« Physics - 10th grade"
Is it possible to explain the properties of a substance in all its states of aggregation by the structure of the substance, the movement and interaction of its particles?
Interaction forces between molecules.
Molecules interact with each other. Without this interaction there would be no solids or liquids.
It is not difficult to prove the existence of significant interaction forces between atoms or molecules. Try to break a thick stick! But it consists of molecules. But alone gravity cannot ensure the existence of stable formations of atoms and molecules. At very small distances between molecules they necessarily act repulsive forces. Thanks to this, molecules do not penetrate each other and pieces of matter are never compressed to sizes on the order of the size of one molecule.
Molecule is a complex system consisting of individual charged particles: electrons and atomic nuclei.
In general, molecules are electrically neutral, however, significant electrical forces act between them at short distances: electrons and atomic nuclei of neighboring molecules interact.
If the molecules are located at distances several times greater than their sizes, then the interaction forces have practically no effect.
At distances exceeding 2-3 molecular diameters, attractive forces act. As the distance between molecules decreases, their strength mutual attraction at first it increases, but at the same time the repulsive force also increases. At a certain distance r 0 the force of attraction becomes equal to the force of repulsion. This distance is considered equal to the diameter of the molecule.
As the distance decreases further, the electron shells of the atoms begin to overlap and the repulsive force quickly increases. Figure 8.5 shows graphs of the potential energy of interaction between molecules (Fig. 8.5, a) and the forces of attraction (1) and repulsion (2) (Fig. 8.5, b) on the distance between the molecules. At r = r 0, the potential energy is minimal, the attractive force is equal to the repulsive force. When r > r 0 the attractive force is greater than the repulsive force; at r< r 0 сила притяжения меньше силы отталкивания.
The molecular kinetic theory makes it possible to understand why a substance can be in gaseous, liquid and solid states.
So, attractive forces act between molecules and they participate in thermal motion. The state of aggregation of a substance is determined by which of these two properties of molecules is the main one.
Gases.
In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves. For example, at atmospheric pressure the volume of a vessel is tens of thousands of times greater than the volume of the molecules in it.
Gases are easily compressed, and the average distance between molecules decreases, but the shape of the molecule does not change.
Gases can expand indefinitely. They retain neither shape nor volume. Numerous impacts of molecules on the walls of the vessel create gas pressure.
Gas 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. Weak forces the attraction of gas molecules is not able to keep them near each other.
In gases, the average kinetic energy of the thermal motion of molecules is greater than the average potential energy of their interaction, so we can often neglect the potential energy of interaction between molecules.
Liquids.
Liquid molecules are located almost close to each other, so a liquid molecule behaves differently than a gas molecule.
In liquids there is a so-called close order, i.e., the ordered arrangement of molecules is maintained over distances equal to several molecular diameters.
The molecule oscillates around its equilibrium position, colliding with neighboring molecules. Only from time to time she makes another “jump”, getting into a new equilibrium position.
In the equilibrium position, the repulsive force is equal to the attractive force, i.e., the total interaction force of the molecule is zero.
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. In the figurative expression of the scientist: “...liquid molecules lead a nomadic lifestyle...” At the same time, time settled life water molecules, i.e., the time of its vibrations 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 residence time of molecules decreases.
Liquid molecules are located directly next to each other. As the volume decreases, the repulsive forces become very large. This explains low compressibility of liquids.
Liquids: 1) low compressibility;
2) fluid, i.e. they do not retain their shape.
The fluidity of liquids can be explained as follows. The external force does not noticeably change the number of molecular jumps per second. But jumps of molecules from one stationary position to another occur predominantly in the direction of the action of the external force. This is why liquid flows and takes the shape of a container.
In liquids, the average kinetic energy of thermal motion of molecules is comparable to the average potential energy of their interaction. The presence of surface tension proves that the interaction forces between liquid molecules are significant and cannot be neglected.
Solids.
Atoms or molecules of solids, unlike atoms and molecules of liquids, vibrate around certain equilibrium positions. For this reason Solids retain not only volume, but also shape.
In solids, the average potential energy of interaction between molecules is much greater than the average kinetic energy of their thermal motion.
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 8.6 and 8.7 show the crystal lattices of table salt and diamond. Internal order in the arrangement of crystal atoms leads to regular external geometric shapes.
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Gases Gas (gaseous state) (from Dutch gas) is a state of aggregation of a substance, characterized by very weak bonds between its constituent particles (molecules, atoms or ions), as well as their high mobility. Gas particles move almost freely and chaotically in the intervals between collisions, during which a sharp change in the nature of their movement occurs. The gaseous state of a substance under conditions where the existence of a stable liquid or solid phase of the same substance is possible is usually called vapor. Like liquids, gases have fluidity and resist deformation. Unlike liquids, gases do not have a fixed volume and do not form a free surface, but tend to fill the entire available volume (for example, a vessel).
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The gaseous state is the most common state of matter in the Universe (interstellar matter, nebulae, stars, planetary atmospheres, etc.). By chemical properties gases and their mixtures are very diverse - from low-active inert gases to explosive gas mixtures. Gases sometimes include not only systems of atoms and molecules, but also systems of other particles - photons, electrons, Brownian particles, as well as plasma
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Gases can expand indefinitely. They do not retain their shape or volume. Numerous impacts of molecules on the walls of the vessel create gas pressure.
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Liquid Liquid is one of the aggregate states of matter. The main property of a liquid, which distinguishes it from other states of aggregation, is the ability to unlimitedly change its shape under the influence of tangential mechanical stresses, even arbitrarily small, while practically maintaining its volume.
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Liquid is physical body, which has two properties: It has fluidity, due to which it has no shape and takes the shape of the vessel in which it is located. It changes shape and volume little with changes in pressure and temperature, in which it is similar to a solid body.
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The liquid state is usually considered intermediate between a solid and a gas: a gas retains neither volume nor shape, but a solid retains both. The shape of liquid bodies can be determined entirely or partly by the fact that their surface behaves like an elastic membrane. So, water can collect in drops. But liquid is capable of flowing even under its stationary surface, and this also means unpreserved forms ( internal parts liquid body). Liquid molecules do not have a definite position, but at the same time they do not have complete freedom of movement. There is an attraction between them, strong enough to keep them close. A substance in a liquid state exists in a certain temperature range, below which it turns into a solid state (crystallization occurs or transformation into a solid-state amorphous state - glass), above which it turns into a gaseous state (evaporation occurs). The boundaries of this interval depend on pressure. As a rule, a substance in the liquid state has only one modification. (The most important exceptions are quantum liquids and liquid crystals.) Therefore, in most cases, a liquid is not only a state of aggregation, but also a thermodynamic phase (liquid phase). All liquids are usually divided into pure liquids and mixtures. Some mixtures of liquids are of great importance for life: blood, sea water, etc. Liquids can act as solvents.
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Formation of a free surface and surface tension Due to the conservation of volume, a liquid is capable of forming a free surface. Such a surface is the interface between the phases of a given substance: on one side there is a liquid phase, on the other there is a gaseous phase (steam), and, possibly, other gases, for example, air. If the liquid and gaseous phases of the same substance come into contact, forces arise that tend to reduce the interface area - surface tension forces. The interface behaves like an elastic membrane that tends to contract. Surface tension can be explained by the attraction between liquid molecules. Each molecule attracts other molecules, strives to “surround” itself with them, and therefore leave the surface. Accordingly, the surface tends to decrease. Therefore, soap bubbles and bubbles tend to take a spherical shape when boiling: for a given volume, a sphere has the minimum surface area. If only surface tension forces act on a liquid, it will necessarily take a spherical shape - for example, water drops in zero gravity. Small objects with a density greater than that of the liquid are able to “float” on the surface of the liquid, since the force of gravity is less than the force that prevents the increase in surface area.
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Evaporation is the gradual transition of a substance from a liquid to the gaseous phase (steam). During thermal movement, some molecules leave the liquid through its surface and become vapor. At the same time, some molecules pass back from vapor to liquid. If more molecules leave a liquid than enter, then evaporation occurs. Condensation is a reverse process, the transition of a substance from a gaseous state to a liquid one. In this case, more molecules pass into the liquid from the vapor than into the vapor from the liquid. Boiling is the process of vaporization inside a liquid. At a sufficiently high temperature, the vapor pressure becomes higher than the pressure inside the liquid, and vapor bubbles begin to form there, which (under the conditions of gravity) float to the top. Wetting is a surface phenomenon that occurs when a liquid comes into contact with a solid surface in the presence of steam, that is, at the interfaces of three phases. Miscibility is the ability of liquids to dissolve in each other. An example of miscible liquids: water and ethyl alcohol, an example of immiscible liquids: water and liquid oil. The transition of liquids from one state to another
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Solids A solid is one of the four states of aggregation of matter, differing from other states of aggregation (liquids, gases, plasma) in the stability of its shape and the nature of the thermal motion of atoms that perform small oscillations around equilibrium positions.
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 in the medium is many times greater than the size of the molecules themselves (Fig. 10). For example, at atmospheric pressure the volume of a vessel is tens
thousand 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, 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 s. The time of one oscillation is much less (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.
The molecules of the liquid are located directly next to each other. Therefore, when you try to change the volume of the liquid, even by a small amount, the molecules themselves begin to deform (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 a 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 equilibrium positions of atoms or ions of a solid, you get a regular spatial lattice, called a crystalline lattice. 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 regular external shapes. 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?
Under certain conditions, all matter on planet Earth is present in one of three states: gaseous, liquid or solid. There is also a fourth state of matter called plasma. Let us consider the question of the structure of gaseous, liquid and solid bodies, as well as their transition from one state to another when external conditions change.
Solid state of matter
Solids are characterized by their ability to resist external forces that act on them to change their shape and volume. Considering the question of the structure of gaseous, liquid and solid bodies and dwelling on the latter, it must be said that the molecules in them are firmly connected to each other. Therefore, an object has a specific shape, which it retains under constant external conditions.
Molecules in a solid can be in an ordered state, then they speak of a crystalline structure. Or they can be in a disordered state, then we are talking about amorphous solids. A striking example of a crystal lattice is the structure of metal systems, which in space forms an ideal lattice of a specific type, at the nodes of which there are atomic ions. An example of a solid object with an amorphous structure is glass.
Solid Matter Sciences
Solids are studied by several sciences, which include the following:
- Physics of condensed matter. She studies solid and liquid matter larger than 10 19 particles, using experimental and theoretical methods.
- Mechanics of deformations. This science studies the mechanical properties of solids, such as stresses in them, elastic and plastic deformations, as well as the relationship of these properties with thermodynamic external parameters. In this discipline, the structure of the solid substance itself does not matter.
- Materials Science. She studies the structure of molecules of solid, liquid and gaseous bodies, as well as phase transitions between these states.
- Solid state chemistry. This discipline specializes in the synthesis of new materials in the solid state.
Some properties of solids
At constant pressure and relatively low temperatures, the substance is in a solid state. The impact of a small external force on a solid state does not lead to externally noticeable deformation of the solid body.
If you increase the force, the body will begin to deform elastically. With an even greater increase in external influence, two options are possible:
- If the body is a metal, then it will begin to experience plastic deformation, that is, significant changes will occur in its shape that will remain after the cessation of external influence.
- If a body has an amorphous structure or a crystalline structure, but the lattice nodes contain ions of different signs, for example, a NaCl crystal, then the body will not deform plastically, but will simply collapse.
Each solid body is characterized by a certain density. The lightest substance in this category is airgel, its density is 3 kg/m3. The densest solid material known to mankind is the metal osmium. One cubic meter of osmium has a mass of 22,600 kg.
Metal materials
A special group of solids are pure metals and their alloys. The difference in this case in the structure of solids from gaseous and liquid states of matter lies in the existence of a spatial periodic lattice, which is called a crystal lattice.
Due to their crystalline structure, metals have a number of important properties, such as ductility and diffraction. Almost all of them exist in three main crystal lattices:
- face-centered cubic, for example, Au, Ag, Al, Cu;
- body-centered cubic, for example, Nb, Mo, W, Fe;
- hexagonal close packed, for example Ti, Zr.
The science of crystallography has been developed to study the characteristics of crystal lattices.
Condensed state of matter - liquid
The liquid state, just like the solid, is incompressible, that is, it retains its volume over a significant range of pressures. However, a liquid does not retain its shape, which distinguishes it from a solid and brings it closer to the gaseous state of matter.
If molecular and atomic forces act in the formation of solids, then a liquid is formed by molecules that are connected to each other only by weak molecular forces. The most common on Earth is water, which, like gas, can take the shape of the container in which it is placed.
If we talk about the structure of gaseous, liquid and solid bodies, it should be mentioned that a liquid, unlike a gas, does not change its density when it is placed in any closed vessel.
Features unique to liquids
Each liquid, due to the presence of molecular forces in it, has properties such as surface tension and capillary effect. If a substance is in the gravitational field, for example, of our Earth, then any body placed in it will be pushed out of the liquid according to the famous Archimedes' law.
If gravity does not act on the liquid, then the buoyant force will be zero. In addition, in the absence of external forces, substances in this state tend to acquire the smallest surface area, thereby reducing the total energy. That is why, under conditions of weightlessness, water drops have a spherical shape, since the ball is the figure with the smallest surface area for this volume of liquid.
Capillary properties are explained by the ability of molecules to enter into bonds not only with each other, but also with atoms and molecules of other bodies. These physical characteristics of a fluid are called cohesion and adhesion, respectively.
Speaking briefly about the structure of gaseous, liquid and solid bodies, we should mention the property of viscosity, which is inherent in the liquid and gaseous state. Viscosity refers to the ability to resist any displacement of layers of a substance relative to each other in the presence of a pressure gradient. For liquids, this indicator depends on the speed of displacement of these layers, temperature and molecular weight. The higher the speed of movement of a body in a liquid, the greater the molecular weight of the liquid particles, and the lower the temperature, the greater the viscosity.
Structure of gases
A gas is a state of matter when its constituent particles are not connected by any forces to each other or these forces are very weak. Therefore, such substances freely change volume and shape, filling the entire vessel in which they are placed. This difference in the structure of gaseous bodies from liquid and solid leads to the fact that they have a lower density. In the case of the gaseous state of water, it is customary to talk about steam.
In real gases there is no absolute disorder. However, the molecules in it move so quickly that they practically do not interact with each other. Therefore, the gas fills absolutely any volume, and the molecules in it will be separated relatively long distances compared to the size of the molecules themselves. Due to the large distance between molecules, gases are easily compressed, thereby increasing their density and internal pressure.
Ideal gas
In physics, thanks to the creation of models of the structure of solid, liquid and gaseous bodies, some reasonable simplifications of real states of matter arise, which make it possible to use a simpler mathematical apparatus for studying these states. One of these models was the concept of an ideal gas.
This term refers to the gaseous state of a substance in which the molecules have point sizes compared to the distances between them, and in which they do not interact with each other.
Under normal conditions, that is, at atmospheric pressure and room temperature, most real gases can be considered ideal. For example, nitrogen, oxygen, hydrogen, noble gases, carbon dioxide and others.
The equation of state for an ideal gas is as follows:
P * V = n * R * T, where:
P, V, T and n are the pressure, volume, temperature and amount of gas substance, respectively,
R = 8.31 J/(mol*K) - universal constant.
Plasma is the fourth state of matter
When considering the structure of gaseous, liquid and solid bodies in grade 10, attention is also paid to another state of matter - plasma, which is a gas consisting of cations and anions, that is, positively and negatively charged particles. A striking example of plasma is the substance that makes up our sun.
In a number of properties, plasma is similar to gas, the only difference is that it is capable of reacting to magnetic fields and also conducting electricity. Plasma can be obtained by heating to high temperatures gas, since this induces collisions between molecules, which leads to their partial or complete ionization.
Change in state of matter
In 10th grade physics, the structure of gaseous, solid and liquid bodies is considered along with the transitions between these states. Transitions between states of substances are possible due to changes in pressure and temperature. Changes occur only in the physical structure of gaseous, liquid and solid bodies, and their chemical composition remains constant.
The following transitions between different states of matter are possible:
- Melting. Endothermic process of transition from solid to liquid.
- Crystallization. An exothermic process in which a liquid becomes a solid as it cools.
- Boiling. A physical endothermic process in which a liquid changes into a gas.
- Condensation. Exothermic transition of gas to liquid.
- Sublimation or sublimation. Endothermic transition from a solid to a gas, bypassing the liquid state. A classic example is the sublimation of dry ice.
It should be noted that all endothermic and exothermic processes of phase transitions occur with a constant temperature of the substance. All these processes, the existence of which is determined by the structural features of gaseous, liquid and solid bodies, are energetic, that is, they require either the supply or removal of energy during their implementation.
Gases. In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves. For example, at atmospheric pressure the volume of a vessel is tens of thousands of times greater than the volume of the molecules in it.
Gases are easily compressed, and the average distance between molecules decreases, but the molecules do not compress each other.
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. Liquid molecules are located almost close to each other, so a liquid molecule behaves differently than a gas molecule. In liquids, there is so-called short-range order, i.e., the ordered arrangement of molecules is maintained over distances equal to several molecular diameters. The molecule vibrates around its position, colliding with neighboring molecules. Only from time to time she makes another “jump”, getting into a new equilibrium position. In this equilibrium position, the repulsive force is equal to the attractive force, i.e. the total interaction force of the molecule is zero. The time of settled life of a water molecule, i.e. the time of its 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 residence time of molecules decreases. The nature of molecular motion in liquids, first established by the Soviet physicist Ya.I. Frenkel, allows you to understand the basic properties of liquids. Liquid molecules are located directly next to each other. As the volume decreases, the repulsive forces become very large. This explains the low compressibility of liquids. As you know, liquids are fluid, that is, they do not retain their shape. This can be explained this way. The external force does not noticeably change the number of molecular jumps per second. But jumps of molecules from one stationary position to another occur predominantly in the direction of the external force (Fig. 8.8). This is why liquid flows and takes the shape of the container.
Solids.
Atoms or molecules of solids vibrate around certain equilibrium positions, therefore solids retain not only volume, but also shape
If you connect the center of equilibrium of atoms or ions of a solid, you get a regular spatial lattice, called a crystalline lattice
Crystalline bodies.
Crystals are solid bodies whose atoms or molecules occupy a certain, orderly position in space. Therefore, the crystals have flat edges. For example, a grain of ordinary table salt has flat edges that form right angles with each other.
Anisotropy of crystals.
The correct external shape is not the only or even the most important consequence of the ordered structure of the crystal. The main thing is the dependence of physical properties on the direction chosen in the crystal. For example, a piece of mica easily delaminates in one direction into thin plates, but it is much more difficult to tear it in the direction perpendicular to the plates. Many crystals conduct heat and electrical current differently in different directions. The optical properties of crystals also depend on the direction. Thus, a quartz crystal refracts light differently depending on the direction of the rays incident on it. The dependence of physical properties on the direction inside the crystal is called anisotropy. All crystalline bodies are anisotropic.
Single crystals and polycrystals.
Metals have a crystalline structure. If you take a large piece of metal, then at first glance its crystalline structure does not appear in any way. appearance piece, nor in its physical properties
Typically, a metal consists of a huge number of small crystals fused together. The properties of each crystal depend on the direction, but the crystals are randomly oriented relative to each other. As a result, in a volume significantly larger than the volume of individual crystals, all directions within metals are equal and the properties of metals are the same in all directions.
A solid consisting of large number small crystals are called polycrystalline. Single crystals are called single crystals.
