Structure of liquid and gas. Structure of gases, liquids and solids

Basic physical characteristics of liquids and gases.

LECTURE 3

The subject of 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. Both droplet liquids (liquids themselves, such as water, gasoline, technical oils) and gases (air, nitrogen, hydrogen, carbon dioxide) have the property of fluidity. A significant difference in the behavior of liquids and gases, explained from the point of view of molecular structure, will be determined by the presence of a free surface of the droplet liquid bordering 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 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 types of internal molecular motion arise.

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

In contrast to a solid body, in gases there are no adhesion forces between molecules. Gas molecules perform random movements, 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 movement. The average distance between two successive collisions of molecules determines free path length. The average speed of thermal motion of molecules is comparable to the speed of propagation of small disturbances (the speed of sound) in a given state of the gas.

Liquid bodies in their molecular structure and thermal movement of molecules they occupy an intermediate state between solid and gaseous bodies. According to existing views around some, central, molecules are grouped by neighboring molecules, performing small vibrations with a frequency close to the frequency of vibrations of molecules in the lattice of a solid and an amplitude of the order of the average distance between the molecules. The central molecule either (when the liquid is at rest) remains motionless or migrates at a speed that in value and direction coincides with the average speed 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. Evidence of the presence of vibrations of molecules in liquids is the “Brownian motion” of the smallest solid particles introduced into the liquid. The vibrations of these particles are easily observed in the field of a microscope and can be considered as a result of the collision of solid particles with liquid molecules. The presence of intermolecular interaction in liquids determines the existence of surface tension of the liquid at its interface with any other medium, which forces it to take a form in which its surface is minimal. Small volumes of liquid usually have the shape of a spherical drop. Because of this, fluids in hydraulics are called drip.

It should be noted that the boundary between solids and liquids is not always clearly defined. Thus, when large forces are applied to a droplet liquid (for example, a liquid jet), with a short interaction time, the latter acquires properties close to the properties 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. Thus, at pressures greater than 10 8 Pa, a water jet cuts a steel plate; at a pressure of about 5·10 7 Pa it cuts granite, at pressures of 1.5·10 7 - 2·10 7 Pa it destroys coal. Pressure (1.5 – 2)·10 6 Pa is sufficient to destroy various soils.

Under certain conditions, there may also be no boundary between liquid and gaseous bodies. Gases fill the entire volume provided to them; their density can vary widely depending on the applied forces. Liquids, filling a vessel with 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 barely noticeable. Recently, the concept of a fluid state has appeared, when liquid particles with dimensions of several nanometers are mixed quite evenly with their vapor. In this case, there is no visual difference between liquid and vapor.

Steam differs from gas in that its state during movement is close to the saturation state. 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 occurs like an avalanche. In this case, the laws of steam flow may differ significantly from the laws of liquid and gas flow.

The properties of solids, liquids and gases are determined by their different molecular structures . 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), filling 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 directional movement of a liquid consists of the movement of a huge number of molecules randomly moving in all directions relative to each other. In the mechanics of liquids and gases, which studies their directed motion, the distribution of all characteristics of the liquid in the space under consideration is assumed to be continuous. Molecular structure is taken into account only when mathematically describing 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, since 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 established by the value of the Knudsen criterion - the ratio of the mean free path of gas molecules l to the characteristic flow size L

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

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

In mechanical engineering, the continuum hypothesis may not be fulfilled when calculating the flow of liquid or gas in narrow gaps. Molecules have dimensions of the order of 10 -10 m; at gaps of the order of 10 -9 m, characteristic of nanotechnology, significant deviations of the calculated data obtained using conventional fluid dynamics equations can be observed

In accordance with the molecular kinetic theory, all bodies consist of molecules. The processes studied in fluid and gas mechanics are the result of the action of a huge number of molecules. For example, it makes no sense to talk about the temperature of one molecule. When the distance between molecules is many times greater than the size of the molecules themselves, they move independently of each other, and as a result of the collision, their speeds and direction of movement constantly change. Such substances are called gases. When the distance between molecules is commensurate with the size of the molecules, then the mutual influence of molecules on each other becomes significant. The molecules perform oscillatory movements around the equilibrium position for some time, then move abruptly to a new equilibrium position (the theory of Ya.I. Frenkel). This structural feature underlies such properties as viscosity and surface tension.

In mechanics, liquids and gases are not studied from the standpoint of their molecular structure. Liquid and gas are considered as a continuous medium, devoid of molecules and intermolecular spaces.

To assess the validity of applying the continuum model for gas, the Knudsen criterion is used:

Where l– free path of molecules, m; L– characteristic size of liquid (gas) flow, m. At Kn < 0,01 гипотеза сплошности справедлива, при Kn> 0.01, rarefied gases flow and the continuity hypothesis cannot be applied.

This hypothesis has been confirmed by numerous experiments. Therefore, it is quite reasonable to consider the continuum hypothesis as the basic theory of fluid and gas mechanics.

The molecular kinetic theory makes it possible to understand why a substance can exist 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 the molecules decreases, but the molecules do not “squeeze” each other (Fig. 11).


Molecules move at enormous speeds—hundreds of meters per second—in space. When they collide, they bounce off each other in different directions like billiard balls.
The weak attractive forces of gas molecules are not able to hold them near each other. Therefore, gases can expand without limit. 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 behaves differently in a liquid than in a gas. Clamped, as in a cage, by other molecules, it “runs in place” (oscillates around the equilibrium position, colliding with neighboring molecules). Only from time to time she makes a “leap”, breaking through the “bars of the cage”, but immediately finds herself in a new “cage” formed by new neighbors. The “settled 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 less (10–12–10–13 s). With increasing temperature, the “settled life” time of molecules decreases. The nature of molecular motion in liquids, first established by the Soviet physicist Ya. I. Frenkel, allows us to understand the basic properties of liquids.

Frenkel Yakov Ilyich (1894 - 1952) is an outstanding Soviet theoretical physicist who made significant contributions 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 liquid even by a small amount, deformation of the molecules themselves begins (Fig. 13). And this requires very great strength. This explains the low compressibility of liquids.

Liquids, as is known, are fluid, that is, they do not retain their shape. This is explained as follows. If the liquid does not flow, then jumps of molecules from one “sedentary” position to another occur with the same frequency 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). This is why liquid flows and takes the shape of the container.
Solids. Atoms or molecules of solids, unlike liquids, vibrate around certain equilibrium positions. True, sometimes molecules change their equilibrium position, but this happens extremely rarely. This 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, individual members of which are restlessly jostling 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 movement), maintain on average certain intervals between themselves. If you connect the centers of the equilibrium positions of atoms or ions of a solid body, you get a regular 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 in crystals leads to geometrically correct external forms. Figure 17 shows Yakut diamonds.

A qualitative explanation of the basic properties of a substance based on 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 will limit ourselves to considering 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 low 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 together? 6. What is the difference between the thermal motion of molecules of gases, liquids and solids?

Liquid- a substance in a state intermediate between solid and gas. This is a state of aggregation of a substance in which molecules (or atoms) are interconnected so much that this allows it to maintain its volume, but not strongly enough to maintain its shape.

Properties of liquids.

Liquids easily change their shape but retain their volume. Under normal conditions, they take the shape of the vessel in which they are located.

The surface of the liquid that is not in contact with the walls of the container is called free surface. It is formed as a result of the action of gravity on liquid molecules.

The structure of liquids.

The properties of liquids are explained by the fact that the spaces between their molecules are small: the molecules in liquids are packed so tightly that the distance between every two molecules is less than the size of the molecules. An explanation of the behavior of liquids based on the nature of the molecular motion of the liquid was given by the Soviet scientist Ya. I. Frenkel. It is as follows. A liquid molecule oscillates around a temporary equilibrium position, colliding with other molecules from its immediate environment. From time to time she manages to make a “leap” in order to leave her neighbors from the immediate environment and continue to oscillate among other neighbors. The time of settled life of a water molecule, i.e. the time of oscillation around one equilibrium position at room temperature, is on average 10 -11 s. The time of one oscillation is much less - 10 -12 - 10 -13.

Since the distances between the molecules of a liquid are small, an attempt to reduce the volume of the liquid leads to deformation of the molecules; they begin to repel each other, which explains the low compressibility of the liquid. The fluidity of a liquid is explained by the fact that “jumps” of molecules from one stationary position to another occur in all directions with the same frequency. An external force does not noticeably change the number of “jumps” per second; it only sets their preferred direction, which explains the fluidity of the liquid and the fact that it takes the shape of a vessel.

The liquid state, occupying an intermediate position between gases and crystals, combines some features of both of these states. In particular, liquids, like crystalline bodies, are characterized by the presence of a certain volume, and at the same time, the liquid, like a gas, takes the shape 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 X-ray studies, liquids also occupy an intermediate position with regard to the nature of the arrangement of particles. 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 you move away from a given particle, the arrangement of other particles in relation to it becomes less and less ordered, and quite quickly the order in the arrangement of particles completely disappears. Long-range order occurs in crystals: an ordered arrangement of particles relative 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 quasicrystalline (crystal-like).

Due to the lack of long-range order, liquids, with few exceptions, do not exhibit the anisotropy 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. In them, only the orientation of the molecules is ordered; the mutual arrangement of the molecules, as in ordinary liquids, does not reveal long-range order.

The intermediate position of liquids is due to the fact that the liquid state turns out to be especially complex in its properties. Therefore, his theory is much less developed than the theory of crystalline and gaseous states. There is still no completely completed and generally accepted theory of liquids. Significant achievements in 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, a molecule changes its place of equilibrium, moving abruptly 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, remaining part of the time near certain places. According to the figurative expression of Ya. I. Frenkel, molecules wander throughout the entire volume of liquid, leading a nomadic lifestyle, in which short-term movements are replaced by relatively long periods of sedentary 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, sharply decreasing 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 are in many respects closer to liquids than to crystals. Such bodies, called amorphous, do not exhibit anisotropy. In the arrangement of their particles, like 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 reason to consider amorphous solids as supercooled liquids, the particles of which, due to their greatly increased viscosity, have limited mobility.

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