The structure of liquid solids and gaseous bodies.

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Gas (gaseous state) 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).

The gaseous state is the most common state of matter in the Universe (interstellar matter, nebulae, stars, planetary atmospheres, etc.). The chemical properties of 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.

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.

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 a liquid is capable of flowing even under its stationary surface, and this also means unpreserved forms (of the internal parts of a 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 transforms into a solid state (crystallization occurs or glass transforms into a solid amorphous state), above which it transforms 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.

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 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 surface area of the surface tension force. 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.

Evaporation is the gradual transition of a substance from a liquid to a 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 within a liquid. At a sufficiently high temperature, the vapor pressure becomes higher than the pressure inside the liquid, and vapor bubbles begin to form there, which (under the conditions of gravity) float to the top. Wetting is a surface phenomenon that occurs when a liquid comes into contact with a solid surface in the presence of steam, that is, at the interfaces of three phases. 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

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 of

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?

Structure of gases, liquids and solids.

Basic principles of molecular kinetic theory:

All substances are made up of molecules, and molecules are made up of atoms,

atoms and molecules are in constant motion,

There are forces of attraction and repulsion between molecules.

IN gases molecules move chaotically, the distances between molecules are large, molecular forces are small, the gas occupies the entire volume provided to it.

IN liquids molecules are arranged in an orderly manner only at short distances, and at large distances the order (symmetry) of the arrangement is violated - “short-range order”. The forces of molecular attraction keep molecules close together. The movement of molecules is “jumping” from one stable position to another (usually within one layer. This movement explains the fluidity of a liquid. A liquid has no shape, but has volume.

Solids are substances that retain their shape, divided into crystalline and amorphous. Crystalline solids bodies have a crystal lattice, in the nodes of which there may be ions, molecules or atoms. They oscillate relative to stable equilibrium positions.. Crystal lattices have a regular structure throughout the entire volume - “long-range order” of arrangement.

Amorphous bodies retain their shape, but do not have a crystal lattice and, as a result, do not have a pronounced melting point. They are called frozen liquids, since they, like liquids, have a “short-range” order of molecular arrangement.

Molecular interaction forces

All molecules of a substance interact with each other through forces of attraction and repulsion. Evidence of the interaction of molecules: the phenomenon of wetting, resistance to compression and tension, low compressibility of solids and gases, etc. The reason for the interaction of molecules is the electromagnetic interactions of charged particles in a substance. How to explain this? An atom consists of a positively charged nucleus and a negatively charged electron shell. The charge of the nucleus is equal to the total charge of all the electrons, so the atom as a whole is electrically neutral. A molecule consisting of one or more atoms is also electrically neutral. Let's consider the interaction between molecules using the example of two stationary molecules. Gravitational and electromagnetic forces can exist between bodies in nature. Since the masses of molecules are extremely small, negligible forces of gravitational interaction between molecules can be ignored. At very large distances there is also no electromagnetic interaction between molecules. But, as the distance between molecules decreases, the molecules begin to orient themselves in such a way that their sides facing each other will have charges of different signs (in general, the molecules remain neutral), and attractive forces arise between the molecules. With an even greater decrease in the distance between molecules, repulsive forces arise as a result of the interaction of negatively charged electron shells of the atoms of the molecules. As a result, the molecule is acted upon by the sum of the forces of attraction and repulsion. At large distances, the force of attraction predominates (at a distance of 2-3 diameters of the molecule, attraction is maximum), at short distances the force of repulsion prevails. There is a distance between molecules at which the attractive forces become equal to the repulsive forces. This position of the molecules is called the position of stable equilibrium. Molecules located at a distance from each other and connected by electromagnetic forces have potential energy. In a stable equilibrium position, the potential energy of the molecules is minimal. In a substance, each molecule interacts simultaneously with many neighboring molecules, which also affects the value of the minimum potential energy of the molecules. In addition, all molecules of a substance are in continuous motion, i.e. have kinetic energy. Thus, the structure of a substance and its properties (solid, liquid and gaseous bodies) are determined by the relationship between the minimum potential energy of interaction of molecules and the reserve of kinetic energy of thermal motion of molecules.

Structure and properties of solid, liquid and gaseous bodies

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

Solid

Solids have a constant shape and volume and are practically incompressible. The minimum potential energy of interaction of molecules is greater than the kinetic energy of molecules. Strong particle interaction. The thermal motion of molecules in a solid is expressed only by vibrations of particles (atoms, molecules) around a stable equilibrium position.

Due to the large forces of attraction, molecules practically cannot change their position in matter, this explains the invariability of the volume and shape of solids. Most solids have a spatially ordered arrangement of particles that form a regular crystal lattice. Particles of matter (atoms, molecules, ions) are located at the vertices - nodes of the crystal lattice. The nodes of the crystal lattice coincide with the position of stable equilibrium of the particles. Such solids are called crystalline.

Liquid

Liquids have a certain volume, but do not have their own shape; they take the shape of the vessel in which they are located. The minimum potential energy of interaction between molecules is comparable to the kinetic energy of molecules. Weak particle interaction. The thermal motion of molecules in a liquid is expressed by vibrations around a stable equilibrium position within the volume provided to the molecule by its neighbors. Molecules cannot move freely throughout the entire volume of a substance, but transitions of molecules to neighboring places are possible. This explains the fluidity of the liquid and the ability to change its shape.

In liquids, molecules are quite firmly bound to each other by forces of attraction, which explains the invariance of the volume of the liquid. In a liquid, the distance between molecules is approximately equal to the diameter of the molecule. When the distance between molecules decreases (compression of the liquid), the repulsive forces increase sharply, so liquids are incompressible. In terms of their structure and the nature of thermal movement, liquids occupy an intermediate position between solids and gases. Although the difference between a liquid and a gas is much greater than between a liquid and a solid. For example, during melting or crystallization, the volume of a body changes many times less than during evaporation or condensation.

Gases do not have a constant volume and occupy the entire volume of the vessel in which they are located. The minimum potential energy of interaction between molecules is less than the kinetic energy of molecules. Particles of matter practically do not interact. Gases are characterized by complete disorder in the arrangement and movement of molecules.

The distance between gas molecules is many times greater than the size of the molecules. Small attractive forces cannot keep molecules close to each other, so gases can expand without limit. Gases are easily compressed under the influence of external pressure, because the distances between molecules are large, and the interaction forces are negligible. The gas pressure on the walls of the vessel is created by the impacts of moving gas molecules.

The purpose of the lesson: Consider the structural features and properties of gaseous, liquid and solid bodies from the point of view of molecular kinetic theory.

Lesson objectives:

Educational

To contribute to the acquisition of knowledge on the topic “Structure of gaseous, liquid and solid bodies”;
Establish the nature of the dependence of the forces of attraction and repulsion on the distance between molecules;
Learn to solve quality problems.

Developmental

Develop:

observation, independence;
logical thinking
ability to apply theoretical knowledge in practice;
promote the development of speech and thinking

Educational:

Formation of ideas about the unity and interconnection of natural phenomena.
Form a positive attitude towards the subject

Lesson type: A lesson in learning new material.

Lesson format: combined

Comprehensive methodological support: Computer, screen, multimedia projector, author's presentation, crystal samples, test tasks.

Interdisciplinary connections:

chemistry
Informatics

During the classes

Organizational stage

Teacher: Hello. Napoleon I also said: “Imagination rules the world.” And Democritus argued that “Nothing exists except atoms.”

The stage of setting the goals and objectives of the lesson.

Agree! The world is amazing and diverse. Man has long tried to explain the inexplicable, to see the invisible, to hear the inaudible. Looking around him, he reflected on nature and tried to solve the riddles that it posed to him.

Russian poet Fyodor Ivanovich Tyutchev wrote.

Not what you think, nature:
Not a cast, not a soulless face -
She has a soul, she has freedom,
It has love, it has language.

But over time, people began to understand that it is the law that stands at the head of everything that surrounds us.

You, of course, encounter various physical phenomena governed by law every day, and in most cases you can predict how they will end. For example, predict how the following events will end:

If you open a bottle of perfume, then...;
If you heat ice, then...;
If you squeeze two pieces of plasticine tightly, then...;
If you drop a drop of oil on water, then...;
If you put a thermometer in hot water, then...

Teacher: So, in giving your answers, you were guided by certain knowledge acquired earlier. Every day we observe a whole range of objects around us: tables, chairs, books, pens, notebooks, cars, etc. Tell me, do they just seem solid to us or are they actually so?

Student: They only seem to.

Teacher: Then tell me, what do all substances consist of?

Student: Made from molecules or atoms

Teacher: What do you think, are the molecules of different substances the same or not? Prove it.

Student: No. They have different chemical compounds.

Teacher: Are ice, water and water vapor made of the same molecules or not?

Student: Yes.

Teacher: Why?

Student: Because it is the same substance, but in a different form

Teacher: Here, guys, we come to the topic of our lesson. Open your workbooks, write down the date and topic of our lesson: “Structure of gaseous, liquid and solid bodies.”

(Slide 2).

There are no two completely identical objects in the world. It is impossible to find two identical grains of sand in a mountain of sand or two identical leaves on a tree, but the molecules of the same substance are exactly the same. For example, we are used to seeing water in a liquid state. The chemical formula of water is H 2 O. In the gaseous state it is water vapor. (What is the chemical formula?). In a solid state, it is ice or snow. Still the same chemical formula - H 2 O.

Then the question arises: if the molecules of the same substance are exactly the same, then why can this substance be in different states of aggregation?

This is the question you and I will have to answer today in class. (Slide 3)

There are four states of matter:

Solid
Liquid
Gaseous
Plasma

Today we will talk about three of them. First, let's get acquainted with the concept of phase transition. (Slide 4)

Phase transition is the transition of a system from one state of aggregation to another. During a phase transition, any physical quantity changes abruptly (density, internal energy)

The realization of the state of aggregation of a substance depends on the ratio of the kinetic and potential energy of the molecules included in its composition.

Stage of explaining new material

There are supporting notes on the tables in front of you. (Appendix 3) . What does each drawing symbolize? (Different states of aggregation)

A cloud is a gaseous state of a substance, a bottle is a liquid state, a cube is a solid state. We will step by step analyze the structure of gaseous, liquid and solid bodies. We will write down our conclusions in notebooks.

GASES (Slide 6, 10)

The distance between atoms or molecules in gases is on average many times greater than the size of the molecules themselves. 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. When they collide, they bounce off each other in different directions. 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 (Slide 11, 14)

Liquid molecules are located almost close to each other, so a liquid molecule behaves differently than a gas molecule. 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 “jump”, breaking through the “bars of the cage,” but immediately finds herself in a new cell 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 residence time of molecules decreases.

Liquid molecules are located directly next to each other. When you try to change the volume of a liquid (even by a small amount), the molecules themselves begin to deform; this requires very large forces. This explains the low compressibility of liquids.

As you know, liquids are fluid, that is, they do not retain their shape, they take the shape of a vessel.

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. (Appendix 5)

SOLIDS. (Slide 15)

Atoms or molecules of solids, unlike atoms and molecules of liquids, vibrate around certain equilibrium positions. True, sometimes molecules change their equilibrium position, but this happens 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 of people, where individual individuals are restlessly jostling in place, and a solid body is like a slender cohort of the same individuals who, although they do not stand at attention, 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.

The drawings depict crystal lattices of table salt and diamond. The internal order in the arrangement of atoms in crystals leads to regular external geometric shapes.

So, the time has come to answer the question posed at the beginning of the lesson: what determines that the same substance can be in different states of aggregation?

Student answers: From the distance between particles, from interaction forces, that is, from how the molecules are located, how they move and how they interact with each other.

The stage of consolidating the material covered.

Game “What condition is this?”

A grade of “5” is given to the student who scores the most points. The stage of testing the knowledge acquired in the lesson.
Test. (Appendix 4)

The final stage.

Now let's summarize our work in today's lesson. What new did you learn in the lesson? What grades did you receive? Homework: § 62, answer the questions after the paragraph, fill out the table.

Teacher:

(Slide 38)
You can solve riddles forever.
The universe is infinite.
Thanks to all of us for the lesson,

And the main thing is that it will be used for future use!

Literature:
G.V. Markina, Uchitel Publishing House, Volgograd, 97
V.A. Volkov, Moscow “Waco”, 2006 To help a school teacher
Internet resources
G.Ya. Myakishev, physics, Moscow -2007.