State of aggregation- this is a state of matter in a certain range of temperatures and pressures, characterized by properties: the ability (solid body) or inability (liquid, gas) to maintain volume and shape; the presence or absence of long-range (solid) or short-range (liquid) order and other properties.
A substance can be in three states of aggregation: solid, liquid or gaseous, currently an additional plasma (ionic) state is isolated.
AT gaseous state, the distance between atoms and molecules of a substance is large, the interaction forces are small, and the particles, moving randomly in space, have a large kinetic energy exceeding the potential energy. The material in the gaseous state has neither its shape nor volume. The gas fills all available space. This state is typical for substances with low density.
AT liquid state, only the short-range order of atoms or molecules is preserved, when separate sections with an ordered arrangement of atoms periodically appear in the volume of a substance, however, the mutual orientation of these sections is also absent. The short-range order is unstable and can either disappear or reappear under the action of thermal vibrations of atoms. The molecules of a liquid do not have a definite position, and at the same time they do not have full freedom of movement. The material in the liquid state does not have its own shape, it retains only volume. The liquid can occupy only a part of the volume of the vessel, but freely flow over the entire surface of the vessel. The liquid state is usually considered intermediate between a solid and a gas.
AT solid substance, the arrangement of atoms becomes strictly defined, regularly ordered, the interaction forces of particles are mutually balanced, so the bodies retain their shape and volume. The regularly ordered arrangement of atoms in space characterizes the crystalline state, the atoms form a crystal lattice.
Solids have an amorphous or crystalline structure. For amorphous Bodies are characterized only by a short-range order in the arrangement of atoms or molecules, a chaotic arrangement of atoms, molecules or ions in space. Examples of amorphous bodies are glass, pitch, and pitch, which appear to be in a solid state, although in reality they flow slowly, like a liquid. Amorphous bodies, unlike crystalline ones, do not have a definite melting point. Amorphous bodies occupy an intermediate position between crystalline solids and liquids.
Most solids have crystalline a structure that is characterized by an ordered arrangement of atoms or molecules in space. The crystal structure is characterized by a long-range order, when the elements of the structure are periodically repeated; there is no such regular repetition in the short-range order. characteristic feature crystalline body is the ability to retain shape. A sign of an ideal crystal, the model of which is a spatial lattice, is the property of symmetry. Symmetry is understood as the theoretical ability of the crystal lattice solid body coincide with itself when its points are mirrored from a certain plane, called the plane of symmetry. The symmetry of the external form reflects the symmetry of the internal structure of the crystal. For example, all metals have a crystalline structure, which are characterized by two types of symmetry: cubic and hexagonal.
In amorphous structures with a disordered distribution of atoms, the properties of the substance are the same in different directions, i.e. glassy (amorphous) substances are isotropic.
All crystals are characterized by anisotropy. In crystals, the distances between atoms are ordered, but the degree of order may be different in different directions, which leads to a difference in the properties of the crystal substance in different directions. The dependence of the properties of a crystal substance on the direction in its lattice is called anisotropy properties. Anisotropy manifests itself when measuring both physical and mechanical and other characteristics. There are properties (density, heat capacity) that do not depend on the direction in the crystal. Most of the characteristics depend on the choice of direction.
It is possible to measure the properties of objects that have a certain material volume: sizes - from a few millimeters to tens of centimeters. These objects with a structure identical to the crystal cell are called single crystals.
The anisotropy of properties is manifested in single crystals and is practically absent in a polycrystalline substance consisting of many small randomly oriented crystals. Therefore, polycrystalline substances are called quasi-isotropic.
Crystallization of polymers, whose molecules can be arranged in an orderly manner with the formation of supramolecular structures in the form of bundles, coils (globules), fibrils, etc., occurs in a certain temperature range. complex structure molecules and their aggregates determines the specific behavior of polymers upon heating. They cannot go into a liquid state with low viscosity, they do not have a gaseous state. In solid form, polymers can be in glassy, highly elastic and viscous states. Polymers with linear or branched molecules can change from one state to another with a change in temperature, which manifests itself in the process of deformation of the polymer. On fig. 9 shows the dependence of deformation on temperature.
Rice. 9 Thermomechanical curve of amorphous polymer: t c , t t, t p - glass transition temperature, fluidity and the beginning of chemical decomposition, respectively; I - III - zones of a glassy, highly elastic and viscous state, respectively; Δ l- deformation.
The spatial structure of the arrangement of molecules determines only the glassy state of the polymer. At low temperatures all polymers are elastically deformed (Fig. 9, zone I). Above glass transition temperature t c an amorphous polymer with a linear structure passes into a highly elastic state ( zone II), and its deformation in the glassy and highly elastic states is reversible. Heating above pour point t t transforms the polymer into a viscous state ( zone III). The deformation of the polymer in the viscous state is irreversible. An amorphous polymer with a spatial (network, cross-linked) structure does not have a viscous state, the temperature region of the highly elastic state expands to the temperature of polymer decomposition t R. This behavior is typical for rubber-type materials.
The temperature of a substance in any aggregate state characterizes the average kinetic energy of its particles (atoms and molecules). These particles in bodies have mainly the kinetic energy of oscillatory motions relative to the center of equilibrium, where the energy is minimal. When a certain critical temperature is reached, the solid material loses its strength (stability) and melts, and the liquid turns into steam: it boils and evaporates. These critical temperatures are the melting and boiling points.
When a crystalline material is heated at a certain temperature, the molecules move so vigorously that the rigid bonds in the polymer are broken and the crystals are destroyed - they pass into a liquid state. The temperature at which crystals and liquid are in equilibrium is called the melting point of the crystal, or the solidification point of the liquid. For iodine, this temperature is 114 o C.
Everyone chemical element has its own melting point t pl separating the existence of a solid and a liquid, and the boiling point t kip, corresponding to the transition of liquid into gas. At these temperatures, the substances are in thermodynamic equilibrium. A change in the state of aggregation may be accompanied by a jump-like change in free energy, entropy, density, and others. physical quantities.
To describe the various states in physics uses a broader concept thermodynamic phase. Phenomena that describe transitions from one phase to another are called critical.
When heated, substances undergo phase transformations. When melted (1083 o C), copper turns into a liquid in which the atoms have only short-range order. At a pressure of 1 atm, copper boils at 2310 ° C and turns into gaseous copper with randomly arranged copper atoms. Melting point pressure saturated steam crystal and liquid are equal.
The material as a whole is a system.
System- a group of substances combined physical, chemical or mechanical interactions. phase called homogeneous part system separated from other parts physical interfaces (in cast iron: graphite + iron grains; in ice water: ice + water).Components systems are the various phases that make up a given system. System Components- these are substances that form all phases (components) of this system.
Materials consisting of two or more phases are dispersed systems . Disperse systems are divided into sols, whose behavior resembles the behavior of liquids, and gels with characteristic properties solid bodies. In sols, the dispersion medium in which the substance is distributed is liquid; in gels, the solid phase predominates. Gels are semi-crystalline metal, concrete, a solution of gelatin in water at a low temperature (at a high temperature, gelatin turns into a sol). A hydrosol is a dispersion in water, an aerosol is a dispersion in air.
State diagrams.
In a thermodynamic system, each phase is characterized by parameters such as temperature T, concentration with and pressure R. To describe phase transformations, a single energy characteristic is used - the Gibbs free energy ΔG(thermodynamic potential).
Thermodynamics in the description of transformations is limited to consideration of the state of equilibrium. equilibrium state thermodynamic system is characterized by the invariance of thermodynamic parameters (temperature and concentration, as in technological processing R= const) in time and the absence of flows of energy and matter in it - with the constancy of external conditions. Phase balance- equilibrium state of a thermodynamic system consisting of two or more phases.
For the mathematical description of the equilibrium conditions of the system, there is phase rule given by Gibbs. It connects the number of phases (F) and components (K) in an equilibrium system with the variance of the system, i.e., the number of thermodynamic degrees of freedom (C).
The number of thermodynamic degrees of freedom (variance) of a system is the number of independent variables as internal ( chemical composition phases), and external (temperature), which can be given various arbitrary (in a certain interval) values so that new phases do not appear and old phases do not disappear.
Gibbs phase rule equation:
C \u003d K - F + 1.
In accordance with this rule, in a system of two components (K = 2), the following degrees of freedom are possible:
For a single-phase state (F = 1) C = 2, i.e., you can change the temperature and concentration;
For a two-phase state (F = 2) C = 1, i.e., you can change only one external parameter (for example, temperature);
For a three-phase state, the number of degrees of freedom is zero, i.e., it is impossible to change the temperature without disturbing the equilibrium in the system (the system is invariant).
For example, for a pure metal (K = 1) during crystallization, when there are two phases (F = 2), the number of degrees of freedom is zero. This means that the crystallization temperature cannot be changed until the process ends and one phase remains - a solid crystal. After the end of crystallization (F = 1), the number of degrees of freedom is 1, so you can change the temperature, i.e., cool solid without breaking the balance.
The behavior of systems depending on temperature and concentration is described by a state diagram. The state diagram of water is a system with one component H 2 O, therefore largest number there are three phases that can simultaneously be in equilibrium (Fig. 10). These three phases are liquid, ice, steam. The number of degrees of freedom in this case is equal to zero, i.e. it is impossible to change either the pressure or the temperature so that none of the phases disappears. regular ice, liquid water and water vapor can exist in equilibrium simultaneously only at a pressure of 0.61 kPa and a temperature of 0.0075°C. The point where the three phases coexist is called the triple point ( O).
Curve OS separates the regions of vapor and liquid and represents the dependence of the pressure of saturated water vapor on temperature. The OC curve shows those interrelated values of temperature and pressure at which liquid water and water vapor are in equilibrium with each other, therefore it is called the liquid-vapor equilibrium curve or the boiling curve.
Fig 10 Water state diagram
Curve OV separates the liquid region from the ice region. It is a solid-liquid equilibrium curve and is called the melting curve. This curve shows those interrelated pairs of temperatures and pressures at which ice and liquid water are in equilibrium.
Curve OA is called the sublimation curve and shows the interconnected pairs of pressure and temperature values at which ice and water vapor are in equilibrium.
A state diagram is a visual way of representing the regions of existence of various phases depending on external conditions, such as pressure and temperature. State diagrams are actively used in materials science at various technological stages of obtaining a product.
A liquid differs from a solid crystalline body by low values of viscosity (internal friction of molecules) and high values of fluidity (the reciprocal of viscosity). A liquid consists of many aggregates of molecules, within which the particles are arranged in a certain order, similar to the order in crystals. Nature structural units and interparticle interaction determines the properties of the liquid. There are liquids: monoatomic (liquefied noble gases), molecular (water), ionic (molten salts), metallic (molten metals), liquid semiconductors. In most cases, a liquid is not only a state of aggregation, but also a thermodynamic (liquid) phase.
Liquid substances are most often solutions. Solution homogeneous, but not chemically pure substance, consists of a solute and a solvent (examples of a solvent are water or organic solvents: dichloroethane, alcohol, carbon tetrachloride, etc.), therefore it is a mixture of substances. An example is a solution of alcohol in water. However, solutions are also mixtures of gaseous (for example, air) or solid (metal alloys) substances.
Upon cooling under conditions of a low rate of formation of crystallization centers and a strong increase in viscosity, a glassy state can occur. Glasses are isotropic solid materials obtained by supercooling molten inorganic and organic compounds.
Many substances are known whose transition from a crystalline state to an isotropic liquid occurs through an intermediate liquid-crystal state. It is characteristic of substances whose molecules are in the form of long rods (rods) with an asymmetric structure. Such phase transitions, accompanied by thermal effects, cause an abrupt change in mechanical, optical, dielectric, and other properties.
liquid crystals, like a liquid, can take the form of an elongated drop or the shape of a vessel, have high fluidity, and are capable of merging. They received wide application in different areas science and technology. Their optical properties are highly dependent on small changes in external conditions. This feature is used in electro-optical devices. In particular, liquid crystals are used in the manufacture of electronic watches, visual equipment, etc.
Among the main states of aggregation is plasma- partially or fully ionized gas. According to the method of formation, two types of plasma are distinguished: thermal, which occurs when a gas is heated to high temperatures, and gaseous, which forms when electrical discharges in a gas environment.
Plasma-chemical processes have taken a firm place in a number of branches of technology. They are used for cutting and welding of refractory metals, synthesis different substances, plasma light sources are widely used, the use of plasma in thermonuclear power plants is promising, etc.
Definition 1
Aggregate states of matter(from the Latin “aggrego” means “I attach”, “I bind”) - these are the states of the same substance in solid, liquid and gaseous form.
During the transition from one state to another, an abrupt change in energy, entropy, density and other properties of matter is observed.
Solid and liquid bodies
Definition 2Solids- These are bodies that are distinguished by the constancy of their shape and volume.
In solids, intermolecular distances are small, and the potential energy of molecules can be compared with the kinetic energy.
Solid bodies are divided into 2 types:
- crystalline;
- Amorphous.
Only crystalline bodies are in a state of thermodynamic equilibrium. Amorphous bodies, in fact, are metastable states, which are similar in structure to non-equilibrium, slowly crystallizing liquids. In an amorphous body, an excessively slow process of crystallization takes place, a process of gradual transformation of a substance into a crystalline phase. The difference between a crystal and an amorphous solid lies primarily in the anisotropy of its properties. The properties of a crystalline body are determined depending on the direction in space. Various processes (for example, thermal conductivity, electrical conductivity, light, sound) propagate in different directions of a solid body in different ways. But amorphous bodies (for example, glass, resins, plastics) are isotropic, like liquids. The difference between amorphous bodies and liquids lies only in the fact that the latter are fluid, static shear deformations do not occur in them.
Crystalline bodies have the correct molecular structure. It is due to the correct structure that the crystal has anisotropic properties. The correct arrangement of crystal atoms creates the so-called crystal lattice. In different directions, the location of atoms in the lattice is different, which leads to anisotropy. Atoms (ions or whole molecules) in the crystal lattice perform random oscillatory motion near the middle positions, which are considered as nodes of the crystal lattice. The higher the temperature, the higher the energy of oscillations, and hence the average amplitude of oscillations. Depending on the amplitude of oscillations, the size of the crystal is determined. An increase in the amplitude of oscillations leads to an increase in the size of the body. Thus, the thermal expansion of solids is explained.
Definition 3
liquid bodies- These are bodies that have a certain volume, but do not have an elastic shape.
A substance in the liquid state is characterized by strong intermolecular interaction and low compressibility. A liquid occupies an intermediate position between a solid and a gas. Liquids, like gases, have isotopic properties. In addition, the liquid has the property of fluidity. In it, as in gases, there is no shear stress (shear stress) bodies. Liquids are heavy, that is, their specific gravity can be compared with the specific gravity of solids. Near the crystallization temperatures, their heat capacities and other thermal properties are close to those of solids. In liquids observed to a given extent correct location atoms, but only in small areas. Here, the atoms also perform an oscillatory motion around the nodes of the quasicrystalline cell, but, unlike the atoms of a solid body, they periodically jump from one node to another. As a result, the movement of atoms will be very complex: oscillatory, but at the same time, the center of oscillations moves in space.
Definition 4Gas This is a state of matter in which the distances between molecules are huge.
The forces of interaction between molecules at low pressures can be neglected. Gas particles fill the entire volume that is provided for gas. Gases are considered as highly superheated or unsaturated vapors. A special type of gas is plasma (a partially or fully ionized gas in which the densities of positive and negative charges are almost the same). That is, plasma is a gas of charged particles interacting with each other using electrical forces at a great distance, but not having near and far particles.
As you know, substances are able to move from one state of aggregation to another.
Definition 5
Evaporation- this is the process of changing the state of aggregation of a substance, in which molecules fly out from the surface of a liquid or solid body, the kinetic energy of which converts the potential energy of the interaction of molecules.
Evaporation is a phase transition. During evaporation, part of the liquid or solid is converted into vapor.
Definition 6
A substance in a gaseous state that is in dynamic equilibrium with a liquid is called saturated ferry. In this case, the change in the internal energy of the body is equal to:
∆ U = ± m r (1) ,
where m is the mass of the body, r is the specific heat of vaporization (J / k g).
Definition 7
Condensation is the reverse process of vaporization.
The change in internal energy is calculated by formula (1) .
Definition 8
Melting- This is the process of converting a substance from a solid state to a liquid state, the process of changing the state of aggregation of a substance.
When a substance is heated, its internal energy increases, therefore, the speed of thermal movement of molecules increases. When a substance reaches its melting point crystal cell the solid is destroyed. Bonds between particles are also destroyed, and the energy of interaction between particles increases. The heat that is transferred to the body goes to increase the internal energy of this body, and part of the energy is spent on doing work to change the volume of the body when it melts. For many crystalline bodies, the volume increases when melted, but there are exceptions (for example, ice, cast iron). Amorphous bodies do not have a specific melting point. Melting is a phase transition, which is characterized by an abrupt change in heat capacity at the melting temperature. The melting point depends on the substance and remains constant during the process. Then the change in the internal energy of the body is equal to:
∆ U = ± m λ (2) ,
where λ is the specific heat of fusion (D f / k g) .
Definition 9
Crystallization is the reverse process of melting.
The change in internal energy is calculated by formula (2) .
The change in the internal energy of each body of the system during heating or cooling is calculated by the formula:
∆ U = m c ∆ T (3) ,
where c is the specific heat capacity of the substance, J to g K, △ T is the change in body temperature.
Definition 10
When considering the transformations of substances from one state of aggregation to another, one cannot do without the so-called heat balance equations: the total amount of heat released in a thermally insulated system is equal to the amount of heat (total) that is absorbed in this system.
Q 1 + Q 2 + Q 3 + . . . + Q n = Q " 1 + Q " 2 + Q " 3 + . . . + Q " k .
In essence, the heat balance equation is the energy conservation law for heat transfer processes in thermally insulated systems.
Example 1
In a heat-insulated vessel are water and ice with a temperature t i = 0 ° C. The mass of water m υ and ice m i is respectively equal to 0.5 kg and 60 g. Water vapor of mass m p = 10 g is let into the water at a temperature t p = 100 ° C. What will be the temperature of the water in the vessel after thermal equilibrium is established? In this case, the heat capacity of the vessel does not need to be taken into account.
Picture 1
Decision
Let us determine which processes are carried out in the system, which aggregate states of matter we observed and which ones we obtained.
Water vapor condenses, giving off heat.
Thermal energy is spent on melting ice and, perhaps, heating the water available and obtained from ice.
First of all, let's check how much heat is released during the condensation of the available mass of steam:
Q p = - r m p ; Q p \u003d 2, 26 10 6 10 - 2 \u003d 2, 26 10 4 (D w),
here from reference materials we have r \u003d 2, 26 10 6 J k g - the specific heat of vaporization (it is also used for condensation).
To melt ice, you need the following amount of heat:
Q i \u003d λ m i Q i \u003d 6 10 - 2 3, 3 10 5 ≈ 2 10 4 (D w),
here, from reference materials, we have λ = 3, 3 10 5 J k g - the specific heat of ice melting.
It turns out that the steam gives off more heat than necessary, only to melt the existing ice, which means that we write the heat balance equation as follows:
r m p + c m p (T p - T) = λ m i + c (m υ + m i) (T - T i) .
Heat is released during condensation of steam of mass m p and cooling of water formed from steam from temperature T p to the desired T . Heat is absorbed when ice with mass m i melts and water with mass m υ + m i is heated from temperature T i to T . Denote T - T i = ∆ T for the difference T p - T we get:
T p - T = T p - T i - ∆ T = 100 - ∆ T .
The heat balance equation will look like:
r m p + c m p (100 - ∆ T) = λ m i + c (m υ + m i) ∆ T ; c (m υ + m i + m p) ∆ T = r m p + c m p 100 - λ m i ; ∆ T = r m p + c m p 100 - λ m i c m υ + m i + m p .
Let's make calculations, taking into account the fact that the heat capacity of water is tabular
c \u003d 4, 2 10 3 J k g K, T p \u003d t p + 273 \u003d 373 K, T i \u003d t i + 273 \u003d 273 K: ∆ T \u003d 2, 26 10 6 10 - 2 + 4, 2 10 3 10 - 2 10 2 - 6 10 - 2 3, 3 10 5 4, 2 10 3 5, 7 10 - 1 ≈ 3 (K),
then T = 273 + 3 = 276 K
Answer: The temperature of the water in the vessel after the establishment of thermal equilibrium will be 276 K.
Example 2
Figure 2 shows a section of the isotherm, which corresponds to the transition of a substance from a crystalline to a liquid state. What corresponds to this section on the p, T diagram?
Picture 2
Answer: The entire set of states that are shown on the p , V diagram as a horizontal line segment on the p , T diagram is shown by one point, which determines the values of p and T , at which the transformation from one state of aggregation to another takes place.
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In everyday practice, one has to deal not separately with individual atoms, molecules and ions, but with real substances - an aggregate a large number particles. Depending on the nature of their interaction, four types of aggregate state are distinguished: solid, liquid, gaseous and plasma. A substance can transform from one state of aggregation to another as a result of a corresponding phase transition.
The presence of a substance in a particular state of aggregation is due to the forces acting between the particles, the distance between them and the features of their movement. Each state of aggregation characterized by a set of certain properties.
Properties of substances depending on the state of aggregation:
| condition | property |
| gaseous |
|
| liquid |
|
| solid |
|
In accordance with the degree of order in the system, each state of aggregation is characterized by its own ratio between the kinetic and potential energies of the particles. In solids, the potential predominates over the kinetic, since the particles occupy certain positions and only oscillate around them. For gases, there is an inverse relationship between potential and kinetic energies, as a consequence of the fact that gas molecules always move randomly, and there are almost no cohesive forces between them, so the gas occupies the entire volume. In the case of liquids, the kinetic and potential energies of the particles are approximately the same, a non-rigid bond acts between the particles, therefore fluidity and a constant volume are inherent in liquids.
When the particles of a substance form a regular geometric structure, and the energy of bonds between them is greater than the energy of thermal vibrations, which prevents the destruction of the existing structure, it means that the substance is in a solid state. But starting from a certain temperature, the energy of thermal vibrations exceeds the energy of bonds between particles. In this case, the particles, although they remain in contact, move relative to each other. As a result, the geometric structure is broken and the substance passes into a liquid state. If the thermal fluctuations increase so much that the connection between the particles is practically lost, the substance acquires a gaseous state. In an "ideal" gas, particles move freely in all directions.
When the temperature rises, the substance passes from an ordered state (solid) to a disordered state (gaseous); the liquid state is intermediate in terms of the ordering of particles.
The fourth state of aggregation is called plasma - a gas consisting of a mixture of neutral and ionized particles and electrons. Plasma is formed at ultrahigh temperatures (10 5 -10 7 0 C) due to the significant collision energy of particles that have the maximum disorder of motion. A mandatory feature of plasma, as well as other states of matter, is its electrical neutrality. But as a result of the disordered motion of particles in the plasma, separate charged microzones can appear, due to which it becomes a source of electromagnetic radiation. In the plasma state, there is matter on, stars, other space objects, as well as in thermonuclear processes.
Each state of aggregation is determined primarily by the range of temperatures and pressures, therefore, for a visual quantitative characteristic, a phase diagram of a substance is used, which shows the dependence of the state of aggregation on pressure and temperature.
Diagram of the state of matter with phase transition curves: 1 - melting-crystallization, 2 - boiling-condensation, 3 - sublimation-desublimation
The state diagram consists of three main areas, which correspond to the crystalline, liquid and gaseous states. Individual regions are separated by curves reflecting phase transitions:
- solid to liquid and vice versa, liquid to solid (melting-crystallization curve - dotted green graph)
- liquid to gaseous and reverse conversion of gas to liquid (boiling-condensation curve - blue graph)
- solid to gaseous and gaseous to solid (sublimation-desublimation curve - red graph).
The coordinates of the intersection of these curves are called the triple point, in which, under conditions of a certain pressure P \u003d P in and a certain temperature T \u003d T in, a substance can coexist in three states of aggregation at once, and the liquid and solid states have the same vapor pressure. The coordinates Pv and Tv are the only values of pressure and temperature at which all three phases can coexist simultaneously.
The point K on the phase diagram of the state corresponds to the temperature T k - the so-called critical temperature, at which the kinetic energy of the particles exceeds the energy of their interaction and therefore the line of separation between the liquid and gas phases is erased, and the substance exists in the gaseous state at any pressure.
It follows from the analysis of the phase diagram that at a high pressure greater than at the triple point (P c), the heating of a solid ends with its melting, for example, at P 1, melting occurs at the point d. A further increase in temperature from T d to T e leads to the boiling of the substance at a given pressure P 1 . At a pressure Р 2 less than the pressure at the triple point Р в, heating the substance leads to its transition directly from the crystalline to the gaseous state (point q), that is, to sublimation. For most substances, the pressure at the triple point is lower than the saturation vapor pressure (P in
P saturated steam, therefore, when the crystals of such substances are heated, they do not melt, but evaporate, that is, they undergo sublimation. For example, iodine crystals or "dry ice" (solid CO 2) behave this way.
State Diagram Analysis gaseous state
Under normal conditions (273 K, 101325 Pa), both simple substances, the molecules of which consist of one (He, Ne, Ar) or several simple atoms (H 2, N 2, O 2), and complex substances with a low molar mass (CH 4, HCl, C 2 H 6).
Since the kinetic energy of gas particles exceeds their potential energy, the molecules in the gaseous state are constantly moving randomly. Due to the large distances between the particles, the forces of intermolecular interaction in gases are so small that they are not enough to attract particles to each other and keep them together. It is for this reason that gases do not have their own shape and are characterized by low density and high ability to compress and expand. Therefore, the gas constantly presses on the walls of the vessel in which it is located, equally in all directions.
To study the relationship between the most important gas parameters (pressure P, temperature T, amount of substance n, molar mass M, mass m), the simplest model of the gaseous state of matter is used - ideal gas, which is based on the following assumptions:
- the interaction between gas particles can be neglected;
- the particles themselves are material points that do not have their own size.
The most general equation describing the ideal gas model is considered to be the equations Mendeleev-Clapeyron for one mole of a substance:
However, the behavior of a real gas differs, as a rule, from the ideal one. This is explained, firstly, by the fact that between the molecules of a real gas there are still insignificant forces of mutual attraction that compress the gas to a certain extent. With this in mind, the total gas pressure increases by the value a/v2, which takes into account the additional internal pressure due to the mutual attraction of molecules. As a result, the total gas pressure is expressed by the sum P+ a/v2. Secondly, the molecules of a real gas have, albeit a small, but quite definite volume b , so the actual volume of all gas in space is V- b . When substituting the considered values into the Mendeleev-Clapeyron equation, we obtain the equation of state of a real gas, which is called van der Waals equation:
where a and b are empirical coefficients that are determined in practice for each real gas. It is established that the coefficient a has a large value for gases that are easily liquefied (for example, CO 2, NH 3), and the coefficient b - on the contrary, the higher in size, the larger the gas molecules (for example, gaseous hydrocarbons).
The van der Waals equation describes the behavior of a real gas much more accurately than the Mendeleev-Clapeyron equation, which, nevertheless, is widely used in practical calculations due to its clear physical meaning. Although the ideal state of a gas is a limiting, imaginary case, the simplicity of the laws that correspond to it, the possibility of their application to describe the properties of many gases at low pressures and high temperatures, makes the ideal gas model very convenient.
Liquid state of matter
The liquid state of any particular substance is thermodynamically stable in a certain range of temperatures and pressures characteristic of the nature (composition) of the substance. The upper temperature limit of the liquid state is the boiling point above which a substance under conditions of stable pressure is in a gaseous state. The lower limit of the stable state of the existence of a liquid is the temperature of crystallization (solidification). Boiling and crystallization temperatures measured at a pressure of 101.3 kPa are called normal.
For ordinary liquids, isotropy is inherent - the uniformity of physical properties in all directions within the substance. Sometimes other terms are also used for isotropy: invariance, symmetry with respect to the choice of direction.
In the formation of views on the nature of the liquid state, the concept of the critical state, which was discovered by Mendeleev (1860), is of great importance:
A critical state is an equilibrium state in which the separation limit between a liquid and its vapor disappears, since the liquid and its saturated vapor acquire the same physical properties.
In the critical state, the values of both densities and specific volumes of the liquid and its saturated vapor become the same.
The liquid state of matter is intermediate between gaseous and solid. Some properties bring the liquid state closer to the solid. If solids are characterized by a rigid ordering of particles, which extends over a distance of hundreds of thousands of interatomic or intermolecular radii, then in the liquid state, as a rule, no more than a few tens of ordered particles are observed. This is explained by the fact that orderliness between particles in different places of a liquid substance quickly arises, and is just as quickly “blurred” again by thermal vibrations of particles. At the same time, the overall density of the “packing” of particles differs little from that of a solid, so the density of liquids does not differ much from the density of most solids. In addition, the ability of liquids to compress is almost as small as in solids (about 20,000 times less than that of gases).
Structural analysis confirmed that the so-called short range order, which means that the number of nearest "neighbors" of each molecule and their mutual arrangement are approximately the same throughout the volume.
A relatively small number of particles of different composition, connected by forces of intermolecular interaction, is called cluster . If all particles in a liquid are the same, then such a cluster is called associate . It is in clusters and associates that short-range order is observed.
The degree of order in various liquids depends on temperature. At low temperatures slightly above the melting point, the degree of order in the placement of particles is very high. As the temperature rises, it decreases and, as the temperature rises, the properties of the liquid approach the properties of gases more and more, and when the critical temperature is reached, the difference between the liquid and gaseous states disappears.
The proximity of the liquid state to the solid state is confirmed by the values of the standard enthalpies of vaporization DH 0 of evaporation and melting DH 0 of melting. Recall that the value of DH 0 evaporation shows the amount of heat that is needed to convert 1 mole of liquid into vapor at 101.3 kPa; the same amount of heat is spent on the condensation of 1 mole of vapor into a liquid under the same conditions (i.e. DH 0 evaporation = DH 0 condensation). The amount of heat required to convert 1 mole of a solid to a liquid at 101.3 kPa is called standard enthalpy of fusion; the same amount of heat is released during the crystallization of 1 mole of liquid under normal pressure conditions (DH 0 melting = DH 0 crystallization). It is known that DH 0 evaporation<< DН 0 плавления, поскольку переход из твердого состояния в жидкое сопровождается меньшим нарушением межмолекулярного притяжения, чем переход из жидкого в газообразное состояние.
However, other important properties of liquids are more like those of gases. So, like gases, liquids can flow - this property is called fluidity . They can resist the flow, that is, they are inherent viscosity . These properties are influenced by attractive forces between molecules, the molecular weight of the liquid substance, and other factors. The viscosity of liquids is about 100 times greater than that of gases. Just like gases, liquids can diffuse, but at a much slower rate because liquid particles are packed more densely than gas particles.
One of the most interesting properties of the liquid state, which is not characteristic of either gases or solids, is surface tension .
Diagram of the surface tension of a liquid A molecule located in a liquid volume is uniformly acted upon by intermolecular forces from all sides. However, on the surface of the liquid, the balance of these forces is disturbed, as a result of which the surface molecules are under the action of some resultant force, which is directed inside the liquid. For this reason, the liquid surface is in a state of tension. Surface tension is the minimum force that keeps the particles of a liquid inside and thereby prevents the surface of the liquid from contracting.
Structure and properties of solids
Most of the known substances, both natural and artificial, are in the solid state under normal conditions. Of all the compounds known today, about 95% are solids, which have become important, since they are the basis of not only structural, but also functional materials.
- Structural materials are solids or their compositions that are used to make tools, household items, and various other structures.
- Functional materials are solids, the use of which is due to the presence of certain useful properties in them.
For example, steel, aluminum, concrete, ceramics belong to structural materials, and semiconductors, phosphors belong to functional ones.
In the solid state, the distances between the particles of matter are small and have the same order of magnitude as the particles themselves. The interaction energies between them are large enough, which prevents the free movement of particles - they can only oscillate about certain equilibrium positions, for example, around the nodes of the crystal lattice. The inability of particles to move freely leads to one of the most characteristic features of solids - the presence of their own shape and volume. The ability to compress solids is very small, and the density is high and little dependent on temperature changes. All processes occurring in solid matter occur slowly. The laws of stoichiometry for solids have a different and, as a rule, broader meaning than for gaseous and liquid substances.
The detailed description of solids is too voluminous for this material and is therefore covered in separate articles:, and.
Depending on temperature and pressure, any substance is capable of taking on various states of aggregation. Each such state is characterized by certain qualitative properties that remain unchanged within the framework of temperatures and pressures required for a given state of aggregation.
The characteristic properties of aggregate states include, for example, the ability of a body in a solid state to maintain its shape, or vice versa, the ability of a liquid body to change shape. However, sometimes the boundaries between different states of matter are quite blurred, as in the case of liquid crystals, or the so-called "amorphous bodies", which can be elastic like solids and fluid like liquids.
The transition between states of aggregation can occur with the release of free energy, changes in density, entropy, or other physical quantities. The transition from one state of aggregation to another is called a phase transition, and the phenomena accompanying such transitions are called critical phenomena.
List of known aggregate states
Solid |
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Solids whose atoms or molecules do not form a crystal lattice. |
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Solids whose atoms or molecules form a crystal lattice. |
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mesophase |
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A liquid crystal is a phase state during which a substance simultaneously possesses both the properties of liquids and the properties of crystals. |
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Liquid |
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The state of matter at temperatures above the melting point and below the boiling point. |
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A liquid whose temperature exceeds its boiling point. |
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A liquid whose temperature is less than the crystallization temperature. |
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The state of a liquid substance under negative pressure caused by van der Waals forces (forces of attraction between molecules). |
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The state of a liquid at a temperature above the critical point. |
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A liquid whose properties are affected by quantum effects. |
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A state of matter that has very weak bonds between molecules or atoms. Does not lend itself to the mathematical description of an ideal gas. |
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A gas whose properties are affected by quantum effects. |
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Aggregate state, represented by a set of individual charged particles, the total charge of which in any volume of the system is equal to zero. |
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A state of matter in which it is a collection of gluons, quarks, and antiquarks. |
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A momentary state during which gluon force fields are stretched between nuclei. Preceded by quark-gluon plasma. |
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quantum gas |
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A gas composed of fermions whose properties are affected by quantum effects. |
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A gas composed of bosons whose properties are affected by quantum effects. |
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A feature of hydraulic and pneumatic drives is that to create forces, moments of forces and movements in machines, these types of drives use the energy of a liquid or air or other gas, respectively.
The fluid used in a hydraulic drive is called working fluid (WF).
To understand the peculiarities of the use of RJ and gases in drives, it is necessary to recall some basic information about the states of aggregation of matter known from the course of physics.
According to modern views, the aggregate states of a substance (from the Latin aggrego - I attach, connect) - are understood to be the states of the same substance, the 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 four aggregate states of matter: solid, liquid, gaseous and plasma.
SOLID STATE(crystalline solid state of matter) is a state of aggregation, which is characterized by large forces of interaction between particles of matter (atoms, molecules, ions). The particles of solids oscillate around the average equilibrium positions, called the nodes of the crystal lattice; the structure of these substances is characterized by a high degree of order (long-range and short-range order) - order in arrangement (coordination order), in orientation (orientation order) of structural particles or order in physical properties.
LIQUID STATE- This is a 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 fluctuations around equilibrium positions and frequent jumps from one equilibrium position to another. Simultaneously, there are slow movements of molecules and their vibrations inside small volumes. Frequent jumps of molecules break the long-range order in the arrangement of particles and cause the fluidity of liquids, while small fluctuations around equilibrium positions cause the existence of short-range order in liquids.
Liquids and solids, unlike gases, can be regarded as highly condensed media. In them, the 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, obviously cannot occupy an arbitrary volume, and at constant pressure and temperature they retain their volume, no matter in what volume they are placed.
GAS STATE(from the French gaz, which, in turn, came from the Greek chaos - chaos) is an aggregate state of matter in which the interaction forces of its particles filling the entire volume provided to them are negligible. In gases, the intermolecular distances are large and the molecules move almost freely.
Gases can be thought of as highly superheated or low-saturated vapors of liquids. Above the surface of each liquid due to evaporation is vapor. When the vapor pressure rises to a certain limit, called the 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 some of the steam to condense, rather than an increase in pressure. Therefore, the vapor pressure cannot be higher than the saturation 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 the volume is increased or the 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 A partially or fully ionized gas is called, in which the densities of positive and negative charges are almost the same. The sun, stars, clouds of interstellar matter are composed of gases - neutral or ionized (plasma). Unlike other states of aggregation, plasma is a gas of charged particles (ions, electrons) that electrically interact with each other at 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 able to maintain volume, but are not able to independently maintain their shape. The first property brings the liquid closer to the solid, the second - to the gas. Both of these properties are not absolute. All liquids are compressible, although much weaker than gases. All liquids resist the change in shape, the displacement of one part of the volume relative to another, although less than solids.
