The transition from liquid to solid formula. What is the state of aggregation of matter. What factors depend on

Any body can be in different states of aggregation at certain temperatures and pressures - in solid, liquid, gaseous and plasma states.

The transition from one state of aggregation to another occurs under the condition that the heating of the body from the outside occurs faster than its cooling. And vice versa, if the cooling of the body from the outside occurs faster than the heating of the body due to its internal energy.

During the transition to another state of aggregation, the substance remains the same, the same molecules remain, only their mutual arrangement, the speed of movement and the forces of interaction with each other will change.

Those. a change in the internal energy of the particles of the body transfers it from one phase of the state to another. Moreover, this state can be maintained in a large temperature range of the external environment.

When the state of aggregation changes, a certain amount of energy is needed. And in the process of transition, energy is spent not on changing the body temperature, but on changing the internal energy of the body.

Let us display on the graph the dependence of the body temperature T (at constant pressure) on the amount of heat Q supplied to the body during the transition from one state of aggregation to another.

Consider a body with mass m, which is in a solid state with a temperature T 1.

The body does not immediately pass from one state to another. First, energy is needed to change the internal energy, and this takes time. The speed of the transition depends on the body weight and its heat capacity.

Let's start heating the body. Through formulas, you can write like this:

Q \u003d c⋅m⋅ (T 2 -T 1)

So much heat must be absorbed by the body in order to heat up from temperature T 1 to T 2.

Solid to liquid transition

Further, at the critical temperature T 2, which is different for each body, intermolecular bonds begin to collapse and the body passes into another state of aggregation - liquid, i.e. intermolecular bonds weaken, molecules begin to move with a greater amplitude, greater speed and greater kinetic energy. Therefore, the temperature of one and the same body in a liquid state is higher than in a solid one.

In order for the whole body to pass from a solid to a liquid state, it takes time to accumulate internal energy. At this time, all the energy is spent not on heating the body, but on the destruction of old intermolecular bonds and the creation of new ones. The amount of energy needed:

λ - specific heat of melting and crystallization of a substance in J / kg, for each substance its own.

After the whole body has passed into a liquid state, this liquid again begins to heat up according to the formula: Q \u003d c⋅m⋅ (T-T 2); [J].

The transition of a body from a liquid to a gaseous state

When a new critical temperature T 3 is reached, a new process of transition from a liquid to a vapor state begins. To move further from liquid to vapor, you need to expend energy:

r is the specific heat of gas formation and condensation of a substance in J / kg, for each substance its own.

Note that a transition from a solid state to a gaseous state is possible, bypassing the liquid phase. This process is called sublimation, and the reverse process is desublimation.

The transition of a body from a gaseous state to a plasma state

Plasma - partially or completely ionized gas, in which the densities of positive and negative charges are practically the same.

Plasma usually occurs at high temperatures, from several thousand ° C and above. According to the method of formation, two types of plasma are distinguished: thermal, which occurs when the gas is heated to high temperatures, and gaseous, which is formed during electric discharges in a gaseous medium.

This process is very complex and has a simple description, and it is not achievable for us in everyday conditions. Therefore, we will not dwell on this issue in detail.

The most common knowledge about three states of aggregation: liquid, solid, gaseous, sometimes remember about plasma, less often liquid crystal. Recently, a list of 17 phases of a substance, taken from the famous () Stephen Fry, has spread on the Internet. Therefore, we will tell you more about them, because you should know a little more about matter, if only in order to better understand the processes taking place in the Universe.

The list of aggregate states of matter given below increases from the coldest states to the hottest, and so on. can be continued. At the same time, it should be understood that the degree of compression of the substance and its pressure (with some reservations for such unexplored hypothetical states, such as quantum, radial, or weakly symmetric) increase from the gaseous state (No. 11), the most "unclenched", to both sides of the list. a visual graph of the phase transitions of matter is shown.

1. Quantum - the aggregate state of matter, achieved when the temperature drops to absolute zero, as a result of which internal bonds disappear and matter disintegrates into free quarks.

2. Bose-Einstein condensate - the aggregate state of matter, which is based on bosons cooled to temperatures close to absolute zero (less than a millionth of a degree above absolute zero). In such a strongly cooled state, a sufficiently large number of atoms find themselves in their minimum possible quantum states and quantum effects begin to manifest themselves at the macroscopic level. Bose-Einstein condensate (often called "Bose condensate", or simply "back") occurs when you cool a chemical element to extremely low temperatures (usually to a temperature slightly above absolute zero, minus 273 degrees Celsius , Is the theoretical temperature at which everything stops moving).
Here completely strange things begin to happen to the substance. Processes normally seen only at the atomic level now occur on scales large enough to be observed with the naked eye. For example, if you put the “backing” in a beaker and provide the required temperature, the substance will begin to crawl up the wall and eventually will get out by itself.
Apparently, here we are dealing with a futile attempt by the substance to lower its own energy (which is already at the lowest of all possible levels).
Slowing down the atoms using cooling equipment produces a singular quantum state known as a Bose condensate, or Bose-Einstein condensate. This phenomenon was predicted in 1925 by A. Einstein, as a result of a generalization of the work of S. Bose, where statistical mechanics was built for particles ranging from massless photons to atoms with a mass (Einstein's manuscript, which was considered lost, was discovered in the Leiden University library in 2005 ). The result of the efforts of Bose and Einstein was the concept of Bose gas obeying Bose - Einstein statistics, which describes the statistical distribution of identical particles with integer spin, called bosons. Bosons, which are, for example, and individual elementary particles - photons, and whole atoms, can be with each other in the same quantum states. Einstein suggested that cooling the atoms - bosons to very low temperatures would force them to go (or, in other words, condense) into the lowest possible quantum state. The result of such condensation will be the emergence of a new form of matter.
This transition occurs below the critical temperature, which is for a homogeneous three-dimensional gas consisting of non-interacting particles without any internal degrees of freedom.

3. Fermion condensate - the state of aggregation of a substance, similar to the back, but different in structure. When approaching absolute zero, atoms behave differently depending on the magnitude of the proper angular momentum (spin). Bosons have integer spins, while fermions have multiples of 1/2 (1/2, 3/2, 5/2). Fermions obey the Pauli exclusion principle, according to which two fermions cannot have the same quantum state. There is no such prohibition for bosons, and therefore they have the opportunity to exist in one quantum state and thereby form the so-called Bose-Einstein condensate. The formation of this condensate is responsible for the transition to the superconducting state.
Electrons have spin 1/2 and are therefore fermions. They combine into pairs (called Cooper pairs), which then form a Bose condensate.
American scientists have attempted to obtain a kind of molecule from fermion atoms with deep cooling. The difference from real molecules was that there was no chemical bond between the atoms - they just moved together, in a correlated manner. The bond between atoms turned out to be even stronger than between electrons in Cooper pairs. For the resulting pairs of fermions, the total spin is no longer a multiple of 1/2, therefore, they already behave like bosons and can form a Bose condensate with a single quantum state. In the course of the experiment, a gas of potassium-40 atoms was cooled to 300 nanokelvin, while the gas was contained in a so-called optical trap. Then an external magnetic field was imposed, with the help of which it was possible to change the nature of interactions between atoms - instead of a strong repulsion, a strong attraction began to be observed. When analyzing the influence of the magnetic field, it was possible to find such a value at which the atoms began to behave like Cooper pairs of electrons. At the next stage of the experiment, scientists expect to obtain the effects of superconductivity for fermion condensate.

4. Superfluid substance - a state in which a substance has virtually no viscosity, and during flow it does not experience friction with a solid surface. The consequence of this is, for example, such an interesting effect as the complete spontaneous "creeping" of superfluid helium from the vessel along its walls against the force of gravity. Of course, there is no violation of the law of conservation of energy. In the absence of friction forces, only gravity, the forces of interatomic interaction between helium and the walls of the vessel and between helium atoms act on helium. So, the forces of interatomic interaction exceed all other forces combined. As a result, helium tends to spread as much as possible over all possible surfaces, and therefore "travels" along the walls of the vessel. In 1938, the Soviet scientist Pyotr Kapitsa proved that helium can exist in a superfluid state.
It is worth noting that many of the unusual properties of helium have been known for quite some time. However, in recent years this chemical element has been “spoiling” us with interesting and unexpected effects. So, in 2004, Moses Chan and Eun-Siong Kim from the University of Pennsylvania intrigued the scientific world with the statement that they managed to obtain a completely new state of helium - a superfluid solid. In this state, some helium atoms in the crystal lattice can flow around others, and thus helium can flow through itself. The "superhardness" effect was theoretically predicted back in 1969. And then in 2004 - as if an experimental confirmation. However, later and very interesting experiments showed that not everything is so simple, and, perhaps, such an interpretation of the phenomenon, which was previously taken for the superfluidity of solid helium, is incorrect.
The experiment of scientists led by Humphrey Maris from Brown University in the United States was simple and elegant. Scientists placed an upside-down test tube in a closed tank filled with liquid helium. Some of the helium in the test tube and in the reservoir was frozen in such a way that the boundary between liquid and solid inside the test tube was higher than in the reservoir. In other words, in the upper part of the test tube there was liquid helium, in the lower part - solid, it smoothly passed into the solid phase of the reservoir, over which a little liquid helium was poured - lower than the liquid level in the test tube. If liquid helium began to seep through solid, then the level difference would decrease, and then we can talk about solid superfluid helium. And in principle, in three of the 13 experiments, the level difference actually decreased.

5. Superhard substance - the state of aggregation in which matter is transparent and can "flow" like a liquid, but in fact it is devoid of viscosity. Such fluids have been known for many years and are called superfluids. The fact is that if the superfluid is stirred, it will circulate almost forever, while the normal liquid will eventually calm down. The first two superfluids were created by the researchers using helium-4 and helium-3. They were cooled to almost absolute zero - to minus 273 degrees Celsius. And from helium-4, American scientists managed to get a superhard body. They compressed the frozen helium by more than 60 times pressure, and then the glass filled with the substance was placed on a rotating disk. At a temperature of 0.175 degrees Celsius, the disk suddenly began to spin more freely, which, according to scientists, indicates that helium has become a superbody.

6. Solid - the aggregate state of matter, characterized by the stability of the form and the nature of the thermal motion of atoms, which perform small oscillations around the equilibrium positions. The stable state of solids is crystalline. Distinguish between solids with ionic, covalent, metallic, and other types of bonds between atoms, which determines the variety of their physical properties. Electrical and some other properties of solids are mainly determined by the nature of the movement of the outer electrons of its atoms. According to their electrical properties, solids are divided into dielectrics, semiconductors and metals, according to their magnetic properties - into diamagnets, paramagnets, and bodies with an ordered magnetic structure. Studies of the properties of solids have united into a large area - solid state physics, the development of which is stimulated by the needs of technology.

7. Amorphous solid - condensed aggregate state of matter, characterized by isotropy of physical properties due to the disordered arrangement of atoms and molecules. In amorphous solids, atoms vibrate around randomly located points. In contrast to the crystalline state, the transition from solid amorphous to liquid occurs gradually. Various substances are in the amorphous state: glasses, resins, plastics, etc.

8. Liquid crystal Is a specific aggregate state of a substance in which it simultaneously exhibits the properties of a crystal and a liquid. Immediately it is necessary to make a reservation that not all substances can be in a liquid crystal state. However, some organic substances with complex molecules can form a specific aggregate state - liquid crystal. This state occurs when crystals of certain substances melt. When they melt, a liquid crystal phase is formed that differs from ordinary liquids. This phase exists in the range from the melting point of the crystal to some higher temperature, when heated to which the liquid crystal transforms into an ordinary liquid.
How does a liquid crystal differ from a liquid and an ordinary crystal and how is it similar to them? Like an ordinary liquid, a liquid crystal is fluid and takes the form of a vessel in which it is placed. In this it differs from the crystals known to all. However, despite this property, which unites it with a liquid, it has a property characteristic of crystals. This is the ordering in space of the molecules that form the crystal. True, this ordering is not as complete as in ordinary crystals, but, nevertheless, it significantly affects the properties of liquid crystals, which distinguishes them from ordinary liquids. Incomplete spatial ordering of the molecules that form a liquid crystal is manifested in the fact that in liquid crystals there is no complete order in the spatial arrangement of the centers of gravity of molecules, although there may be a partial order. This means that they do not have a rigid crystal lattice. Therefore, liquid crystals, like ordinary liquids, have the property of fluidity.
An obligatory property of liquid crystals that brings them closer to ordinary crystals is the presence of the order of the spatial orientation of molecules. This order in orientation can manifest itself, for example, in the fact that all long axes of molecules in a liquid crystal sample are oriented in the same way. These molecules must be elongated. In addition to the simplest named ordering of the molecular axes, a more complex orientational order of molecules can be realized in a liquid crystal.
Depending on the type of ordering of the molecular axes, liquid crystals are divided into three types: nematic, smectic, and cholesteric.
Research on the physics of liquid crystals and their applications is currently being carried out on a broad front in all the most developed countries of the world. Domestic research is concentrated in both academic and industrial research institutions and has a long tradition. The works of V.K. Fredericks to V.N. Tsvetkova. In recent years, the explosive study of liquid crystals, Russian researchers have also made a significant contribution to the development of the theory of liquid crystals in general and, in particular, of the optics of liquid crystals. Thus, the works of I.G. Chistyakova, A.P. Kapustina, S.A. Brazovsky, S.A. Pikina, L.M. Blinov and many other Soviet researchers are widely known to the scientific community and serve as the foundation for a number of effective technical applications of liquid crystals.
The existence of liquid crystals was established a very long time ago, namely in 1888, that is, almost a century ago. Although scientists faced this state of matter before 1888, it was officially discovered later.
The first to discover liquid crystals was the Austrian botanist Reinitzer. Investigating the new substance he synthesized, cholesteryl benzoate, he found that at a temperature of 145 ° C the crystals of this substance melt, forming a turbid liquid that scatters light strongly. As the heating continues, upon reaching a temperature of 179 ° C, the liquid clears up, that is, it begins to behave optically, like an ordinary liquid, for example water. Cholesteryl benzoate exhibited unexpected properties in a cloudy phase. Examining this phase under a polarizing microscope, Rey-nitzer discovered that it has birefringence. This means that the refractive index of light, that is, the speed of light in this phase, depends on the polarization.

9. Liquid - the aggregate state of a substance, combining the features of a solid state (retention of volume, a certain tensile strength) and gaseous (variability of shape). A liquid is characterized by short-range order in the arrangement of particles (molecules, atoms) and a small difference in the kinetic energy of the thermal motion of molecules and their potential interaction energy. The thermal motion of liquid molecules consists of oscillations about equilibrium positions and relatively rare jumps from one equilibrium position to another, which is associated with the fluidity of the liquid.

10. Supercritical fluid (GFR) - the state of aggregation of a substance in which the difference between the liquid and gas phases disappears. Any substance at a temperature and pressure above the critical point is a supercritical fluid. The properties of a substance in a supercritical state are intermediate between its properties in the gas and liquid phases. So, SCF has a high density, close to a liquid, and a low viscosity, like gases. In this case, the diffusion coefficient has an intermediate value between liquid and gas. Supercritical substances can be used as substitutes for organic solvents in laboratory and industrial processes. Supercritical water and supercritical carbon dioxide have received the greatest interest and distribution in connection with certain properties.
One of the most important properties of the supercritical state is the ability to dissolve substances. By changing the temperature or pressure of the fluid, you can change its properties in a wide range. So, you can get a fluid that is close in properties to either a liquid or a gas. Thus, the dissolving power of the fluid increases with increasing density (at constant temperature). Since the density increases with increasing pressure, changing the pressure can affect the dissolving ability of the fluid (at constant temperature). In the case of temperature, the envy of the properties of the fluid is somewhat more complicated - at a constant density, the dissolving ability of the fluid also increases, however, near the critical point, a slight increase in temperature can lead to a sharp drop in density, and, accordingly, in the dissolving ability. Supercritical fluids mix indefinitely with each other, therefore, when the critical point of the mixture is reached, the system will always be single-phase. The approximate critical temperature of a binary mixture can be calculated as the arithmetic mean of the critical parameters of substances Tc (mix) \u003d (mole fraction A) x TcA + (mole fraction B) x TcB.

11. Gaseous - (French gaz, from the Greek chaos - chaos), the state of aggregation of matter, in which the kinetic energy of the thermal motion of its particles (molecules, atoms, ions) significantly exceeds the potential energy of interactions between them, and therefore the particles move freely, evenly filling in the absence of external fields the entire volume provided to them.

12. Plasma - (from the Greek. Plasma - sculpted, shaped), the state of matter, which is an ionized gas, in which the concentrations of positive and negative charges are equal (quasineutrality). The vast majority of the substance of the Universe is in the state of plasma: stars, galactic nebulae and the interstellar medium. Plasma exists near the Earth in the form of the solar wind, magnetosphere and ionosphere. High-temperature plasma (T ~ 106 - 108K) from a mixture of deuterium and tritium is being investigated for the purpose of controlled thermonuclear fusion. Low-temperature plasma (T Ј 105K) is used in various gas-discharge devices (gas lasers, ion devices, MHD generators, plasmatrons, plasma engines, etc.), as well as in technology (see Plasma metallurgy, Plasma drilling, Plasma technology) ...

13. Degenerate substance - is an intermediate stage between plasma and neutronium. It is observed in white dwarfs and plays an important role in the evolution of stars. When atoms are under extremely high temperatures and pressures, they lose their electrons (they go into electron gas). In other words, they are completely ionized (plasma). The pressure of such a gas (plasma) is determined by the pressure of the electrons. If the density is very high, all particles are forced to approach each other. Electrons can be in states with certain energies, and two electrons cannot have the same energy (unless their spins are opposite). Thus, in a dense gas, all the lower energy levels are filled with electrons. This gas is called degenerate. In this state, electrons exert degenerate electron pressure, which opposes the forces of gravity.

14. Neutronium - the state of aggregation, into which matter passes under ultrahigh pressure, which is unattainable in the laboratory, but exists inside neutron stars. During the transition to the neutron state, the electrons of the substance interact with protons and turn into neutrons. As a result, the substance in the neutron state consists entirely of neutrons and has a density of the order of the nuclear one. In this case, the temperature of the substance should not be too high (in energy equivalent, no more than a hundred MeV).
With a strong increase in temperature (hundreds of MeV and above), various mesons begin to be produced and annihilated in the neutron state. With a further increase in temperature, deconfinement occurs, and the substance passes into the state of a quark-gluon plasma. It no longer consists of hadrons, but of quarks and gluons constantly being born and disappearing.

15. Quark-gluon plasma (chromoplasm) - the aggregate state of matter in high-energy physics and elementary particle physics, in which hadronic matter passes into a state similar to the state in which electrons and ions are found in ordinary plasma.
Usually matter in hadrons is in the so-called colorless ("white") state. That is, quarks of different colors cancel each other out. Ordinary matter has a similar state - when all atoms are electrically neutral, that is,
positive charges in them are compensated by negative ones. At high temperatures, ionization of atoms can occur, while the charges are separated, and the substance becomes, as they say, "quasineutral". That is, the entire cloud of matter as a whole remains neutral, and its individual particles cease to be neutral. Exactly the same, apparently, can happen with hadronic matter - at very high energies, color is released and makes matter "quasi-colorless".
Presumably, the substance of the Universe was in the state of a quark-gluon plasma in the first moments after the Big Bang. Now quark-gluon plasma can be formed for a short time in collisions of particles of very high energies.
Quark-gluon plasma was obtained experimentally at the RHIC accelerator at Brookhaven National Laboratory in 2005. The maximum plasma temperature of 4 trillion degrees Celsius was obtained there in February 2010.

16. Strange substance - the state of aggregation, in which matter is compressed to the limit values \u200b\u200bof density, it can exist in the form of a "quark soup". A cubic centimeter of matter in this state will weigh billions of tons; besides, it will transform any normal substance with which it comes in contact into the same "strange" form with the release of a significant amount of energy.
The energy that can be released during the transformation of the matter of the star's core into "strange matter" will lead to a super-powerful explosion of the "quark nova" - and, according to Leahy and Wyed, it was his astronomers who observed in September 2006.
The process of formation of this substance began with an ordinary supernova, into which a massive star turned. As a result of the first explosion, a neutron star was formed. But, according to Leahy and Uyed, it did not last long - as its rotation seemed to be slowed down by its own magnetic field, it began to contract even more, with the formation of a clot of "strange matter", which led to an even more powerful than in an ordinary supernova explosion, the release of energy - and the outer layers of the substance of the former neutron star, scattering into the surrounding space at a speed close to the speed of light.

17. Strongly symmetrical substance Is a substance compressed to such an extent that the microparticles inside it are layered on top of each other, and the body itself collapses into a black hole. The term "symmetry" is explained by the following: Let's take the aggregate states of matter known to everyone from school - solid, liquid, gaseous. For definiteness, consider an ideal infinite crystal as a solid. It has a certain, so-called discrete symmetry with respect to transfer. This means that if you move the crystal lattice by a distance equal to the interval between two atoms, nothing will change in it - the crystal will coincide with itself. If the crystal is melted, then the symmetry of the resulting liquid will be different: it will increase. In the crystal, only points were equivalent, which were distant from each other at certain distances, the so-called nodes of the crystal lattice, in which there were identical atoms.
The liquid is homogeneous throughout the volume, all its points are indistinguishable from one another. This means that the liquid can be displaced at any arbitrary distance (and not only at some discrete, as in a crystal) or rotated at any arbitrary angles (which cannot be done in crystals at all) and it will coincide with itself. The degree of its symmetry is higher. The gas is even more symmetrical: the liquid occupies a certain volume in the vessel and asymmetry is observed inside the vessel, where there is liquid, and points where it is not. Gas occupies the entire volume provided to it, and in this sense, all its points are indistinguishable from one another. Yet here it would be more correct to speak not about points, but about small, but macroscopic elements, because there are still differences at the microscopic level. At some points at a given time there are atoms or molecules, while others do not. Symmetry is observed only on average, either over some macroscopic volume parameters, or over time.
But there is still no instantaneous symmetry at the microscopic level. If the substance is compressed very strongly, up to pressures that are unacceptable in everyday life, compress so that the atoms were crushed, their shells penetrated each other, and the nuclei began to touch, symmetry arises at the microscopic level. All nuclei are the same and are pressed against each other, not only interatomic, but also internuclear distances are absent and the substance becomes homogeneous (strange substance).
But there is also a submicroscopic level. Nuclei are made up of protons and neutrons that move inside the nucleus. There is also some space between them. If you continue to squeeze so that the nuclei are also crushed, the nucleons will tightly press against each other. Then, at the submicroscopic level, symmetry will appear, which is not even inside ordinary nuclei.
From what has been said, one can see a quite definite tendency: the higher the temperature and the higher the pressure, the more symmetrical the substance becomes. Based on these considerations, the substance compressed to the maximum is called strongly symmetric.

18. Weakly symmetric substance - a state opposite to a strongly symmetric substance in its properties, which was present in a very early Universe at a temperature close to the Planck temperature, perhaps 10-12 seconds after the Big Bang, when strong, weak and electromagnetic forces were a single superpower. In this state, matter is compressed to such an extent that its mass turns into energy, which begins to influence, that is, to expand indefinitely. It is not yet possible to achieve energies for the experimental obtaining of superpower and transfer of matter into this phase under terrestrial conditions, although such attempts were made at the Large Hadron Collider in order to study the early universe. Due to the absence of gravitational interaction in the composition of the super-force that forms this substance, the super-force is not sufficiently symmetric in comparison with the supersymmetric force, which contains all 4 types of interactions. Therefore, this state of aggregation has received such a name.

19. Beam matter - this, in fact, is not a substance at all, but energy in its pure form. However, it is this hypothetical state of aggregation that a body that has reached the speed of light will assume. It can also be obtained by heating the body to the Planck temperature (1032K), that is, by accelerating the molecules of the substance to the speed of light. As follows from the theory of relativity, upon reaching a speed of more than 0.99 s, the body's mass begins to grow much faster than during "normal" acceleration, in addition, the body lengthens, heats up, that is, begins to radiate in the infrared spectrum. Upon crossing the threshold of 0.999 s, the body changes dramatically and begins a rapid phase transition up to the ray state. As follows from Einstein's formula, taken in full form, the growing mass of the final substance consists of masses that are separated from the body in the form of thermal, X-ray, optical and other radiation, the energy of each of which is described by the next term in the formula. Thus, a body approaching the speed of light will begin to emit in all spectra, grow in length and slow down in time, thinning to the Planck length, that is, upon reaching speed c, the body will turn into an infinitely long and thin ray moving at the speed of light and consisting photons that have no length, and its infinite mass is completely converted into energy. Therefore, such a substance is called ray.

Bodies can be liquid, solid or gaseous depending on conditions. These conditions are called aggregate states of matter .

In gases, the distance between molecules is much greater than the size of molecules. If the walls of the vessel do not interfere with the gas, its molecules scatter.

In liquids and solids, molecules are located closer to each other and therefore cannot move far away from each other.

The transition from one state of aggregation to another is called phase transition .

The transition of a substance from a solid state to a liquid is called melting , and the temperature at which this occurs is melting point ... The transition of a substance from a liquid to a solid state is called crystallization , and the transition temperature is crystallization temperature .

The amount of heat that is released during the crystallization of a body or is absorbed by the body during melting, per unit mass of the body, is called specific heat of fusion (crystallization) λ:

Crystallization produces the same amount of heat that is absorbed during melting.

The transition of a substance from a liquid to a gaseous state is called vaporization ... The transition of a substance from a gaseous state to a liquid is called condensation ... The amount of heat required for vaporization (released during condensation):

Q \u003d Lm,

where L - specific heat of vaporization (condensation).

Vaporization occurring from the surface of a liquid is called evaporation ... Evaporation can occur at any temperature. The transition of liquid into vapor, occurring throughout the body, is called boiling , and the temperature at which the liquid boils is boiling point .

Finally, sublimation Is the transition of a substance from a solid state directly to a gaseous state, bypassing the liquid stage.

If other parameters of the external environment (in particular, pressure) remain constant, then the body temperature during melting (crystallization) and boiling does not change.

If the number of molecules leaving the liquid is equal to the number of molecules returning to the liquid, then it is said that a dynamic equilibrium has come between the liquid and its vapor. Steam in dynamic equilibrium with its liquid is called

The establishment of an ideal order in the arrangement of atoms, that is, the formation of a solid, is hindered by thermal movements, the main feature of which is, as we know, chaos and disorder. Therefore, in order for a substance to be in a solid state, its temperature must be low enough - so low that the energy of thermal movements is less than the potential energy of interaction of atoms.

A completely ideal crystal, in which all atoms are in equilibrium and have minimum energy, a body can be only at absolute zero. In fact, all substances become solid at much higher temperatures. The only exception is helium, which remains liquid even at absolute zero, but this is due to some quantum effects, which we will briefly discuss below.

A substance can go into a solid state both from a liquid and from a gaseous state. In both cases, such a transition is a transition from a state devoid of symmetry to a state in which symmetry exists (this in any case refers to the long-range order that exists in crystals, but does not exist in either liquid or gaseous substances) ... Therefore, the transition to the solid state must occur abruptly, that is, at a certain temperature, in contrast to the transition from gas to liquid, which, as we know, can also occur in a continuous manner.

Let us first consider the liquid-solid transformation. The process of formation of a solid during cooling of a liquid is the process of crystal formation (crystallization), (and it occurs at a certain temperature of crystallization or solidification. Since during such a transformation the energy decreases, it is accompanied by the release of energy in the form of latent heat of crystallization. - also occurs abruptly at the same temperature and is accompanied by the absorption of energy in the form

that heat of fusion, equal in magnitude to the heat of crystallization.

This is clearly seen from the graph of the dependence of the temperature of the cooling liquid on time, shown in Fig. 179 (curve a). Section 1 of curve a gives the course of a monotonic decrease in the temperature of the liquid due to heat removal from it. Horizontal section 2 shows that at a certain temperature, its decrease stops, despite the fact that heat removal continues. After a while, the temperature starts to drop again (section 3). The temperature corresponding to section 2 is the crystallization temperature. The heat released during crystallization compensates for the heat removal from the substance and therefore the temperature decrease is temporarily stopped. After the end of the crystallization process, the temperature of the now solid body begins to decrease again.

Such a course of the graph of temperature decrease is typical for crystalline bodies. When cooling liquids that do not crystallize (amorphous substances), no latent heat is released and the cooling graph is a monotonic curve without stopping the cooling.

In the reverse process of the transition of a substance from a solid to a liquid state (melting) on \u200b\u200bthe heating curve, there is also a stop in the increase in temperature due to the absorption of the latent heat of melting - heat, due to which the destruction of the crystal lattice occurs (curve in Fig. 179).

To start crystallization, the presence of a center or centers of crystallization is necessary. Such centers could serve as random accumulations of liquid particles adhering to each other, to which more and more particles could join until all the liquid turns into a solid. However, the formation of such accumulations in the liquid itself is hampered by thermal movements, which destroy them even before they have time to acquire any noticeable size. Crystallization is greatly facilitated if, from the very beginning, sufficiently large solid particles in the form of dust grains and bodies are present in the liquid, which become centers of crystallization.

The formation of crystallization centers in the liquid itself is facilitated, of course, with decreasing temperature. Therefore, crystallization of a pure liquid devoid of extraneous formations,

usually starts at a temperature slightly lower than the true crystallization temperature. Under normal conditions, there are many crystallization centers in a crystallizing liquid, so that many crystals are formed in the liquid, growing together, and the solidified substance is polycrystalline.

Only under special conditions, which are usually difficult to achieve, it is possible to obtain a single crystal - a single crystal, growing from a single crystallization center. If in this case the same conditions for the accumulation of particles are provided for all directions, then the crystal is obtained correctly faceted according to its symmetry properties.

The liquid - solid transition, as well as the reverse transformation, is a phase transition, since the liquid and solid states can be considered as two phases of a substance. At the crystallization (melting) temperature, both phases can come into contact with each other, being in equilibrium (ice, for example, can float in water without melting), just as liquid and its saturated vapor can be in equilibrium.

Just as the boiling point depends on pressure, the crystallization temperature (and its equivalent melting point) also depends on pressure, usually increasing with pressure. It grows because the external pressure brings the atoms closer to each other, and for the destruction of the crystal lattice during melting, the atoms must be separated from each other: at higher pressure, this requires more energy of thermal movements, i.e., a higher temperature.

In fig. 180 shows a plot of melting (crystallization) temperature versus pressure. A solid curve divides the entire area in two. The area to the left of the curve corresponds to the solid state, and the area to the right of the curve corresponds to the liquid state. Any point lying on the melting curve itself corresponds to the equilibrium of the solid and liquid phases: at these pressures and temperatures, the substance in the liquid and solid states is in equilibrium, in contact with each other, and the liquid does not solidify, and the solid does not melt.

The dotted line in Fig. 180 shows the melting curve for those few substances (bismuth, antimony, ice, germanium), in which the volume does not decrease, but increases during solidification. Such

substances, naturally, the melting point decreases with increasing pressure.

A change in the melting point is associated with a change in pressure by the Clapeyron - Clausius ratio:

Here is the melting (crystallization) temperature, and are the molar volumes of the liquid and solid phases and the molar heat of fusion, respectively.

This formula is valid for other phase transitions as well. In particular, for the case of evaporation and condensation, the Clapeyron-Clausius formula was derived in Ch. VII [see. (105.6)].

From the Clapeyron - Clausius formula, it can be seen that the sign of the change in the melting temperature with a change in pressure is determined by which of the two quantities is or more. The slope of the curve also depends on the value of the latent heat of transition, the less the less the melting point changes with pressure. Table 20 shows the values \u200b\u200bof the specific (i.e., per unit mass) heats of fusion for some substances.

Table 20 (see scan) Specific heat of fusion for some substances

The Clapeyron - Clausius equation can be written in the following form:

This equation shows how the pressure under which both equilibrium phases are located changes with temperature.

A solid can be formed not only by crystallization of a liquid, but also by condensation of a gas (vapor) into a crystal, bypassing the liquid phase. In this case, the latent heat of transition is also released, which, however, is always higher than the latent heat of fusion. After all, the formation of a solid at a certain temperature and pressure can occur both directly from the gaseous state, and by preliminary liquefaction, In both

cases, the initial and final states are the same. It means that the difference in the energies of these states is the same. Meanwhile, in the second case, firstly, the latent heat of condensation is released during the transition from the gaseous to the liquid state and, secondly, the latent heat of crystallization during the transition from the liquid to the solid state. It follows from this that the latent heat in the direct formation of a solid from the gaseous phase should be equal to the sum of the heat of condensation and crystallization from the liquid. This only applies to heats measured at the melting point. At lower temperatures, the heat of condensation from the gas increases.

The reverse process of evaporation of a solid is usually called sublimation or sublimation. The evaporating particles of a solid form vapor above it in exactly the same way as it happens when a liquid evaporates. At certain pressures and temperatures, steam and solids can be in equilibrium. Steam in equilibrium with a solid is also called saturated steam. As in the case of a liquid, the elasticity of saturated vapor over a solid body depends on temperature, rapidly decreasing with decreasing temperature, so that for many solids at ordinary temperatures the elasticity of saturated vapor is negligible.

In fig. 181 shows a plot of saturated vapor pressure versus temperature. This curve is the line of equilibrium between the solid and gaseous phases. The region to the left of the curve corresponds to the solid state, to the right of it to the gaseous state. Sublimation, like melting, is associated with the destruction of the lattice and requires the expenditure of the necessary energy for this. This energy manifests itself as the latent heat of sublimation (sublimation), equal, of course, to the latent heat of condensation. The heat of sublimation is therefore equal to the sum of the heats of fusion and vaporization.

In the world around us, a huge variety of different physical phenomena and processes are constantly and continuously occurring. One of the most important can be considered the evaporation process. There are several prerequisites for this phenomenon. In this article, we will break down each of them in more detail.

It is the process of converting substances into a gaseous or vapor state. It is characteristic only of the consistency. However, something similar is observed in solids, only this phenomenon is called sublimation. This can be seen by careful observation of the bodies. For example, a bar of soap dries out over time and begins to crack, this is due to the fact that water droplets in its composition evaporate and pass into the gaseous state of H 2 O.

Definition in physics

Evaporation is an endothermic process in which heat is the source of absorbed energy.It includes two components:

• certain necessary to overcome the molecular forces of attraction when there is a gap between the connected molecules;
• the heat required by the expansion of molecules in the process of converting liquid substances into steam or gas.

How does this happen?

The transition of a substance from a liquid to a gaseous state can occur in two ways:

1. Evaporation is a process in which molecules volatilize from the surface of a liquid substance.
2. Boiling is the process of vaporization from a liquid by bringing the temperature to the specific boiling point of a substance.

Despite the fact that both of these phenomena convert liquid matter into gas, there are significant differences between them. Boiling is an active process that takes place only at a certain temperature, while evaporation occurs under any conditions. Another difference is that boiling is characteristic of the entire thickness of the liquid, while the second phenomenon occurs only on the surface of liquid substances.

Molecular kinetic theory of evaporation

If we consider this process at the molecular level, then it occurs as follows:

1. Molecules in liquid substances are in constant chaotic motion, they all have completely different speeds. Meanwhile, the particles are attracted to each other due to the forces of attraction. Each time they collide with each other, their speeds change. At some point, some develop a very high speed, which allows them to overcome the forces of gravity.
2. These elements, which are on the surface of the liquid, have such kinetic energy that they are able to overcome intermolecular bonds and leave the liquid.
3. It is these fastest molecules that fly out from the surface of a liquid substance, and this process occurs constantly and continuously.
4. Once in the air, they turn into steam - this is called vaporization.
5. As a consequence, there are fewer and fewer remaining particles. This explains the cooling of the liquid. Remember how in childhood we were taught to blow on hot liquid so that it cools down quickly. It turns out that we accelerated the process and the temperature drop occurred much faster.

What factors does it depend on?

There are many conditions required for this process to occur. It comes from everywhere where water particles are present: these are lakes, seas, rivers, all wet objects, the covers of the bodies of animals and people, as well as plant leaves. It can be concluded that evaporation is a very significant and irreplaceable process for the surrounding world and all living beings.

Here are the factors that influence this phenomenon:

1. The evaporation rate directly depends on the composition of the liquid itself. It is known that each of them has its own characteristics. For example, those substances with a lower heat of vaporization will transform faster. Let's compare two processes: evaporation of alcohol and ordinary water. In the first case, the transformation into a gaseous state occurs faster, because the specific heat of vaporization and condensation for alcohol is 837 kJ / kg, and for water it is almost three times higher - 2260 kJ / kg.
2. The speed also depends on the initial temperature of the liquid: the higher it is, the faster steam is generated. As an example, take a glass of water, when there is boiling water inside the vessel, then vaporization occurs at a much higher rate than when the water temperature is lower.
3. Another factor that determines the rate of this process is the surface area of \u200b\u200bthe liquid. Remember that hot soup cools faster in a large bowl than in a small saucer.
4. The rate of distribution of substances in the air largely determines the rate of evaporation, that is, the faster diffusion occurs, the sooner vaporization occurs. For example, in strong winds, water droplets evaporate faster from the surface of lakes, rivers and reservoirs.
5. Indoor air temperature also plays an important role. We will talk about this in more detail below.

What is the role of air humidity?

Due to the fact that the evaporation process occurs from everywhere continuously and constantly, water particles are always present in the air. In molecular form, they look like a group of elements H 2 O. Liquids can evaporate depending on the volume of water vapor in the atmosphere, this coefficient is called air humidity. It is of two types:

1. Relative humidity is the ratio of the amount of water vapor in the air to the density of saturated vapor at the same temperature as a percentage. For example, an indicator of 100% indicates that the atmosphere is completely saturated with H 2 O molecules.
2. The absolute one characterizes the density of water vapor in the air, denoted by the letter f and shows what mass of water molecules is contained in 1m 3 of air.

The relationship between the evaporation process and air humidity can be determined as follows. The lower the indicator, the faster the evaporation from the surface of the earth and other objects will occur.

Evaporation of various substances

For different substances, this process proceeds in different ways. For example, alcohol evaporates faster than many liquids due to its low specific heat of vaporization. Often, such liquid substances are called volatile, because water vapor literally evaporates from them at almost any temperature.

Alcohol can also evaporate even at room temperature. In the process of preparing wine or vodka, alcohol is driven through the moonshine, only after reaching the boiling point, it is approximately equal to 78 degrees. However, the real evaporation temperature of alcohol will be slightly higher, because in the initial product (for example, mash), it is a compound with various aromatic oils and water.

Condensation and sublimation

The following phenomenon can be observed every time the water in the kettle boils. Note that when boiling, water changes from a liquid to a gaseous state. It happens in this way: a hot jet of water vapor at high speed flies out of the kettle through its spout. In this case, the formed steam is not visible directly at the exit from the spout, but at a short distance from it. This process is called condensation, that is, water vapor thickens to such an extent that it becomes visible to our eyes.

The evaporation of a solid is called sublimation. At the same time, they pass from an aggregate state to a gaseous state, bypassing the liquid stage. The most famous case of sublimation is associated with ice crystals. In its original form, ice is a solid, at temperatures above 0 ° it begins to melt, taking on a liquid state. However, in some cases, at negative temperatures, ice turns into a vaporous form, bypassing the liquid phase.

Effect of evaporation on the human body

Thanks to evaporation, thermoregulation occurs in our body. This process takes place through a self-cooling system. On a hot sultry day, a person who is engaged in certain physical labor becomes very hot. This means that internal energy increases in it. And as you know, at temperatures above 42 °, the protein in the human blood begins to coagulate, if this process is not stopped in time, it will lead to death.

The self-cooling system is designed just in such a way as to regulate the temperature for normal life. When the temperature becomes the maximum permissible, active perspiration begins through the pores on the skin. And then evaporation occurs from the skin surface, which absorbs excess body energy. In other words, evaporation is a process that helps to cool the body to normal.