PQ in contact with the film.

Due to the presence of surface tension, any increase in the surface area of a liquid is associated with energy costs. That is why small liquid drops take on a spherical shape: the ratio of their surface area to volume becomes minimal, and after that, the potential energy is also minimized. Large drops are deformed under the influence of gravity.

capillary phenomena.

A drop of water on a clean glass plate loses its spherical shape and spreads, forming a thin film. This happens because the cohesive forces between water and glass molecules exceed similar forces between water molecules - water wets the glass. A drop of mercury on the same plate remains spherical: the cohesive forces between mercury molecules are greater than the cohesive forces between mercury and glass molecules - mercury does not wet glass. This explains the so-called capillary phenomena observed in a thin glass capillary tube (Fig. 5). If you lower the capillary into a vessel with water, then the water will rise through it above the level in the vessel, and its surface (meniscus) will have a concave shape. The level of mercury in the same capillary, on the contrary, will be lower than the level in the vessel itself, and the meniscus will be convex. Since the adhesion between the molecules of water and glass is stronger than between the molecules of water themselves, water, as it were, “climbs” along the walls of the capillary until the pressure of its column in the capillary is balanced by the pressure due to intermolecular forces. A concave meniscus is formed because water molecules near the walls of the capillary are affected by a nonzero force directed towards the wall. For mercury, the picture is reversed.

Boiling liquids.

When a liquid boils in an open vessel, the pressure inside the vapor bubbles formed in the liquid must be at least equal to atmospheric pressure - otherwise the bubbles will simply collapse. Therefore, at the boiling point, the vapor pressure of a liquid is equal to atmospheric pressure. At a sufficiently high altitude, the boiling point of a liquid is lower than at sea level, since the barometric pressure decreases with altitude. Thus, the boiling point of water at an altitude of 4000 m is only about 85 ° C, while at sea level it is 100 ° C.

Boiling is the intense evaporation of a liquid, which occurs not only from the surface, but throughout its entire volume, inside the resulting vapor bubbles. To go from liquid to vapor, molecules must acquire the energy needed to overcome the attractive forces that hold them in the liquid. For example, to evaporate 1 g of water at a temperature of 100 ° C and a pressure corresponding to atmospheric pressure at sea level, it is required to spend 2258 J, of which 1880 go to separate molecules from the liquid, and the rest go to work to increase the volume occupied by the system, against atmospheric pressure forces (1 g of water vapor at 100 ° C and normal pressure occupies a volume of 1.673 cm 3, while 1 g of water under the same conditions is only 1.04 cm 3).

The boiling point of a solution of a non-volatile substance is usually higher than that of a pure solvent. Since a liquid boils when its vapor pressure becomes equal to atmospheric pressure, this pattern means that the vapor pressure of a solution of a non-volatile substance at a given temperature is lower than that of a pure solvent.

Solidification of liquids.

Usually, when liquids solidify, their volume decreases somewhat (by about 10%), although there are exceptions to this rule. For example, water gallium and bismuth expand when solidified, so that the solidified substance floats on the surface of the liquid. The behavior of liquids near the solidification temperature can show other anomalies, for example, when the temperature rises in the range from 0 to 4 ° C, water contracts. To explain these experimental facts, let us first consider the transition from a liquid state to a solid state for "normal" substances, such as aluminum. As x-ray diffraction analysis shows, aluminum crystallizes with the formation of a face-centered cubic lattice (Fig. 6), in which each atom is surrounded by twelve nearest neighbors located at a distance of 2.86 Å (2.86 × 10–8 cm) from it. If the atoms are considered spheres, then this arrangement corresponds to their most dense packing (“close-packed” structure). In liquid aluminum, there is no long-range order, but some short-range order still remains. According to X-ray diffraction data, each atom in it is surrounded by 10–11 nearest neighbors located at a distance of 2.96 Å from it, i.e. the structure of liquid aluminum near the solidification temperature is similar to the structure of solid aluminum, but somewhat more “loose”. For water, gallium and bismuth, the opposite picture is observed: near the solidification temperature, their structure is more “loose” not in the liquid, but in the solid state. The answer to the question about the causes of such an anomaly should be sought in the structural features of their molecules and the bonds between them in different states of aggregation. Consider, for example, water and ice. Both of them are built from the same molecules, which consist of doubly ionized negative oxygen ions (O 2–) and two singly ionized positive hydrogen ions (H +). In a water molecule, these three ions form a triangle with two protons at the base and oxygen at the top (respectively, two small circles and one large circle in Fig. 7); the angle between the O–H bonds is 104°. In the structure of ice, H 2 O molecules are arranged in such a way that each oxygen atom is surrounded by four hydrogen atoms located at the vertices of the tetrahedron. This provides the maximum energy gain due to the attraction between the positive and negative ions, but the structure becomes much looser. When ice melts, this rather uneconomical packing of H 2 O molecules is gradually replaced by a denser one, and in the range from 0 to 4 ° C, the volume of the substance gradually decreases. The loose structure of solid gallium and bismuth is also due to the peculiarities of interactions between atoms, but the nature of these bonds is much more complicated than that of ice.

Dissolution of liquids.

It is well known that water dissolves alcohol in any amount, while it does not mix with mercury and oil at all. Similarly, benzene dissolves hydrocarbons but does not dissolve water. What is the reason for this phenomenon? Here, a general answer can be given: liquids mix if their electronic structures are similar, and differences in electronic structure make mixing difficult. To clarify what we mean by "electronic structure", let's look at water again. When a water molecule is formed, the charge is redistributed between its constituent atoms: the hydrogen atoms donate their valence electrons, and the oxygen atom accepts them. Thus, the water molecule has a nonzero electric dipole moment, i.e. is polar. This explains, in particular, the fact that water has a very high dielectric constant and salts dissolve well in it, dissociating into ions. The dipole-dipole interaction holds the water molecules together, as a result of which its boiling point rises. Another example of a polar liquid is alcohol C 2 H 5 OH; it is easily miscible with water, since the dipole moment of its molecules is similar to the dipole moment of water molecules.

Along with polar liquids, the molecules of which are largely interconnected, there are also non-polar liquids with weaker intermolecular bonds. An example of such liquids is hydrocarbons - benzene, naphthalene, etc. The molecules of these liquids are built from carbon and hydrogen atoms, which socialize their valence electrons instead of giving or adding them. The relative weakness of bonds between hydrocarbon molecules is evidenced by their low boiling point. Between liquids with clearly defined polar properties (water) and absolutely non-polar ones (hydrocarbons), there is a whole range of classes of liquids, so it is not always possible to say in advance whether two given liquids will mix or not. But in most cases, the rule formulated at the beginning of the section is followed.

In addition to the electronic structure, the miscibility of liquids can significantly depend on the size of the molecules, as well as on temperature. For example, nicotine is miscible with water in any proportion below 60°C and above 208°C; at intermediate temperatures, the mutual solubility of nicotine and water is very limited.

Osmosis.

In 1748, J. Nollet discovered that some plant cells shrink in a concentrated saline solution - water leaves them through the cell membrane. If the same cells are then transferred to water, they swell and restore their size. Such movement of a substance (diffusion) through a semi-permeable partition separating a solution and a pure solvent or two solutions of different concentrations is called osmosis. This phenomenon can be explained by the fact that the solvent molecules, as a rule, are smaller than the molecules of the solute, and therefore more easily pass through the pores in the partition. Since the number of solvent molecules in a dilute solution (or pure solvent) is greater than in a concentrated one, diffusion transfer of these molecules occurs towards the latter.

Liquids and solids.

Earlier we talked about the relationship of liquids and their vapors near the critical temperature T c. Similar relationships exist between liquids and solids - at least near the melting point Tm.

Usually, when a solid is melted, its volume increases by about 10%, i.e. the average distance between neighboring molecules in the solid and liquid states is almost the same. The cohesion between atoms or molecules in solid and liquid states does not differ very much, and the plasticity of solids can be considered analogous to the fluidity of liquids. Thus, in terms of their physical properties, solids and liquids do not differ as radically as it seems. Accordingly, there are two types of theories of the liquid state: some are based on the ideas of modern solid state theory, and others are based on ideas borrowed from the theory of gases. Theories of the first type are more adequate near the melting point Tm, and the second - near the critical point T c.

liquid metals.

Many physical properties of solid metals change little upon melting. In this regard, more general theories are being developed in which the properties of liquid and solid metals are considered from a unified standpoint. In these theories, an important role is played by the structural factor determined by the mutual arrangement of atoms. It turns out that due to rather strong vibrations of the atoms of a solid at elevated temperatures, the structure factor of a solid near the melting point does not differ very much from that of a liquid. Metals with a low melting point, such as sodium, are used as coolants in nuclear reactors at nuclear power plants.

The attraction and repulsion of particles determine their mutual arrangement in matter. And the properties of substances significantly depend on the location of the particles. So, looking at a transparent very hard diamond (brilliant) (Fig. 111, a) and soft black graphite (Fig. 111, b) (pencil rods are made from it), we do not guess that both substances consist of exactly the same atoms carbon. It's just that these atoms are arranged differently in graphite than in diamond.

Rice. 111

Note that the figures do not show the atoms themselves, but their models - balls, and in reality there are no connecting rods or wires between them. This is a conventional representation of the arrangement of atoms in a substance.

The interaction of particles of a substance leads to the fact that it can be in three states: solid, liquid and gaseous. For example, ice, water, steam (Fig. 112). Any substance can be in three states, but certain conditions are needed for this: pressure, temperature. For example, oxygen in the air is a gas, but when cooled below -193°C it turns into a liquid, and at a temperature of -219°C oxygen is a solid. Iron at normal pressure and room temperature is in a solid state. At temperatures above 1539°C, iron becomes liquid, and at temperatures above 3050°C, it becomes gaseous. Liquid mercury used in medical thermometers becomes solid when cooled to temperatures below -39°C. At temperatures above 357 ° C, mercury turns into vapor (gas).

Rice. 112

Turning metallic silver into gas, it is sprayed onto glass and get "mirror" glasses.

What are the properties of substances in different states?

Let's start with gases, in which the behavior of molecules (Fig. 113) resembles the movement of bees in a swarm. However, the bees in the swarm independently change the direction of movement and practically do not collide with each other. At the same time, for molecules in a gas, such collisions are not only inevitable, but occur almost continuously. As a result of collisions, the directions and values of the velocities of the molecules change.

Rice. 113

The result of this motion and the lack of particle interaction in motion is that gas does not retain volume or shape, but occupies the entire volume provided to it. Each of you will consider the statements “Air occupies half the volume of the room” and “I pumped air into two-thirds of the volume of a rubber ball” as sheer absurdity. Air, like any gas, occupies the entire volume of the room and the entire volume of the ball.

What are the properties of liquids? Let's do an experiment.

Rice. 114

Pour the water from beaker 1 into beaker 2. The shape of the liquid has changed, but volume water stayed the same(Fig. 114). The molecules did not scatter throughout the volume, as would be the case with a gas. This means that the mutual attraction of liquid molecules exists, but it does not rigidly hold neighboring molecules. They oscillate and jump from one place to another (Fig. 115), which explains the fluidity of liquids.

Fig.115

The strongest is the interaction of particles in a solid. It does not allow the particles to disperse. Particles only perform chaotic oscillatory motions around certain positions (Fig. 116). So solids retain both volume and shape. A rubber ball will retain its ball shape and volume wherever it is placed: in a jar, on a table, etc.

Rice. 116

Think and answer

What are the main properties of a gas?
Why does a liquid not retain its shape?
What is the difference between solid state of matter and liquid and gaseous?
Are water molecules different from ice molecules?
Which of the following substances under normal conditions (at room temperature and normal pressure) are in a gaseous state, and which are in a liquid or solid state: tin, gasoline, oxygen, iron, mercury, air, glass, plastic?
Can mercury be in a solid state and air in a liquid state? Under what conditions?

Homework

Fill a plastic bottle (0.5 l) to the top with water and close the lid tightly. Try squeezing water in a bottle. Then pour out the water and close the bottle again. Now squeeze the air in it. Based on the results of the experiment, express a hypothesis about the structure of gases and liquids.
Task-competition: make a table in which you compare the nature of the movement, the interaction of particles, as well as the properties of matter in gaseous, solid and liquid states. The winner of the competition will be the one whose table contains the most complete and correct information.

Let's repeat the main thing in the studied

All substances consist of individual particles (atoms, molecules), between which there are distances.
Particles of matter are constantly and randomly moving.
The speed of movement of particles is greater, the higher the temperature of the body.
Diffusion is the phenomenon of mutual penetration of substances into each other. Diffusion proceeds especially rapidly in gases, more slowly in liquids, and very slowly in solids. As the temperature increases, diffusion proceeds faster.
At distances greater than the size of the particles themselves, the attraction of the particles prevails. At distances smaller than the size of the particles themselves, there is repulsion. The attraction of particles very quickly weakens as they move away from each other.
The change in the size of a body when it is heated is called thermal expansion.
The thermal expansion of different solid and liquid substances is different, and all gases are the same.