Antipyretics for children are prescribed by a pediatrician. But there are emergency situations with fever when the child needs to be given medicine immediately. Then the parents take responsibility and use antipyretic drugs. What is allowed to be given to infants? How can you lower the temperature in older children? What medications are the safest?
Lesson objectives:
- deepen and generalize knowledge about the aggregate states of matter, study in what states substances can exist.
Lesson objectives:
Educational – formulate an idea of the properties of solids, gases, liquids.
Developmental – development of students’ speech skills, analysis, conclusions on the material covered and studied.
Educational - instilling mental work, creating all the conditions to increase interest in the subject studied.
Key terms:
State of aggregation- this is a state of matter that is characterized by certain qualitative properties: - the ability or inability to maintain shape and volume; - presence or absence of short-range and long-range order; - by others.
Fig.6. Aggregate state of a substance when temperature changes.
When a substance passes from a solid state to a liquid state, this is called melting; the reverse process is called crystallization. When a substance passes from a liquid to a gas, this process is called vaporization, and into a liquid from a gas - condensation. And the transition directly to gas from a solid, bypassing the liquid, is sublimation, the reverse process is desublimation.
1.Crystallization; 2. Melting; 3. Condensation; 4. Vaporization;
5. Sublimation; 6. Desublimation.
We see these examples of transitions all the time in everyday life. When ice melts, it turns into water, and the water in turn evaporates, creating steam. If we look at it in the opposite direction, the steam, condensing, begins to turn back into water, and the water, in turn, freezes and becomes ice. The smell of any solid body is sublimation. Some molecules escape from the body, and a gas is formed, which gives off the smell. An example of the reverse process is patterns on glass in winter, when vapor in the air freezes and settles on the glass.
The video shows a change in the state of aggregation of a substance.
Control block.
1.After freezing, the water turned into ice. Did the water molecules change?
2.Medical ether is used indoors. And because of this, it usually smells strongly of him there. What state is the ether in?
3.What happens to the shape of the liquid?
4.Ice. What state of water is this?
5.What happens when water freezes?
Homework.
Answer the questions:
1. Is it possible to fill half the volume of a vessel with gas? Why?
2.Can nitrogen and oxygen exist in a liquid state at room temperature?
3.Can iron and mercury exist in a gaseous state at room temperature?
4. On a frosty winter day, fog formed over the river. What state of matter is this?
We believe that matter has three states of aggregation. In fact, there are at least fifteen of them, and the list of these conditions continues to grow every day. These are: amorphous solid, solid, neutronium, quark-gluon plasma, strongly symmetric matter, weakly symmetric matter, fermion condensate, Bose-Einstein condensate and strange matter.
State of aggregation- this is the state of a substance in a certain range of temperatures and pressures, characterized by properties: the ability (solid) 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 distinguished.
IN gaseous In this state, the distance between the atoms and molecules of the substance is large, the interaction forces are small and the particles, moving chaotically in space, have a large kinetic energy that exceeds the potential energy. A material in a gaseous state has neither its own shape nor volume. Gas fills all available space. This state is typical for substances with low density.
IN liquid state, only short-range order of atoms or molecules is preserved, when individual areas with an ordered arrangement of atoms periodically appear in the volume of the substance, but the mutual orientation of these areas is also absent. Short-range order is unstable and under the influence of thermal vibrations of atoms it can either disappear or appear again. Liquid molecules do not have a specific position, and at the same time they do not have complete freedom of movement. The material in the liquid state does not have its own shape; it retains only its volume. The liquid can occupy only part of the volume of the vessel, but flow freely over the entire surface of the vessel. The liquid state is usually considered intermediate between a solid and a gas.
IN hard In a substance, the arrangement of atoms becomes strictly defined, naturally ordered, the forces of interaction between 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 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, var, which are outwardly in a solid state, although in fact they flow slowly, like a liquid. Amorphous bodies, unlike crystalline ones, do not have a specific melting point. Amorphous solids occupy an intermediate position between crystalline solids and liquids.
Most solids have crystalline a structure characterized by the orderly arrangement of atoms or molecules in space. The crystal structure is characterized by long-range order, when the elements of the structure are periodically repeated; with short-range order there is no such correct repetition. A characteristic feature of a crystalline body is the ability to maintain its shape. A sign of an ideal crystal, the model of which is a spatial lattice, is the property of symmetry. Symmetry refers to the theoretical ability of the crystal lattice of a solid body to align with itself when its points are mirrored from a certain plane, called the plane of symmetry. The symmetry of the external shape reflects the symmetry of the internal structure of the crystal. For example, all metals have a crystalline structure and are characterized by two types of symmetry: cubic and hexagonal.
In amorphous structures with a disordered distribution of atoms, the properties of the substance in different directions are the same, that is, glassy (amorphous) substances are isotropic.
All crystals are characterized by anisotropy. In crystals, the distances between atoms are ordered, but in different directions the degree of ordering may not be the same, which leads to differences 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 properties of objects that have a certain material volume: sizes - from several millimeters to tens of centimeters. These objects with a structure identical to the crystal cell are called single crystals.
Anisotropy of properties manifests itself 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, the molecules of which can be arranged in an orderly manner with the formation of supramolecular structures in the form of packs, coils (globules), fibrils, etc., occurs in a certain temperature range. The complex structure of molecules and their aggregates determines the specific behavior of polymers when heated. They cannot go into a liquid state with low viscosity and 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 when the temperature changes, which manifests itself in the process of deformation of the polymer. In Fig. Figure 9 shows the dependence of deformation on temperature.
Rice. 9 Thermomechanical curve of an amorphous polymer: t c , t T, t p - glass transition, fluidity and onset of chemical decomposition temperatures, respectively; I - III - zones of 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 deform elastically (Fig. 9, zone I). Above glass transition temperature t c an amorphous polymer with a linear structure transforms into a highly elastic state ( zone II), and its deformation in the glassy and highly elastic states is reversible. Heating above the pour point t t transfers the polymer to a viscous flow state ( zone III). The deformation of a polymer in a viscous flow state is irreversible. An amorphous polymer with a spatial (network, cross-linked) structure does not have a viscous flow state; the temperature region of the highly elastic state expands to the temperature of polymer decomposition t R. This behavior is typical for materials such as rubber.
The temperature of a substance in any state of aggregation characterizes the average kinetic energy of its particles (atoms and molecules). These particles in bodies possess mainly the kinetic energy of vibrational movements 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 energetically that the rigid bonds in the polymer are broken and the crystals are destroyed - they turn into a liquid state. The temperature at which the 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.
Each chemical element has an individual 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, substances are in thermodynamic equilibrium. A change in the state of aggregation can be accompanied by an abrupt 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 copper melts (1083 o C) it turns into a liquid in which the atoms have only short-range order. At a pressure of 1 atm, copper boils at 2310 o C and turns into gaseous copper with randomly arranged copper atoms. At the melting point, the saturated vapor pressures of the crystal and the liquid are equal.
The material as a whole is a system.
System- a group of substances combined physical, chemical or mechanical interactions. Phase called a homogeneous part of a system, separated from other parts physical interface boundaries (in cast iron: graphite + iron grains; in water with ice: ice + water).Components systems are the different phases that make up a given system. System components- these are the substances that form all the phases (components) of a given system.
Materials consisting of two or more phases are dispersed systems Dispersed systems are divided into sols, whose behavior resembles the behavior of liquids, and gels with the characteristic properties of solids. 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 low temperatures (at high temperatures gelatin turns into a sol). A hydrosol is a dispersion in water, an aerosol is a dispersion in air.
Status 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 describing transformations is limited to considering the equilibrium state. Equilibrium state thermodynamic system is characterized by the invariance of thermodynamic parameters (temperature and concentration, since in technological treatments R= const) in time and the absence of flows of energy and matter in it - with constant external conditions. Phase equilibrium- the equilibrium state of a thermodynamic system consisting of two or more phases.
To mathematically describe the equilibrium conditions of a system, there is phase rule, derived by Gibbs. It connects the number of phases (F) and components (K) in an equilibrium system with the variability 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, both internal (chemical composition of phases) and external (temperature), to which various arbitrary (in a certain range) values can be given so that new phases do not appear and old phases do not disappear .
Gibbs phase rule equation:
C = 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., only one external parameter can be changed (for example, temperature);
For a three-phase state, the number of degrees of freedom is zero, i.e., the temperature cannot be changed 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 is completed and one phase remains - a solid crystal. After the end of crystallization (Ф = 1), the number of degrees of freedom is 1, so you can change the temperature, i.e., cool the solid without disturbing the equilibrium.
The behavior of systems depending on temperature and concentration is described by a phase diagram. The phase diagram of water is a system with one component H 2 O, therefore the largest number of phases that can simultaneously be in equilibrium is three (Fig. 10). These three phases are liquid, ice, steam. The number of degrees of freedom in this case is zero, i.e. Neither the pressure nor the temperature can be changed without any of the phases disappearing. Ordinary 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 three phases coexist is called the triple point ( O).
Curve OS separates the vapor and liquid regions and represents the dependence of saturated water vapor pressure on temperature. The OS 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 boiling curve.
Fig 10 Diagram of the state of water
Curve OB separates the liquid region from the ice region. It is the solid-liquid equilibrium curve and is called the melting curve. This curve shows those interrelated pairs of temperature and pressure values at which ice and liquid water are in equilibrium.
Curve O.A. called a sublimation curve and shows the interrelated pairs of pressure and temperature values at which ice and water vapor are in equilibrium.
A phase diagram is a visual way of representing the regions of existence of different phases depending on external conditions, such as pressure and temperature. State diagrams are actively used in materials science at various technological stages of product production.
A liquid differs from a crystalline solid by low viscosity values (internal friction of molecules) and high fluidity values (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. The nature of structural units and interparticle interactions 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, liquid is not only a state of aggregation, but also a thermodynamic (liquid) phase.
Liquid substances are most often solutions. Solution homogeneous, but not a chemically pure substance, consists of a dissolved substance 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.
When cooled under conditions of low rate of formation of crystallization centers and a strong increase in viscosity, a glassy state may occur. Glasses are isotropic solid materials obtained by supercooling molten inorganic and organic compounds.
There are many known substances whose transition from a crystalline state to an isotropic liquid occurs through an intermediate liquid crystalline state. It is typical for substances whose molecules have the shape of long rods (rods) with an asymmetric structure. Such phase transitions, accompanied by thermal effects, cause abrupt changes 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 are widely used in various fields of 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 wristwatches, visual equipment, etc.
The main states of aggregation include plasma- partially or fully ionized gas. Based on the method of formation, two types of plasma are distinguished: thermal, which occurs when gas is heated to high temperatures, and gaseous, which is formed during electrical discharges in a gaseous environment.
Plasma-chemical processes have taken a strong place in a number of branches of technology. They are used for cutting and welding refractory metals, synthesis of various substances, plasma light sources are widely used, the use of plasma in thermonuclear power plants is promising, etc.
In everyday practice, we have to deal not separately with individual atoms, molecules and ions, but with real substances - a collection of a large number of particles. Depending on the nature of their interaction, four types of state of aggregation are distinguished: solid, liquid, gaseous and plasma. A substance can transform from one state of aggregation to another as a result of an appropriate phase transition.
The presence of a substance in one or another state of aggregation is determined by the forces acting between particles, the distance between them and the characteristics of their movement. Each state of aggregation is characterized by a set of certain properties.
Properties of substances depending on their state of aggregation:
| state | property |
| gaseous |
|
| liquid |
|
| hard |
|
In accordance with the degree of order in the system, each state of aggregation is characterized by its own relationship between the kinetic and potential energies of the particles. In solids, potential prevails over kinetic, since particles occupy certain positions and only vibrate 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 chaotically, 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, there is a non-rigid connection between the particles, therefore liquids are characterized by fluidity and a constant volume.
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 disrupted and the substance passes into a liquid state. If thermal vibrations increase so much that the connection between particles is practically lost, the substance acquires a gaseous state. In an “ideal” gas, particles move freely in all directions.
As the temperature increases, a substance passes from an ordered state (solid) to a disordered state (gaseous); the liquid state is intermediate in the order 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 ultra-high temperatures (10 5 -10 7 0 C) due to the significant collision energy of particles that have maximum disorder of motion. A mandatory feature of plasma, like other states of matter, is its electrical neutrality. But as a result of the disordered movement of particles in the plasma, individual charged microzones can appear, due to which it becomes a source of electromagnetic radiation. In the plasma state, matter exists on stars and other space objects, as well as during thermonuclear processes.
Each state of aggregation is determined, first of all, 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.
State diagram of a substance with phase transition curves: 1 - melting-crystallization, 2 - boiling-condensation, 3 - sublimation-desublimation
The phase diagram consists of three main regions, which correspond to the crystalline, liquid and gaseous states. Individual areas are separated by curves reflecting phase transitions:
- solid state into liquid and, conversely, liquid into 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 intersection coordinates of these curves are called the triple point, in which, under conditions of a certain pressure P = P in and a certain temperature T = T in, a substance can coexist in three states of aggregation at once, with the liquid and solid states having the same vapor pressure. The coordinates P in and T in are the only values of pressure and temperature at which all three phases can simultaneously coexist.
Point K on the phase diagram of the state corresponds to temperature Tk - the so-called critical temperature at which the kinetic energy of 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 a gaseous state at any pressure.
From the analysis of the phase diagram it follows that at a high pressure greater than at the triple point (P in), heating of a solid substance ends with its melting, for example, at P 1 melting occurs at the point d. A further increase in temperature from Td to Te leads to boiling of the substance at a given pressure P1. At a pressure P 2 less than the pressure at the triple point P in, 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 saturated vapor pressure (P in
P is saturated steam, therefore, when 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.
Analysis of the phase diagram of matter Gaseous state
Under normal conditions (273 K, 101325 Pa), both simple substances, whose molecules consist of one (He, Ne, Ar) or several simple atoms (H 2, N 2, O 2), and complex ones can be in the gaseous state substances with 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 continuously move randomly. Due to the large distances between particles, the forces of intermolecular interaction in gases are so insignificant that they are not enough to attract particles to each other and hold 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 parameters of a gas (pressure P, temperature T, amount of substance n, molar mass M, mass m), the simplest model of the gaseous state of a substance 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 equation Mendeleev-Clapeyron for one mole of substance:
However, the behavior of a real gas differs, as a rule, from an ideal one. This is explained, firstly, by the fact that there are still insignificant forces of mutual attraction between the molecules of a real gas, which compress the gas to a certain extent. Taking this into account, the total gas pressure increases by the amount a/V 2, which takes into account the additional internal pressure caused by the mutual attraction of molecules. As a result, the total gas pressure is expressed by the sum P+ A/V 2. Secondly, the molecules of a real gas have, although small, a well-defined volume b , therefore 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 — empirical coefficients that are determined in practice for each real gas. It has been established that the coefficient a has a larger value for gases that are easily liquefied (for example, CO 2, NH 3), and the coefficient b - on the contrary, the higher in magnitude, 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, due to its clear physical meaning, is widely used in practical calculations. 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 under conditions of low pressure 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 this substance. The upper temperature limit of the liquid state is the boiling point, above which a substance is in a gaseous state under stable pressure conditions. The lower limit of the stable state of existence of a liquid is the crystallization (solidification) temperature. Boiling and crystallization temperatures measured at a pressure of 101.3 kPa are called normal.
Ordinary liquids are characterized by isotropy—uniformity of physical properties in all directions within a substance. Sometimes other terms are used for isotropy: invariance, symmetry with respect to the choice of direction.
In shaping views on the nature of the liquid state, the idea of a critical state, which was discovered by Mendeleev (1860), is important:
A critical state is an equilibrium state in which the limit of separation between a liquid and its vapor disappears because the liquid and its saturated vapor acquire the same physical properties.
In a critical state, the values of both the densities and specific volumes of the liquid and its saturated vapor become the same.
The liquid state of a substance is intermediate between gaseous and solid. Some properties bring the liquid state closer to the solid state. If solids are characterized by a rigid ordering of particles, which extends over distances of up to hundreds of thousands of interatomic or intermolecular radii, then in the liquid state, as a rule, no more than several tens of ordered particles are observed. This is explained by the fact that order between particles in different places of a liquid substance quickly arises, and just as quickly is “eroded” again by thermal vibrations of the 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 is not very different from the density of most solids. In addition, the ability of liquids to compress is almost as low as that of solids (about 20,000 times less than that of gases).
Structural analysis confirmed that liquids exhibit the so-called close order, which means that the number of nearest “neighbors” of each molecule and their relative positions are approximately the same throughout the entire volume.
A relatively small number of particles of different compositions connected by intermolecular interaction forces is called cluster . If all particles in a liquid are identical, 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 orderliness in the arrangement of particles is very high. As the temperature rises, it decreases and as it heats up, the properties of the liquid become more and more similar to the properties of gases, and when the critical temperature is reached, the difference between the liquid and gaseous states disappears.
The closeness of the liquid state to the solid state is confirmed by the values of the standard enthalpies of evaporation DН 0 evaporation and melting DН 0 melting. Let us 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 steam into liquid under the same conditions (i.e. DH 0 evaporation = DH 0 condensation). The amount of heat expended to convert 1 mole of a solid into 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 more closely resemble those of gases. So, like gases, liquids can flow - this property is called fluidity . They can resist the flow, that is, they have an inherent viscosity . These properties are influenced by the forces of attraction between molecules, the molecular weight of the liquid substance and other factors. The viscosity of liquids is approximately 100 times greater than that of gases. Just like gases, liquids can diffuse, but much more slowly because liquid particles are packed more tightly together 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 .
Liquid surface tension diagram A molecule located in a liquid volume is uniformly acted on 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 influence of some resultant force, which is directed inside the liquid. For this reason, the surface of the liquid is in a state of tension. Surface tension is the minimum force that holds liquid particles inside and thereby prevents the surface of the liquid from contracting.
Structure and properties of solids
Most known substances, both natural and artificial, are in a solid state under normal conditions. Of all the compounds known today, about 95% are solids, which have become important because they are the basis of not only structural but also functional materials.
- Construction materials are solid substances or their compositions that are used for the manufacture of tools, household items, and various other structures.
- Functional materials are solid substances, the use of which is determined by the presence of certain beneficial properties in them.
For example, steel, aluminum, concrete, and ceramics belong to structural materials, while semiconductors and phosphors belong to functional materials.
In the solid state, the distances between the particles of the substance are small and have the same order of magnitude as the particles themselves. The interaction energies between them are quite high, which prevents the free movement of particles - they can only oscillate around certain equilibrium positions, for example, around the nodes of a 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 compressibility of solids is very low, and the density is high and depends little on changes in temperature. 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.
A detailed description of solids is too voluminous for this material and is therefore discussed in separate articles:, and.
Aggregate states. Liquids. Phases in thermodynamics. Phase transitions.
Lecture 1.16
All substances can exist in three states of aggregation - solid, liquid And gaseous. Transitions between them are accompanied by abrupt changes in a number of physical properties (density, thermal conductivity, etc.).
The state of aggregation depends on the physical conditions in which the substance is located. The existence of several states of aggregation in a substance is due to differences in the thermal motion of its molecules (atoms) and in their interaction under different conditions.
Gas- the state of aggregation of a substance in which the particles are not connected or are very weakly connected by interaction forces; the kinetic energy of the thermal motion of its particles (molecules, atoms) significantly exceeds the potential energy of interactions between them, therefore the particles move almost freely, completely filling the vessel in which they are located and taking its shape. In the gaseous state, a substance has neither its own volume nor its own shape. Any substance can be converted into a gas by changing pressure and temperature.
Liquid- state of aggregation of a substance, intermediate between solid and gaseous. It is characterized by high mobility of particles and small free space between them. This causes liquids to maintain their volume and take the shape of the container. In a liquid, the molecules are located very close to each other. Therefore, the density of liquid is much greater than the density of gases (at normal pressure). The properties of a liquid are the same (isotropic) in all directions, with the exception of liquid crystals. When heated or the density decreases, the properties of the liquid, thermal conductivity, and viscosity change, as a rule, towards the properties of gases.
The thermal motion of liquid molecules consists of a combination of collective vibrational movements and jumps of molecules that occur from time to time from one equilibrium position to another.
Solid (crystalline) bodies- the state of aggregation of a substance, characterized by stability of shape and the nature of the thermal movement of atoms. This movement is the vibration of the atoms (or ions) that make up the solid. The vibration amplitude is usually small compared to the interatomic distances.
Properties of liquids.
The molecules of a substance in a liquid state are located almost close to each other. Unlike solid crystalline bodies, in which molecules form ordered structures throughout the entire volume of the crystal and can perform thermal vibrations around fixed centers, liquid molecules have greater freedom. Each molecule of a liquid, just like in a solid, is “sandwiched” on all sides by neighboring molecules and undergoes thermal vibrations around a certain equilibrium position. However, from time to time any molecule may move to a nearby vacant site. Such jumps in liquids occur quite often; therefore, the molecules are not tied to specific centers, as in crystals, and can move throughout the entire volume of the liquid. This explains the fluidity of liquids. Due to the strong interaction between closely located molecules, they can form local (unstable) ordered groups containing several molecules. This phenomenon is called close order.
Due to the dense packing of molecules, the compressibility of liquids, i.e., the change in volume with a change in pressure, is very small; it is tens and hundreds of thousands of times less than in gases. For example, to change the volume of water by 1%, you need to increase the pressure approximately 200 times. This increase in pressure compared to atmospheric pressure is achieved at a depth of about 2 km.
Liquids, like solids, change their volume with changes in temperature. For not very large temperature ranges, the relative change in volume Δ V / V 0 is proportional to the temperature change Δ T:
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The coefficient β is called temperature coefficient of volumetric expansion. This coefficient for liquids is tens of times greater than for solids. For water, for example, at a temperature of 20 °C β ≈ 2 10 –4 K –1, for steel - β st ≈ 3.6 10 –5 K –1, for quartz glass - β kV ≈ 9 10 – 6 K –1.
The thermal expansion of water has an interesting and important anomaly for life on Earth. At temperatures below 4 °C, water expands as the temperature decreases (β< 0). Максимум плотности ρ в = 10 3 кг/м 3 вода имеет при температуре 4 °С.
When water freezes, it expands, so ice remains floating on the surface of a freezing body of water. The temperature of freezing water under the ice is 0 °C. In denser layers of water at the bottom of the reservoir, the temperature is about 4 °C. Thanks to this, life can exist in the water of freezing reservoirs.
The most interesting feature of liquids is the presence free surface. Liquid, unlike gases, does not fill the entire volume of the container into which it is poured. An interface is formed between liquid and gas (or vapor), which is in special conditions compared to the rest of the liquid. Molecules in the boundary layer of a liquid, unlike molecules in its depth, are not surrounded by other molecules of the same liquid on all sides. The forces of intermolecular interaction acting on one of the molecules inside a liquid from neighboring molecules are, on average, mutually compensated. Any molecule in the boundary layer is attracted by molecules located inside the liquid (the forces acting on a given liquid molecule from gas (or vapor) molecules can be neglected). As a result, a certain resultant force appears, directed deep into the liquid. Surface molecules are drawn into the liquid by forces of intermolecular attraction. But all molecules, including molecules of the boundary layer, must be in a state of equilibrium. This equilibrium is achieved by slightly reducing the distance between the molecules of the surface layer and their nearest neighbors inside the liquid. As the distance between molecules decreases, repulsive forces arise. If the average distance between molecules inside a liquid is r 0, then the molecules of the surface layer are packed somewhat more densely, and therefore they have an additional reserve of potential energy compared to the internal molecules. It should be borne in mind that due to the extremely low compressibility, the presence of a more densely packed surface layer does not lead to any noticeable change in the volume of the liquid. If a molecule moves from the surface into the liquid, the forces of intermolecular interaction will do positive work. On the contrary, in order to pull a certain number of molecules from the depth of the liquid to the surface (i.e., increase the surface area of the liquid), external forces must do positive work A external, proportional to the change Δ S surface area:
| A ext = σΔ S. |
The coefficient σ is called the surface tension coefficient (σ > 0). Thus, the coefficient of surface tension is equal to the work required to increase the surface area of a liquid at constant temperature by one unit.
In SI, the coefficient of surface tension is measured in joules per meter square (J/m2) or in newtons per meter (1 N/m = 1 J/m2).
Consequently, the molecules of the surface layer of a liquid have an excess of potential energy. Potential energy E p of the liquid surface is proportional to its area: 
(1.16.1)
It is known from mechanics that the equilibrium states of a system correspond to the minimum value of its potential energy. It follows that the free surface of the liquid tends to reduce its area. For this reason, a free drop of liquid takes on a spherical shape. The liquid behaves as if forces acting tangentially to its surface are contracting (pulling) this surface. These forces are called surface tension forces.
The presence of surface tension forces makes the surface of a liquid look like an elastic stretched film, with the only difference that the elastic forces in the film depend on its surface area (i.e., on how the film is deformed), and the surface tension forces do not depend on the surface area liquids.
Surface tension forces tend to reduce the surface of the film. Therefore we can write: (1.16.2)
Thus, the surface tension coefficient σ can be defined as the modulus of the surface tension force acting per unit length of the line bounding the surface ( l- the length of this line).
Due to the action of surface tension forces in drops of liquid and inside soap bubbles, excess pressure Δ arises p. If you mentally cut a spherical drop of radius R into two halves, then each of them must be in equilibrium under the action of surface tension forces applied to the cut boundary of length 2π R and excess pressure forces acting on the area π R 2 sections (Fig. 1.16.1). The equilibrium condition is written as
Near the boundary between a liquid, a solid and a gas, the shape of the free surface of the liquid depends on the forces of interaction between liquid molecules and solid molecules (interaction with gas (or vapor) molecules can be neglected). If these forces are greater than the forces of interaction between the molecules of the liquid itself, then the liquid wets surface of a solid. In this case, the liquid approaches the surface of the solid at a certain acute angle θ, characteristic of a given liquid-solid pair. The angle θ is called contact angle. If the forces of interaction between liquid molecules exceed the forces of their interaction with solid molecules, then the contact angle θ turns out to be obtuse (Fig. 1.16.2(2)). In this case they say that the liquid does not wet surface of a solid. Otherwise (angle - acute) liquid wets surface (Fig. 1.16.2(1)). At full wettingθ = 0, at complete non-wettingθ = 180°.
Capillary phenomena called the rise or fall of liquid in small diameter tubes - capillaries. Wetting liquids rise through the capillaries, non-wetting liquids descend.
Figure 1.16.3 shows a capillary tube of a certain radius r, lowered at the lower end into a wetting liquid of density ρ. The upper end of the capillary is open. The rise of liquid in the capillary continues until the force of gravity acting on the column of liquid in the capillary becomes equal in magnitude to the resultant F n surface tension forces acting along the boundary of contact of the liquid with the surface of the capillary: F t = F n, where F t = mg = ρ hπ r 2 g, F n = σ2π r cos θ.
This implies: 
With complete wetting θ = 0, cos θ = 1. In this case
With complete non-wetting θ = 180°, cos θ = –1 and, therefore, h < 0. Уровень несмачивающей жидкости в капилляре опускается ниже уровня жидкости в сосуде, в которую опущен капилляр.
Water almost completely wets the clean glass surface. On the contrary, mercury does not completely wet the glass surface. Therefore, the level of mercury in the glass capillary drops below the level in the vessel.
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: 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.
