Aggregate states of matter(from the Latin aggrego - I attach, connect) - these are states of the same substance, transitions between which correspond to abrupt changes in free energy, density and other physical parameters of the substance.
Gas (French gaz, derived from the Greek chaos - chaos)- This state of matter, in which the interaction forces of its particles, filling the entire volume provided by it, are negligible. In gases, intermolecular distances are large and molecules move almost freely.

Gases can be considered as significantly superheated or undersaturated vapors. There is vapor above the surface of every liquid. When the vapor pressure increases to a certain limit, called saturated vapor pressure, the evaporation of the liquid stops, since the liquid becomes the same. A decrease in the volume of saturated steam causes parts of the steam rather than an increase in pressure. Therefore, the vapor pressure cannot be higher. The saturation state is characterized by the saturation mass contained in 1m mass of saturated vapor, which depends on temperature. Saturated steam can become unsaturated if its volume is increased or its temperature is increased. If the temperature of the steam is much higher than the point corresponding to a given pressure, the steam is called superheated.

Plasma is a partially or fully ionized gas in which the densities of positive and negative charges are almost equal. The sun, stars, clouds of interstellar matter consist of gases - neutral or ionized (plasma). Unlike other states of aggregation, plasma is a gas of charged particles (ions, electrons), which electrically interact with each other over large distances, but have neither short-range nor long-range orders in the arrangement of particles.

Liquids and solids, unlike gases, can be considered highly condensed media. In them, molecules (atoms) are located much closer to each other and the interaction forces are several orders of magnitude greater than in gases. Therefore, liquids and solids have significantly limited possibilities for expansion; they obviously cannot occupy an arbitrary volume, but at constants they retain their volume, no matter what volume they are placed in. Transitions from a more structurally ordered state of aggregation to a less ordered state can also occur continuously. In this regard, instead of the concept of a state of aggregation, it is advisable to use a broader concept - the concept of phase.

Phase is the collection of all parts of a system that have the same chemical composition and are in the same state. This is justified by the simultaneous existence of thermodynamically equilibrium phases in a multiphase system: liquid with its saturated vapor; water and ice at melting point; two immiscible liquids (a mixture of water with triethylamine), differing in concentrations; the existence of amorphous solids that retain the structure of a liquid (amorphous state).

Amorphous solid state of matter is a type of supercooled state of liquid and differs from ordinary liquids in its significantly higher viscosity and numerical values ​​of kinetic characteristics.
Crystalline solid state of matter is a state of aggregation that is characterized by large interaction forces between particles of matter (atoms, molecules, ions). Particles of solids oscillate around average equilibrium positions, called lattice nodes; the structure of these substances is characterized by a high degree of order (long- and short-range order) - order in the arrangement (coordination order), in the orientation (orientational order) of structural particles, or order in physical properties (for example, in the orientation of magnetic moments or electric dipole moments). The region of existence of the normal liquid phase for pure liquids, liquid and liquid crystals is limited from low temperatures by phase transitions, respectively, into the solid (crystallization), superfluid and liquid-anisotropic state.

The state of aggregation of a substance is usually called its ability to maintain its shape and volume. An additional feature is the methods of transition of a substance from one state of aggregation to another. Based on this, three states of aggregation are distinguished: solid, liquid and gas. Their visible properties are:

A solid body retains both shape and volume. It can pass either into a liquid by melting or directly into a gas by sublimation.
- Liquid – retains volume, but not shape, that is, it has fluidity. Spilled liquid tends to spread indefinitely over the surface on which it is poured. A liquid can become a solid by crystallization, and a gas by evaporation.
- Gas – does not retain either shape or volume. Gas outside any container tends to expand unlimitedly in all directions. Only gravity can prevent him from doing this, due to which the earth’s atmosphere does not dissipate into space. Gas passes into a liquid by condensation, and directly into a solid by sedimentation.

Phase transitions

The transition of a substance from one state of aggregation to another is called a phase transition, since the scientific state of aggregation is the phase of matter. For example, water can exist in the solid phase (ice), liquid (plain water) and gaseous phase (water vapor).

The example of water is also well demonstrated. Hung out in the yard to dry on a frosty, windless day, it immediately freezes, but after some time it turns out to be dry: the ice sublimates, directly turning into water vapor.

As a rule, a phase transition from a solid to a liquid and gas requires heating, but the temperature of the medium does not increase: thermal energy is spent on breaking internal bonds in the substance. This is the so-called latent heat. During reverse phase transitions (condensation, crystallization), this heat is released.

This is why steam burns are so dangerous. When it gets on the skin, it condenses. The latent heat of evaporation/condensation of water is very high: water in this regard is an anomalous substance; This is why life on Earth is possible. In a steam burn, the latent heat of condensation of water “scalds” the burned area very deeply, and the consequences of a steam burn are much more severe than from a flame on the same area of ​​the body.

Pseudophases

The fluidity of the liquid phase of a substance is determined by its viscosity, and viscosity is determined by the nature of the internal bonds, which are discussed in the next section. The viscosity of the liquid can be very high, and such liquid can flow unnoticed by the eye.

A classic example is glass. It is not a solid, but a very viscous liquid. Please note that sheets of glass in warehouses are never stored leaning diagonally against the wall. Within a few days they will bend under their own weight and will be unfit for consumption.

Other examples of pseudosolids are shoe polish and construction bitumen. If you forget an angular piece of bitumen on the roof, over the summer it will spread into a cake and stick to the base. Pseudo-solid bodies can be distinguished from real ones by the nature of melting: the real ones either retain their shape until they immediately spread (solder during soldering), or they float, releasing puddles and streams (ice). And very viscous liquids gradually soften, like pitch or bitumen.

Plastics are extremely viscous liquids, the fluidity of which is not noticeable for many years and decades. Their high ability to retain shape is ensured by the huge molecular weight of polymers, many thousands and millions of hydrogen atoms.

Phase structure of matter

In the gas phase, the molecules or atoms of a substance are very far apart from each other, many times greater than the distance between them. They interact with each other occasionally and irregularly, only during collisions. The interaction itself is elastic: they collided like hard balls and immediately scattered.

In a liquid, molecules/atoms constantly “feel” each other due to very weak bonds of a chemical nature. These bonds break all the time and are immediately restored again; the molecules of the liquid continuously move relative to each other, which is why the liquid flows. But to turn it into gas, you need to break all the bonds at once, and this requires a lot of energy, which is why the liquid retains its volume.

In this regard, water differs from other substances in that its molecules in the liquid are connected by so-called hydrogen bonds, which are quite strong. Therefore, water can be a liquid at a temperature normal for life. Many substances with a molecular weight tens and hundreds of times greater than that of water are, under normal conditions, gases, like ordinary household gas.

In a solid, all its molecules are firmly in place due to strong chemical bonds between them, forming a crystal lattice. Crystals of regular shape require special conditions for their growth and therefore are rare in nature. Most solids are conglomerates of small and tiny crystals – crystallites – tightly coupled by mechanical and electrical forces.

If the reader has ever seen, for example, a cracked axle shaft of a car or a cast iron grate, then the grains of crystallites on scrap are visible to the naked eye. And on fragments of broken porcelain or earthenware they can be observed under a magnifying glass.

Plasma

Physicists also identify a fourth state of matter – plasma. In plasma, electrons are separated from atomic nuclei, and it is a mixture of electrically charged particles. Plasma can be very dense. For example, one cubic centimeter of plasma from the interior of stars - white dwarfs - weighs tens and hundreds of tons.

Plasma is isolated into a separate state of aggregation because it actively interacts with electromagnetic fields due to the fact that its particles are charged. In free space, plasma tends to expand, cooling and turning into gas. But under the influence of electromagnetic fields, it can retain its shape and volume outside the vessel, like a solid body. This property of plasma is used in thermonuclear power reactors - prototypes of power plants of the future.

A feature of hydraulic and pneumatic drives is that to create forces, torques and movements in machines, these types of drives use the energy of liquid, air or other gas, respectively.

The fluid used in the hydraulic drive is called working fluid (WF).

To understand the features of the use of liquids and gases in drives, it is necessary to recall some basic information about the aggregate states of matter, known from a physics course.

According to modern views, aggregate states of matter (from the Latin aggrego - I attach, bind) are understood as states of the same substance, transitions between which correspond to abrupt changes in free energy, entropy, density and other physical parameters of this substance.

In physics, it is customary to distinguish between four aggregate states of matter: solid, liquid, gaseous and plasma.

SOLID STATE(crystalline solid state of matter) is a state of aggregation that is characterized by large interaction forces between particles of matter (atoms, molecules, ions). Particles of solids oscillate around average equilibrium positions, called lattice nodes; the structure of these substances is characterized by a high degree of order (long- and short-range order) - order in the arrangement (coordination order), in the orientation (orientational order) of structural particles or order in physical properties.

LIQUID STATE- this is the state of aggregation of a substance, intermediate between solid and gaseous. Liquids have some features of a solid (retains its volume, forms a surface, has a certain tensile strength) and a gas (takes the shape of the vessel in which it is located). The thermal motion of molecules (atoms) of a liquid is a combination of small vibrations around equilibrium positions and frequent jumps from one equilibrium position to another. At the same time, slow movements of molecules and their vibrations occur within small volumes. Frequent jumps of molecules disrupt the long-range order in the arrangement of particles and determine the fluidity of liquids, and small vibrations around equilibrium positions determine the existence of short-range order in liquids.

Liquids and solids, unlike gases, can be considered highly condensed media. In them, molecules (atoms) are located much closer to each other and the interaction forces are several orders of magnitude greater than in gases. Therefore, liquids and solids have significantly limited possibilities for expansion; they obviously cannot occupy an arbitrary volume, and at constant pressure and temperature they retain their volume, no matter what volume they are placed in.

GASEOUS STATE(from the French gaz, which in turn came from the Greek chaos - chaos) is a state of aggregation of a substance in which the forces of interaction of its particles, filling the entire volume provided to them, are negligible. In gases, intermolecular distances are large and molecules move almost freely.

Gases can be considered as significantly superheated or low-saturated vapors of liquids. There is vapor above the surface of each liquid due to evaporation. When the vapor pressure increases to a certain limit, called saturated vapor pressure, the evaporation of the liquid stops, since the pressure of the vapor and liquid becomes the same. A decrease in the volume of saturated steam causes condensation of part of the steam, rather than an increase in pressure. Therefore, the vapor pressure cannot be higher than the saturated vapor pressure. The saturation state is characterized by the saturation mass contained in 1 m3 of saturated steam mass, which depends on temperature. Saturated steam can become unsaturated if its volume is increased or its temperature is increased. If the temperature of the steam is much higher than the boiling point corresponding to a given pressure, the steam is called superheated.

PLASMA is a partially or fully ionized gas in which the densities of positive and negative charges are almost equal. The sun, stars, clouds of interstellar matter consist of gases - neutral or ionized (plasma). Unlike other states of aggregation, plasma is a gas of charged particles (ions, electrons), which electrically interact with each other over large distances, but have neither short-range nor long-range orders in the arrangement of particles.

As can be seen from the above, liquids are capable of maintaining volume, but are not able to independently maintain shape. The first property brings liquid closer to a solid, the second - to a gas. Both of these properties are not absolute. All liquids are compressible, although much less so than gases. All liquids resist changing shape, the displacement of one part of the volume relative to another, although less than solids.

In order to understand what the state of aggregation of a substance is, remember or imagine yourself in the summer near a river with ice cream in your hands. Wonderful picture, isn't it?

So, in this idyll, in addition to receiving pleasure, you can also carry out physical observation. Pay attention to the water. In the river it is liquid, in ice cream it is solid, and in the sky in the form of clouds it is gaseous. That is, it is simultaneously in three different states. In physics, this is called the aggregate state of matter. There are three states of aggregation - solid, liquid and gaseous.

Changes in aggregate states of matter

We can observe changes in the aggregate states of matter with our own eyes in nature. Water from the surface of reservoirs evaporates and clouds form. This is how the liquid turns into gas. In winter, water in reservoirs freezes, turning into a solid state, and in the spring it melts again, turning back into liquid. What happens to the molecules of a substance when it transitions from one state to another? Are they changing? Are ice molecules, for example, different from steam molecules? The answer is clear: no. The molecules remain absolutely the same. Their kinetic energy changes, and, accordingly, the properties of the substance. The energy of the vapor molecules is high enough to fly apart in different directions, and when cooled, the vapor condenses into liquid, and the molecules still have enough energy to move almost freely, but not enough to break away from the attraction of other molecules and fly away. With further cooling, water freezes, becoming a solid, and the energy of the molecules is no longer enough even to move freely inside the body. They vibrate around one place, held by the attractive forces of other molecules.

The nature of the movement and state of molecules in various states of matter can be reflected in the following table:

Processes in which there is a change in the aggregate states of substances, six in total.

The transition of a substance from solid to liquid is called melting, reverse process - crystallization. When a substance changes from a liquid to a gas, it is called vaporization, from gas to liquid - condensation. The transition from a solid state directly to a gas, bypassing the liquid state, is called sublimation, reverse process - desublimation.

• 1. Melting
• 2. Crystallization
• 3. Vaporization
• 4. Condensation
• 5. Sublimation
• 6. Desublimation

Examples of all these transitions You and I have seen this more than once in our lives. Ice melts to form water, water evaporates to form steam. In the opposite direction, the steam, condensing, turns back into water, and water, freezing, becomes ice. And if you think that you do not know the processes of sublimation and desublimation, then do not rush to conclusions. The smell of any solid body is nothing more than sublimation. Some molecules escape from the body, forming a gas that we can smell. An example of the reverse process is patterns on glass in winter, when steam in the air, freezing, settles on the glass and forms bizarre patterns.

: [in 30 volumes] / ch. ed. A. M. Prokhorov; 1969-1978, vol. 1).