Transition from liquid to solid formula. What is the aggregate state of matter. What factors does it depend on

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

For 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.

When moving to another state of aggregation the substance remains the same, the same molecules will remain, only their mutual arrangement, speed of movement and 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. At the same time, this state can be maintained in a wide temperature range. external environment.

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

Let us display on the graph the dependence of 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 of mass m, which is in a solid state with a temperature T1.

The body does not go instantly from one state to another. First, energy is needed to change the internal energy, and this takes time. The rate of transition depends on the mass of the body and its heat capacity.

Let's start heating the body. Formulas can be written like this:

Q = c⋅m⋅(T 2 -T 1)

This is how much heat the body must absorb in order to warm up from temperature T 1 to T 2 .

The transition of a solid to a liquid

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

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 goes not to heat the body, but to destroy old intermolecular bonds and create new ones. The amount of energy you need:

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 = c⋅m⋅(T-T 2); [J].

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

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

r - specific heat of gas formation and condensation of a substance in J / kg, each substance has its own.

Note that the transition from the solid state to the gaseous state is possible, bypassing the liquid phase. Such a 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 fully ionized gas, in which the density of positive and negative charges is almost 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 a gas is heated to high temperatures, and gaseous, which forms during electrical discharges in a gaseous medium.

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

The most widespread knowledge is about three states of aggregation: liquid, solid, gaseous, sometimes they think about plasma, less often liquid crystal. Recently, a list of 17 phases of matter, taken from the famous () Stephen Fry, has spread on the Internet. Therefore, we will talk about them in more detail, because. one 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. may be continued. At the same time, it should be understood that from the gaseous state (No. 11), the most “expanded”, on both sides of the list, the degree of compression of matter and its pressure (with some reservations for such unexplored hypothetical states as quantum, ray, or weakly symmetric) increase. After the text a visual graph of the phase transitions of matter is given.

1. Quantum- the state of aggregation of matter, achieved when the temperature drops to absolute zero, as a result of which internal bonds disappear and matter crumbles 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. A Bose-Einstein condensate (often referred to as a "Bose condensate" or simply "back") occurs when you cool a chemical element to extremely low temperatures (usually just above absolute zero, minus 273 degrees Celsius). , is the theoretical temperature at which everything stops moving).
This is where strange things start to happen. Processes normally only observable at the atomic level now occur on scales large enough to be observed with the naked eye. For example, if you place "back" in a beaker and provide the desired temperature regime, the substance will begin to crawl up the wall and eventually get out on its own.
Apparently, here we are dealing with a futile attempt by matter to lower its own energy (which is already at the lowest of all possible levels).
Slowing down atoms using cooling equipment produces a singular quantum state known as a Bose condensate, or Bose-Einstein. 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 mass (Einstein's manuscript, which was considered lost, was found in the library of Leiden University in 2005 ). The result of the efforts of Bose and Einstein was the Bose concept of a gas, which obeys Bose-Einstein statistics, which describes the statistical distribution of identical particles with integer spin, called bosons. Bosons, which are, for example, both individual elementary particles - photons, and whole atoms, can be with each other in the same quantum states. Einstein suggested that cooling atoms - bosons to very low temperatures, would cause them to go (or, in other words, condense) into the lowest possible quantum state. This condensation will result in new form substances.
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. Fermionic condensate- the state of aggregation of a substance, similar to the backing, but differing in structure. When approaching absolute zero, atoms behave differently depending on the magnitude of their own angular momentum (spin). Bosons have integer spins, while fermions have spins that are multiples of 1/2 (1/2, 3/2, 5/2). Fermions obey the Pauli exclusion principle, which states that two fermions cannot have the same quantum state. For bosons, there is no such prohibition, and therefore they have the opportunity to exist in one quantum state and thereby form the so-called Bose-Einstein condensate. The process of 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 (so-called Cooper pairs), which then form a Bose condensate.
American scientists attempted to obtain a kind of molecule from fermion atoms by 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 pairs of fermions formed, 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. During the experiment, a gas of potassium-40 atoms was cooled down to 300 nanokelvins, while the gas was enclosed in a so-called optical trap. Then an external magnetic field was applied, with the help of which it was possible to change the nature of interactions between atoms - instead of strong repulsion, 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 propose to obtain the effects of superconductivity for the fermionic condensate.

4. Superfluid matter- a state in which the substance actually has no viscosity, and during the 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 out" of superfluid helium from the vessel along its walls against gravity. Of course, there is no violation of the law of conservation of energy here. In the absence of friction forces, only gravity forces act on helium, forces of interatomic interaction between helium and the walls of the vessel and between helium atoms. 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 last years this chemical element “spoils” us with interesting and unexpected effects. So, in 2004, Moses Chan and Eun-Syong Kim from the University of Pennsylvania were intrigued by scientific world claiming that they had succeeded in obtaining a completely new state of helium - a superfluid solid. In this state, some helium atoms in the crystal lattice can flow around others, and helium can thus flow through itself. The effect of "superhardness" was theoretically predicted back in 1969. And in 2004 - as if experimental confirmation. However, later and very curious experiments showed that everything is not 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 USA was simple and elegant. The scientists placed a test tube turned upside down into a closed tank of liquid helium. Part of the helium in the test tube and in the tank was frozen in such a way that the boundary between liquid and solid inside the test tube was higher than in the tank. In other words, there was liquid helium in the upper part of the test tube, and solid helium in the lower part; it smoothly passed into the solid phase of the tank, 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 speak of solid superfluid helium. And in principle, in three out of 13 experiments, the level difference did decrease.

5. Superhard matter- a state of aggregation in which matter is transparent and can "flow" like a liquid, but in fact it is devoid of viscosity. Such liquids 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 researchers using helium-4 and helium-3. They were cooled almost to 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 pressure more than 60 times, and then the glass filled with the substance was installed on a rotating disk. At a temperature of 0.175 degrees Celsius, the disk suddenly began to rotate more freely, which, according to scientists, indicates that helium has become a superbody.

6. Solid- the state of aggregation of matter, characterized by the stability of the form and the nature of the thermal motion of atoms, which make small vibrations around the equilibrium positions. The stable state of solids is crystalline. Distinguish solids with ionic, covalent, metallic, and other types of bonds between atoms, which determines the variety of their physical properties. The electrical and some other properties of solids are mainly determined by the nature of the motion 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, they are divided into diamagnets, paramagnets, and bodies with an ordered magnetic structure. The investigations of the properties of solids have united into a large field—solid-state physics, the development of which is being stimulated by the needs of technology.

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

8. Liquid crystal- this is a specific state of aggregation of a substance in which it simultaneously exhibits the properties of a crystal and a liquid. We must immediately make a reservation that not all substances can be in the liquid crystal state. However, some organic matter, which have complex molecules, can form a specific state of aggregation - liquid crystal. This state is carried out during the melting of crystals of certain substances. When they melt, a liquid-crystalline phase is formed, which differs from ordinary liquids. This phase exists in the range from the melting temperature 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 has fluidity 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. The incomplete spatial ordering of the molecules that form a liquid crystal manifests itself in the fact that in liquid crystals there is no complete order in the spatial arrangement of the centers of gravity of the 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, which brings them closer to ordinary crystals, is the presence of an order in the spatial orientation of molecules. Such an 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 should have an elongated shape. In addition to the simplest named ordering of the axes of molecules, 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 wide front in all the most developed countries of the world. Domestic research is concentrated both in academic and industrial research institutions and has a long tradition. The works of V.K. Frederiks to V.N. Tsvetkov. In recent years, the rapid study of liquid crystals, Russian researchers also make a significant contribution to the development of the theory of liquid crystals in general and, in particular, the optics of liquid crystals. So, 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 had encountered 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 cholesteryl benzoate synthesized by him, he found that at a temperature of 145 ° C, the crystals of this substance melt, forming a cloudy liquid that strongly scatters light. With continued heating, upon reaching a temperature of 179 ° C, the liquid becomes clear, that is, it begins to behave optically like an ordinary liquid, such as water. Cholesteryl benzoate showed unexpected properties in the turbid phase. Examining this phase under a polarizing microscope, Reinitzer found 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 state of aggregation of a substance, combining the features of a solid state (conservation of volume, a certain tensile strength) and a gaseous state (shape variability). A liquid is characterized by a 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 energy of interaction. The thermal motion of liquid molecules consists of oscillations around 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) is 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 the supercritical state are intermediate between its properties in the gas and liquid phases. So, SCF has high density, close to a liquid, and low viscosity, like gases. The diffusion coefficient in this case has an intermediate value between liquid and gas. Substances in the supercritical state 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, one can change its properties in a wide range. Thus, it is possible to obtain a fluid whose properties are close to either a liquid or a gas. Thus, the dissolving power of a fluid increases with increasing density (at a constant temperature). Since the density increases with increasing pressure, changing the pressure can affect the dissolving power of the fluid (at a constant temperature). In the case of temperature, the dependence of fluid properties is somewhat more complicated - at a constant density, the dissolving power 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, dissolving power. Supercritical fluids mix with each other indefinitely, so 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 the substances Tc(mix) = (mole fraction of A) x TcA + (mole fraction of B) x TcB.

11. Gaseous- (French gaz, from Greek chaos - chaos), the aggregate state 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, uniformly filling in the absence of external fields, the entire volume provided to them.

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

13. Degenerate matter- 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 conditions of extremely high temperatures and pressures, they lose their electrons (they go into an electron gas). In other words, they are completely ionized (plasma). The pressure of such a gas (plasma) is determined by the electron pressure. 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 lower energy levels turn out to be filled with electrons. Such a gas is called degenerate. In this state, the electrons exhibit a degenerate electron pressure that opposes the forces of gravity.

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

15. Quark-gluon plasma(chromoplasm) - aggregate state of matter in high energy physics and physics elementary particles, in which the hadronic matter passes into a state similar to the state in which electrons and ions are found in ordinary plasma.
Usually the matter in hadrons is in the so-called colorless ("white") state. That is, quarks of different colors compensate each other. A similar state exists in ordinary matter - 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, "quasi-neutral". That is, the entire cloud of matter as a whole remains neutral, and its individual particles cease to be neutral. Presumably, the same thing can happen with hadronic matter - at very high energies, color is released and makes the substance "quasi-colorless".
Presumably, the matter of the Universe was in the state of 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- state of aggregation, in which matter is compressed to the limit values ​​of density, it can exist in the form of "quark soup". A cubic centimeter of matter in this state would weigh billions of tons; besides, it will turn any normal substance with which it comes into 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 substance of the core of a star into a "strange substance" will lead to a super-powerful explosion of a "quark nova" - and, according to Leahy and Wyed, it was precisely this explosion that astronomers 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 Wyed, it did not last long - as its rotation seemed to be slowed down by its own magnetic field, it began to shrink even more, with the formation of a clot of "strange stuff", which led to an even more powerful than in a normal supernova explosion, the release of energy - and the outer layers of the substance of the former neutron star, flying into the surrounding space at a speed close to the speed of light.

17. Strongly symmetrical matter- this 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 black hole. The term "symmetry" is explained as follows: Let's take the aggregate states of matter known to everyone from the school bench - solid, liquid, gaseous. For definiteness, consider an ideal infinite crystal as a solid. It has a certain, so-called discrete symmetry with respect to translation. This means that if the crystal lattice is shifted 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 a crystal, only points that were distant from each other at certain distances, the so-called nodes of the crystal lattice, in which identical atoms were located, were equivalent.
The liquid is homogeneous throughout its volume, all its points are indistinguishable from one another. This means that liquids can be displaced by any arbitrary distances (and not just some discrete ones, as in a crystal) or rotated by any arbitrary angles (which cannot be done in crystals at all) and it will coincide with itself. Its degree of symmetry is higher. The gas is even more symmetrical: the liquid occupies a certain volume in the vessel and there is an asymmetry inside the vessel, where there is liquid, and points where it is not. The gas, on the other hand, occupies the entire volume provided to it, and in this sense all its points are indistinguishable from one another. Nevertheless, it would be more correct to speak here not about points, but about small, but macroscopic elements, because at the microscopic level there are still differences. At some points in time there are atoms or molecules, while others do not. Symmetry is observed only on average, either in some macroscopic volume parameters, or in time.
But there is still no instantaneous symmetry at the microscopic level. If the substance is compressed very strongly, to pressures that are unacceptable in everyday life, compressed 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 pressed against each other, there are not only interatomic, but also internuclear distances, and the substance becomes homogeneous (strange substance).
But there is also a submicroscopic level. Nuclei are made up of protons and neutrons that move around inside the nucleus. There is also some space between them. If you continue to compress 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 trend: 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 symmetrical.

18. Weakly symmetrical matter- a state opposite to strongly symmetrical matter in its properties, which was present in the 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 superforce. In this state, the matter is compressed to such an extent that its mass is converted into energy, which begins to inflate, that is, expand indefinitely. It is not yet possible to achieve energies for the experimental production of superpower and the 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 superforce that forms this substance, the superforce is not sufficiently symmetrical in comparison with the supersymmetric force, which contains all 4 types of interactions. Therefore, this state of aggregation received such a name.

19. Radiation matter- this, in fact, is no longer a substance, but energy in its purest form. However, it is this hypothetical state of aggregation that a body that has reached the speed of light will take. It can also be obtained by heating the body to the Planck temperature (1032K), that is, by dispersing the molecules of the substance to the speed of light. As follows from the theory of relativity, when the speed reaches more than 0.99 s, the mass of the body begins to grow much faster than with "normal" acceleration, in addition, the body lengthens, warms up, that is, it begins to radiate in the infrared spectrum. When crossing the threshold of 0.999 s, the body changes dramatically and begins a rapid phase transition up to the beam state. As follows from Einstein's formula, taken in full, the growing mass of the final substance is made up 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 radiate 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 beam moving at the speed of light and consisting of photons that have no length, and its infinite mass will completely turn into energy. Therefore, such a substance is called radiation.

Depending on the conditions, bodies can be in a liquid, solid or gaseous state. These states are called aggregate states of matter .

In gases, the distance between molecules is much more sizes molecules. If the walls of the vessel do not interfere with the gas, its molecules fly apart.

In liquids and solids, the molecules are closer together and therefore cannot move far apart.

The transition from one aggregate state to another is called phase transition .

The transition of a substance from a solid to a liquid state 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 absorbed by the body during melting, per unit mass of the body, is called specific heat of fusion (crystallization) λ:

During crystallization, the same amount of heat is released as is absorbed during melting.

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

Q = Lm ,

where L is specific heat of vaporization (condensation).

Vaporization from the surface of a liquid is called evaporation . Evaporation can take place at any temperature. The transition of liquid to vapor, which occurs throughout the volume of the body, is called boiling , and the temperature at which the liquid boils is boiling point .

Finally, sublimation - this 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 does not change during melting (crystallization) and boiling.

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

The establishment of an ideal order in the arrangement of atoms, i.e., the formation of a solid body, is hindered by thermal motions, the main feature of which, as we know, is randomness 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 motions is less than the potential energy of interaction of atoms.

A completely ideal crystal, in which all atoms are in equilibrium and have a minimum energy, the body can only be 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 change from a liquid state to a solid state, as well as 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, i.e., at a certain temperature, in contrast to the gas-liquid transition, which, as we know, can also occur continuously.

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

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

This is clearly seen from the graph of the coolant temperature versus time shown in Fig. 179 (curve a). Section 1 of the curve a gives the course of a monotonous decrease in the temperature of the liquid due to the removal of heat 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 decrease again (section 3). The temperature corresponding to section 2 is the crystallization temperature. The heat released during crystallization compensates for the removal of heat from the substance and therefore the decrease in temperature temporarily stops. After the end of the crystallization process, the temperature, now of a solid body, again begins to decrease.

Such a course of the temperature decrease graph is typical for crystalline bodies. When cooling liquids that do not crystallize (amorphous substances), latent heat is not released and the cooling curve is a monotonic curve without stopping cooling.

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

To start crystallization, the presence of a center or centers of crystallization is necessary. Such centers could be random accumulations of liquid particles stuck to each other, to which more and more particles could join, until the entire liquid turned into a solid body. 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 dimensions. Crystallization is greatly facilitated if sufficiently large solid particles in the form of dust particles and bodies are present in the liquid from the very beginning, which become centers of crystallization.

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

usually begins at a temperature somewhat lower than the true crystallization temperature. IN normal conditions in a crystallizing liquid there are many centers of crystallization, so that many crystals grow together in the liquid, and the solidified substance turns out to be polycrystalline.

Only under special conditions, which are usually difficult to provide, can a single crystal be obtained - 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. Both phases at the crystallization (melting) temperature can come into contact with each other, being in equilibrium (ice, for example, can float in water without melting), just as a 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 increasing pressure. It grows because the external pressure brings the atoms together, and to destroy the crystal lattice during melting, the atoms must be moved away from each other: at a higher pressure, this requires a greater energy of thermal movements, i.e., a higher temperature.

On fig. 180 shows a curve of melting (crystallization) temperature versus pressure. The solid curve divides the entire region into two parts. 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 harden, and the solid does not melt.

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

substances, of course, the melting point decreases with increasing pressure.

The change in melting point is related to the change in pressure by the Clausius-Clapeyron relation:

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

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

From the Clapeyron-Clausius formula, it can be seen that the sign of the change in melting temperature with a change in pressure is determined by which of the two values, or more. The steepness of the curve also depends on the value of the latent heat of transition; the lower the temperature, the less the melting temperature changes with pressure. In table. 20 shows the values ​​of the specific (i.e., per unit mass) heat of fusion for some substances.

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

The Clausius-Clapeyron equation can also be written in this 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 the 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 greater 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 energy difference 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 that the latent heat in the direct formation of a solid from the gaseous phase must be equal to the sum of the heats of condensation and crystallization from the liquid. This applies only 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. Evaporating particles of a solid form vapor above it in exactly the same way as occurs when a liquid evaporates. At certain pressures and temperatures, vapor and solid can be in equilibrium. Steam in equilibrium with a solid is also called saturated steam. As in the case of a liquid, the saturated vapor pressure over a solid depends on temperature, decreasing rapidly with decreasing temperature, so that many solids have negligible saturated vapor pressure at ordinary temperatures.

On fig. 181 shows the curve 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, as well as melting, is associated with the destruction of the lattice and requires the expenditure of the energy necessary 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 melting and vaporization.

In the world around us, a huge variety of different physical phenomena and processes constantly and continuously occur. One of the most important is the process of evaporation. There are several mandatory conditions for this phenomenon. In this article, we will analyze each of them in more detail.

This is the process of converting substances into a gaseous or vaporous state. It is characteristic only for 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 the 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 break between the connected molecules;
• the heat required in the work of expanding molecules in the process of converting liquid substances into vapor or gas.

How does this happen?

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

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

Despite the fact that both of these phenomena convert a liquid substance into a gas, there are significant differences between them. Boiling is an active process that occurs 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. Every time they collide with each other, their speeds change. At some point, some develop a very high speed, allowing 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, the remaining particles become smaller and smaller. This explains the cooling of the liquid. Remember how in childhood we were taught to blow on a hot liquid so that it cools down faster. 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 necessary 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 rate of evaporation directly depends on the composition of the liquid itself. It is known that each of them has its own characteristics. For example, those substances in which the heat of vaporization is lower will be converted faster. Let's compare two processes: evaporation of alcohol and ordinary water. In the first case, the conversion to 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 the vapor is formed. As an example, let's 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 ​​the 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, i.e., the faster diffusion occurs, the faster vaporization occurs. For example, when strong winds water droplets evaporate faster from the surface of lakes, rivers and reservoirs.
5. The air temperature in the room also plays an important role. We'll talk more about this below.

What is the role of air humidity?

Due to the fact that the evaporation process occurs from everywhere continuously and constantly, there are always particles of water 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 characterizes the density of water vapor in the air, denoted by the letter f and shows how much water molecules are contained in 1 m 3 of air.

The relationship between the evaporation process and air humidity can be determined as follows. The lower the indicator, the faster 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 reaching the boiling point, which is approximately equal to 78 degrees. However, the actual evaporation temperature of alcohol will be slightly higher, because in the original product (for example, mash) it is a combination with various aromatic oils and water.

Condensation and sublimation

The following phenomenon can be observed every time the water boils in the kettle. Note that when water boils, it changes from a liquid state to a gaseous state. It happens in this way: a hot jet of water vapor flies out of the kettle through its spout at high speed. 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, i.e., 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 the state of aggregation to the 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 passes into a vaporous form, bypassing the liquid phase.

The 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 it increases the internal energy. And as you know, at temperatures above 42 ° protein in human blood begins to fold, 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 allowable, active sweating begins through the pores on the skin. And then evaporation occurs from the surface of the skin, which absorbs the excess energy of the body. In other words, evaporation is a process that contributes to the cooling of the body to a normal state.