Tambov State University G.R. Derzhavin
Soros Educational Journal, Vol. 6, No. 9, 2000
Water and ice: do we know enough about them?
Yu. I. GOLOVIN
The physical properties of water and ice are described. Mechanisms of various phenomena in these substances are discussed. In spite of the long period of study and simple chemical composition, water and ice – the substances highly valuable for life on earth – harbor many mysteries because of their complicated dynamic proton and molecular structure.Dan short review physical properties of water and ice. The mechanisms of various phenomena in them are considered. It is shown that, despite the centuries-old history of study, the simplest chemical composition and crucial to life on Earth, the nature of water and ice is fraught with many mysteries due to the complex dynamic proton and molecular structure.
Although simplicity is more necessary for people,
Everything complicated is clearer to them.
B.L. Parsnip
Perhaps there is no more common and at the same time more mysterious substance on Earth than water in liquid and solid phases. Indeed, it is enough to remember that all life came out of the water and consists of more than 50% of it, that 71% of the Earth's surface is covered with water and ice, and a significant part of the northern territories of the land is permafrost. To visualize the total amount of ice on our planet, we note that in the event of their melting, the water in the oceans will rise by more than 50 m, which will lead to the flooding of giant land areas throughout the globe. Huge masses of ice have been discovered in the universe, including the solar system. There is not a single more or less significant production, household activity of a person, in which water would not be used. In recent decades, large reserves of fuel have been discovered in the form of solid ice-like hydrates of natural hydrocarbons.
At the same time, after numerous successes in the physics and physicochemistry of water in recent years, it can hardly be argued that the properties of this simple substance are fully understood and predictable. This article provides a brief overview of the most important physical properties of water and ice and unsolved problems related mainly to the physics of their low-temperature states.
This complex molecule
The foundations of the modern understanding of the physical chemistry of water were laid about 200 years ago by Henry Cavendish and Antoine Lavoisier, who discovered that water is not a simple chemical element, as medieval alchemists believed, and the combination of oxygen and hydrogen in a certain ratio. Actually, hydrogen (hydrogene) - giving birth to water - received its name only after this discovery, and water acquired a modern chemical designation, now known to every schoolchild, - H 2 O.
So, the H 2 O molecule is built from two hydrogen atoms and one oxygen atom. As established by studies of the optical spectra of water, in a hypothetical state of complete absence of movement (without vibrations and rotations), hydrogen and oxygen ions should occupy positions at the vertices of an isosceles triangle with an angle at the vertex occupied by oxygen of 104.5° (Fig. 1, a). In the unexcited state, the distances between the H + and O 2− ions are 0.96 Å. Due to this structure, the water molecule is a dipole, since the electron density in the region of the O 2− ion is much higher than in the region of the H + ions, and the simplest model, the sphere model, is poorly suited for describing the properties of water. One can imagine a water molecule in the form of a sphere with two small swellings in the region where protons are located (Fig. 1b). However, this does not help to understand another feature of water - the ability to form directional hydrogen bonds between molecules, which play a huge role in the formation of its loosened, but at the same time very stable spatial structure, which determines most of the physical properties in both liquid and solid states.
Rice. one. Geometric scheme (a), flat model (b) and spatial electronic structure (c) of the H 2 O monomer. Two of the four electrons of the outer shell of the oxygen atom participate in the creation of covalent bonds with hydrogen atoms, and the other two form strongly elongated electron orbits, the plane which is perpendicular to the H–O–H plane
Recall that a hydrogen bond is such a bond between atoms in one molecule or neighboring molecules, which is carried out through a hydrogen atom. It occupies an intermediate position between a covalent and non-valent bond and is formed when a hydrogen atom is located between two electronegative atoms (O, N, F, etc.). An electron in an H atom is relatively weakly bound to a proton, so the maximum electron density shifts to a more electronegative atom, and the proton is exposed and begins to interact with another electronegative atom. In this case, the approach of the atoms О⋅⋅⋅О, N⋅⋅⋅О, etc. occurs. to a distance close to what would be established between them in the absence of an H atom. The hydrogen bond determines not only the structure of water, but also plays an extremely important role in the life of biomolecules: proteins, carbohydrates, nucleic acids, etc.
Obviously, to explain the nature of water, it is necessary to take into account the electronic structure of its molecules. As you know, the upper shell of an oxygen atom has four electrons, while hydrogen has only one electron. Each O–H covalent bond is formed by one electron from oxygen and hydrogen atoms. The two electrons remaining in oxygen are called the lone pair, since in an isolated water molecule they remain free, not participating in the formation of bonds within the H 2 O molecule. But when approaching other molecules, it is these lone electrons that play a decisive role in the formation of the molecular structure of water .
Lone electrons are repelled from the O–H bonds, so their orbits are strongly elongated in the direction opposite to hydrogen atoms, and the planes of the orbits are rotated relative to the plane formed by the O–H–O bonds. Thus, it would be more correct to depict a water molecule in a three-dimensional space of coordinates xyz in the form of a tetrahedron, in the center of which there is an oxygen atom, and in two vertices there is a hydrogen atom each (Fig. 1, c). The electronic structure of H 2 O molecules determines the conditions for their association into a complex three-dimensional network of hydrogen bonds both in water and in ice. Each of the protons can form a bond with the lone electron of another molecule. In this case, the first molecule acts as an acceptor, and the second acts as a donor, forming a hydrogen bond. Since each H 2 O molecule has two protons and two lone electrons, it can simultaneously form four hydrogen bonds with other molecules. Thus, water is a complex associated liquid with a dynamic nature of bonds, and the description of its properties at the molecular level is possible only with the help of quantum mechanical models of varying degrees of complexity and rigor.
Ice and its properties
From point of view ordinary person, ice is more or less the same no matter where it forms: in the atmosphere in the form of hailstones, on the edges of roofs in the form of icicles, or in bodies of water in the form of plates. From the point of view of physics, there are many varieties of ice that differ in their molecular and mesoscopic structure. In ice that exists at normal pressure, each H 2 O molecule is surrounded by four others, that is, the coordination number of the structure is four (the so-called ice I h). The corresponding crystal lattice - hexagonal - is not close-packed, so the density ordinary ice(∼0.9 g/cm 3) is lower than the density of water (∼1 g/cm 3), for the structure of which, as x-ray diffraction studies show, the average coordination number is ∼4.4 (against 4 for ice I h). Fixed positions in the ice structure are occupied only by oxygen atoms. Two hydrogen atoms can occupy different positions on the four bonds of the H 2 O molecule with other neighbors. Due to the hexagonality of the lattice, crystals growing in a free state (for example, snowflakes) have a hexagonal shape.
However, the hexagonal phase is by no means the only form of existence of ice. The exact number of other crystalline phases - polymorphic forms of ice - is still unknown. They are formed when high pressures and low temperatures (Fig. 2). Some researchers consider the presence of 12 such phases to be precisely established, while others count them up to 14. Of course, this is not the only substance that has polymorphism (remember, for example, graphite and diamond, consisting of chemically identical carbon atoms), but the number of different phases ice, which continue to open to this day, is amazing. All of the above referred to the ordered arrangement of oxygen ions in the crystal lattice of ice. As for protons - hydrogen ions - as shown by neutron diffraction, there is a strong disorder in their arrangement. Thus, crystalline ice is both a well-ordered medium (with respect to oxygen) and simultaneously disordered (with respect to hydrogen).
Rice. 2. Phase diagram of crystalline ice.
Roman numerals indicate areas of existence
stable phases. Ice IV is a metastable phase
for, located on the diagram inside the region V
Often it seems that the ice is malleable and fluid. So it is, if the temperature is close to the melting point (that is, t \u003d 0 ° C at atmospheric pressure), and the load acts for a long time. And the most rigid material (for example, metal) at temperatures close to the melting point behaves in a similar way. Plastic deformation of ice, as, indeed, of many other crystalline bodies, occurs as a result of the nucleation and movement through the crystal of various structural imperfections: vacancies, interstitial atoms, grain boundaries, and, most importantly, dislocations. As it was established back in the 1930s, it is the presence of the latter that predetermines a sharp decrease in the resistance of crystalline solids to plastic deformation (by a factor of 102–104 with respect to the resistance of an ideal lattice). To date, all types of dislocations characteristic of the hexagonal structure have been discovered in ice Ih, and their micromechanical and electrical characteristics have been studied.
The influence of the strain rate on the mechanical properties of single-crystal ice is well illustrated in Fig. 3, taken from the book by N. Maeno. It can be seen that with an increase in the strain rate, the mechanical stresses σ necessary for plastic flow increase rapidly, and a giant yield tooth appears on the dependence of the relative strain E on σ.
Rice. 3.(on ). The stress curves are the relative strain for an ice single crystal Ih at t = −15°С (slip along the basal plane oriented at an angle of 45° to the compression axis). The numbers on the curves indicate the relative strain rate ( ∆l– sample length change l during ∆τ
) in units of 10 −7 s −1
Rice. 4. Scheme of the formation of defects in the proton subsystem of ice: (a) a pair of ionic defects H 3 O + and OH − ; b – pair of orientational Bjerrum defects D and L
No less remarkable are the electrical properties of ice. The value of conductivity and its exponentially rapid increase with increasing temperature sharply distinguish ice from metallic conductors and put it on a par with semiconductors. Usually ice is very pure chemically, even if it grows from dirty water or solution (think of clean, transparent pieces of ice in a dirty puddle). This is due to the low solubility of impurities in the ice structure. As a result, during freezing, impurities are pushed aside at the crystallization front into the liquid and do not enter the ice structure. That is why freshly fallen snow is always white, and the water from it is exceptionally pure.
Nature has wisely provided a gigantic water treatment plant on the scale of the entire atmosphere of the Earth. Therefore, one cannot count on a high impurity conductivity (as, for example, in doped silicon) in ice. But there are no free electrons in it, as in metals. It was only in the 1950s that it was established that charge carriers in ice are disordered protons, that is, ice is a proton semiconductor.
The proton hopping mentioned above creates two types of defects in the ice structure: ionic and orientational (Fig. 4). In the first case, the proton hops along the hydrogen bond from one H 2 O molecule to another (Fig. 4, a), resulting in the formation of a pair of ionic defects H 3 O + and OH − , and in the second, to the adjacent hydrogen bond in one H 2 O molecule (Fig. 4b), resulting in a pair of orientational Bjerrum defects, called L and D defects (from German leer - empty and doppelt - double). Formally, such a jump can be considered as a rotation of the H 2 O molecule by 120°.
The flow of direct current due to the movement of only ionic or only orientational defects is impossible. If, for example, an H 3 O + ion has passed through any part of the grid, then the next similar ion will not be able to pass along the same path. However, if a D-defect is passed along this path, then the arrangement of protons will return to the original one and, consequently, the next H 3 O + ion will also be able to pass. OH − and L defects behave similarly. Therefore, the electrical conductivity of chemically pure ice is limited by those defects, which are fewer, namely, ionic ones. Dielectric polarization, on the other hand, is due to more numerous Bjerrum orientational defects. In fact, when an external electric field is applied, both processes proceed in parallel, which allows ice to conduct a direct current and at the same time experience a strong dielectric polarization, that is, to exhibit both the properties of a semiconductor and the properties of an insulator. V last years Attempts continue to be made to detect ferroelectric and piezoelectric properties of pure ice at low temperatures both in the bulk and at interfaces. There is no complete confidence in their existence yet, although several pseudo-piezoelectric effects associated with the presence of dislocations and other structural defects have been discovered.
Physics of the surface and crystallization of ice
In connection with the development of semiconductor technology, the microminiaturization of the element base, and the transition to planar technologies, interest in surface physics has greatly increased in the last decade. Many subtle techniques have been developed for studying near-surface states in solids, which have proved useful in the study of metals, semiconductors, and dielectrics. However, the structure and properties of the ice surface adjoining vapor or liquid remain largely unclear. One of the most intriguing hypotheses, put forward by M. Faraday, is the existence of a quasi-liquid layer on the ice surface with a thickness of tens or hundreds of angstroms even at a temperature well below the melting point. The reason for this is not only speculative constructions and theories of the structure of near-surface layers of strongly polarized H 2 O molecules, but also subtle determinations (using the method of nuclear magnetic resonance) of the phase state of the ice surface, as well as its surface conductivity and its dependence on temperature. However, in most cases of practical importance, the properties of the surface of snow and ice are most likely determined by the presence of a macroscopic water film rather than a quasi-liquid layer.
The melting of near-surface layers of ice under the influence of sunlight, a warmer atmosphere or a solid body sliding on it (skates, skis, sledge runners) is crucial for the realization of a low coefficient of friction. Low sliding friction is not the result of a decrease in the melting point under the action of increased pressure, as is often thought, but a consequence of the release of frictional heat. The calculation shows that the pressure effect, even in the case of sliding a sharply ground skate on ice, under which a pressure of about 1 MPa develops, leads to a decrease in the melting temperature by only ∼0.1°C, which cannot significantly affect the friction value.
An established tradition in describing the properties of water and ice is the ascertainment and discussion of many anomalous properties that distinguish this substance from homologues (H 2 S, H 2 Se, H 2 Te). Perhaps the most important is the very high (among simple substances) specific heat of fusion (crystallization) and heat capacity, that is, it is difficult to melt ice, and it is difficult to freeze water. As a result, the climate on our planet is generally quite mild, but in the absence of water (for example, in the deserts of hot Africa), the contrast between day and night temperatures is much higher than on the ocean coast at the same latitude. Vital for the biosphere is the ability to increase in volume during crystallization, and not decrease, as does the vast majority of known substances. As a result, ice floats in water, rather than sinking, and greatly slows down the freezing of water bodies in cold weather, protecting all living things that hide in it for the winter. This is also facilitated by the nonmonotonic change in the density of water as the temperature drops to 0°C - one of the most well-known anomalous properties of water, discovered more than 300 years ago. The maximum density is reached at t = 4°C, and this prevents subsurface layers of water that have cooled to a temperature below 4°C from sinking to the bottom. The convective mixing of the liquid is blocked, which greatly slows down further cooling. Other anomalies of water have been known for quite a long time: shear viscosity at 20°C, specific heat at 40°C, isothermal compressibility at 46°C, sound propagation velocity at 60°C. The viscosity of water decreases with increasing pressure, and does not increase, as with other liquids. It is clear that the anomalous properties of water are due to the structural features of its molecule and the specifics of intermolecular interactions. Complete clarity regarding the latter has not yet been achieved. The properties described above refer to water, ice and the interface between them, existing in conditions of thermodynamic equilibrium. Problems of a completely different level of complexity arise when trying to describe the dynamics of the water–ice phase transition, especially under conditions that are far from thermodynamic equilibrium.
The thermodynamic cause of any phase transition is the difference between the chemical potentials of particles on one side and the other of the interface ∆µ = µ 1 −µ 2 . The chemical potential µ is a function of state that determines the changes in thermodynamic potentials with a change in the number N of particles in the system, that is, µ = G/N, where G = H − TS is the Gibbs thermodynamic potential, H is the enthalpy, S is the entropy, T is the temperature . The difference in thermodynamic potentials is the driving force of a macroscopic process (just as the difference in electrical potentials at the ends of a conductor is the cause of an electric current). For µ1 = µ2, both phases can coexist in equilibrium for an arbitrarily long time. At normal pressure, the chemical potential of water is equal to the chemical potential of ice at t = 0°C. At t< 0°С более низким химическим потенциалом обладает лед, но это еще не означает, что при любом, самом маленьком переохлаждении начнется кристаллизация. Опыт показывает, что тщательно очищенный от примесей, обезгаженный, деионизированный расплав может быть переохлажден относительно точки равновесия фаз на десятки кельвин (а для некоторых веществ и на сотни). Анализ показывает, что причина заключается в отсутствии зародышей new phase(centers of crystallization, condensation, vaporization, etc.).
Nuclei can also form homogeneously, that is, from the medium itself, which is in a metastable state, but certain conditions must be met for this. We begin the consideration of the situation by taking into account the fact that any interface between a crystal and a melt (or vapor, solution) introduces additional energy Sα, where S is the area of the boundary, α is the surface energy. In addition, N molecules that formed the seed crystal have an energy lower than in a liquid by N∆µ. As a result, the total energy change in the system upon the appearance of the nucleus ∆U = −N∆µ + Sα turns out to be nonmonotonically dependent on N. Indeed, for a spherical nucleus
where A = (36πV 2) 1/3 V is the volume per molecule in the crystal. It follows from the above that ∆U reaches its maximum ∆Uc = - N c ∆µ + AN c 2/3 α when N c = (2Aα/3∆µ) 3 molecules are in the nucleus.
Thus, when molecules are sequentially attached to the nucleus, the system must first climb to the top of a potential hill with a height ∆U s, depending on supercooling, after which further growth of N in the crystal will proceed with a decrease in energy, that is, easier. It would seem that the lower the temperature of the liquid, that is, the stronger the supercooling, the faster the crystallization should proceed. So it really is with not too much hypothermia. However, as t decreases, the viscosity of the liquid also increases exponentially, hindering the movement of molecules. As a result, at high degrees of supercooling, the crystallization process can be delayed for many years (as is the case with glasses of various origins).
Numerical estimates show that for water, under normal degrees of supercooling under natural conditions (∆t = 1–10°С), the nucleus should consist of several tens of molecules, which is much larger than the coordination number in the liquid phase (∼4.4). Thus, the system needs a large number of fluctuation attempts to climb to the top of the energy hill. In not very carefully purified water, strong supercooling is prevented by the presence of already existing centers of crystallization, which can be particles of impurities, dust particles, irregularities in the walls of the vessel, etc. Subsequently, the kinetics of crystal growth depends on the conditions of heat transfer near the interface, as well as on the morphology of the latter at the atomic molecular level.
Highly supercooled water has two characteristic temperatures t h = −36°C and t g = −140°C. Well-purified and degassed water in the temperature range 0°C > t > t h can remain in a state of supercooled liquid for a long time. At t g< t < t h происходит гомогенное зарождение кристалликов льда, и вода не может находиться в переохлажденном состоянии при любой степени очистки. В условиях достаточно быстрого охлаждения при t < tg подвижность молекул воды настолько падает (а вязкость растет), что она образует стеклообразное твердое тело с аморфной структурой, свойственной жидкостям. При этом в области невысоких давлений образуется аморфная фаза низкой плотности, а в области повышенных – аморфная фаза высокой плотности, то есть вода демонстрирует полиаморфизм. При изменениях давления или температуры одна аморфная фаза скачком переходит в другую с неожиданно большим изменением плотности (>20%).
There are several points of view on the nature of water polyamorphism. So, according to , this behavior of strongly supercooled water can be explained if we assume that there is more than one minimum in the potential profile of the interaction of two H2O molecules,
Rice. 5(on ). Hypothetical potential profiles: a – with one energy minimum (for example, the Lennard-Jones potential U(r) = A/r 6 − B/r 12) and b – with two energy minima, which correspond to two stable configurations of a cluster of two interacting molecules water (1 and 2) with different distances between the conditional centers of molecules r H and r L ; the first of them corresponds to a phase with a higher density, the second - with a lower one.
and two (Fig. 5). Then the amorphous phase with high density will correspond to the average distance rH, and the phase with low density - rL. Computer modeling confirms this point of view, but there is still no reliable experimental evidence for this hypothesis, just as there is no rigorous theory confirming the validity of using a double-well potential to describe such unusual properties of supercooled water.
The behavior of supercooled water is of great interest for various reasons. In particular, it determines the climatic conditions, the possibility and mode of navigation in high latitudes, which is relevant for our country. In the process of dynamic crystallization at the interface, a lot of interesting and still little-studied phenomena occur, for example, the redistribution of impurities, separation and subsequent relaxation of electric charges, accompanied by electromagnetic radiation in a wide frequency band, etc. Finally, crystallization in a strongly supercooled liquid is excellent, easily reproduced many times over. a model situation of the behavior of a system far from thermodynamic equilibrium and capable, as a result of the development of instabilities, of the formation of dendrites of various orders and dimensions (typical representatives are snowflakes and ice patterns on windows), convenient for creating and modeling the behavior of fractals.
The processes of melting ice at first glance seem easier to analyze than the processes of crystallization. However, they also leave many questions. So, for example, it is widely believed that melt water for some time has properties that differ from those of ordinary water, at least in relation to biological objects: plants, animals, humans. Probably, these features can be due to high chemical purity (due to the noted low impurity capture coefficient during ice crystallization), differences in the content of dissolved gases and ions, and also memorization of the ice structure in multimolecular clusters of the liquid phase. However, the author does not have reliable information about this, obtained by modern physical methods.
No less difficult is the analysis of the mechanisms of influence of external physical fields, in particular magnetic fields, on the processes and properties of water, ice and phase transitions. All of our lives are in constant action. magnetic field Earth and its weak fluctuations. For many centuries, magnetobiology and magnetic methods of treatment in medicine have been developed. Finally, installations for the magnetization of water used for irrigation in agriculture (in order to increase productivity), power steam boilers (to reduce the rate of scale formation in them), etc. are mass-produced and widely used. However, there is still no satisfactory physical description of the mechanisms of action of a magnetic field in these and other similar cases.
Conclusion
Water, ice and their mutual phase transformations are still fraught with many mysteries. Solving them is not only a very interesting physical problem, but also extremely important for life on Earth, as it is directly related to human health and well-being. Perhaps they provide one of the most striking examples of the role of electronic and molecular structure in the formation of physical properties with the simplest and well-known chemical composition of matter.
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2. Maeno N. The science of ice. M.: Mir, 1988. 231 p.
3. Hobbs P.V. ice physics. Oxford: Univ. Press, 1974. 864 p.
4. Zatsepina G.N. Physical properties and structure of water. M.: Publishing House of Moscow State University, 1998. 184 p.
5. Mishima O., Stanley E. The Relationship between Liquid, Supercooled and Glassy Water // Nature. 1998 Vol. 396. P. 329–335.
6. Zolotukhin I.V. Fractals in solid state physics // Soros Educational Journal. 1998. No. 7. S. 108–113. Article reviewer B.A. Strukov
Yuri Ivanovich Golovin, Doctor of Physical and Mathematical Sciences, Professor, Head. Department of Theoretical and Experimental Physics Tambov state university them. G.R. Derzhavin, Honored Scientist of the Russian Federation. The area of scientific interests is the electronic structure of defects in solids and the macroscopic properties caused by them. Author and co-author of over 200 scientific works, including monographs and 40 inventions.
Water is a familiar and unusual substance. Almost 3/4 of the surface of our planet is occupied by oceans and seas. Solid water - snow and ice - covers 20% of the land. The planet's climate depends on water. Geophysicists say that The earth would have cooled long ago and turned into a lifeless piece of stone, if not for the water. She has a very high heat capacity. When heated, it absorbs heat; cooling down, gives it away. Terrestrial water both absorbs and returns a lot of heat and thus "levels" the climate. And those water molecules that are scattered in the atmosphere - in clouds and in the form of vapors protect the Earth from cosmic cold.
Water is the most mysterious substance in nature after DNA, possessing unique properties that not only have not yet been fully explained, but far from all are known. The longer it is studied, the more new anomalies and mysteries are found in it. Most of these anomalies, which provide the possibility of life on Earth, are explained by the presence of hydrogen bonds between water molecules, which are much stronger than the van der Waals forces of attraction between molecules of other substances, but an order of magnitude weaker than ionic and covalent bonds between atoms in molecules. The same hydrogen bonds are also present in the DNA molecule.
The water molecule (H 2 16 O) consists of two hydrogen atoms (H) and one oxygen atom (16 O). It turns out that almost all the variety of properties of water and the unusual nature of their manifestation is ultimately determined by the physical nature of these atoms, the way they are combined into a molecule and the grouping of the resulting molecules.
Rice. The structure of the water molecule . Geometric scheme (a), flat model (b), and spatial electronic structure (c) of the H2O monomer. Two of the four electrons of the outer shell of the oxygen atom participate in the creation of covalent bonds with hydrogen atoms, and the other two form strongly elongated electron orbits, the plane of which is perpendicular to the H-O-H plane.
The water molecule H 2 O is built in the form of a triangle: the angle between the two oxygen-hydrogen bonds is 104 degrees. But since both hydrogen atoms are located on the same side of oxygen, the electric charges in it disperse. The water molecule is polar, which is the reason for the special interaction between its different molecules. The hydrogen atoms in the H 2 O molecule, having a partial positive charge, interact with the electrons of the oxygen atoms of neighboring molecules. Such a chemical bond is called a hydrogen bond. It combines H 2 O molecules into peculiar associates of the spatial structure; the plane in which the hydrogen bonds are located is perpendicular to the plane of the atoms of the same H 2 O molecule. The interaction between water molecules primarily explains the irregularly high temperatures of its melting and boiling. Additional energy is needed to loosen and then break the hydrogen bonds. And this energy is very significant. That is why the heat capacity of water is so high.
The water molecule has two polar H–O covalent bonds. They are formed due to the overlap of two one-electron p-clouds of an oxygen atom and one-electron S-clouds of two hydrogen atoms.
In accordance with the electronic structure of hydrogen and oxygen atoms, the water molecule has four electron pairs. Two of them are involved in the formation of covalent bonds with two hydrogen atoms, i.e. are binding. The other two electron pairs are free - not bonding. They form an electron cloud. The cloud is inhomogeneous - it is possible to distinguish individual concentrations and rarefaction in it.
There are four poles of charges in a water molecule: two are positive and two are negative. Positive charges are concentrated at hydrogen atoms, since oxygen is more electronegative than hydrogen. Two negative poles fall on two non-bonding electron pairs of oxygen.
An excess of electron density is created at the oxygen nucleus. The internal electron pair of oxygen evenly frames the nucleus: it is schematically represented by a circle with the center - the O 2 - nucleus. The four outer electrons are grouped into two electron pairs, gravitating towards the nucleus, but not partially compensated. Schematically, the total electronic orbitals of these pairs are shown as ellipses, elongated from a common center - the O 2- nucleus. Each of the remaining two oxygen electrons pairs with one hydrogen electron. These vapors also gravitate towards the oxygen core. Therefore, hydrogen nuclei - protons - are somewhat bare, and here there is a lack of electron density.
Thus, four poles of charges are distinguished in a water molecule: two negative (excess electron density in the region of the oxygen nucleus) and two positive (lack of electron density in two hydrogen nuclei). For greater clarity, one can imagine that the poles occupy the vertices of a deformed tetrahedron, in the center of which there is an oxygen nucleus.
Rice. The structure of the water molecule: a – angle between O-H bonds; b - the location of the charge poles; v - appearance electron cloud of the water molecule.
The almost spherical water molecule has a markedly pronounced polarity, since the electric charges in it are located asymmetrically. Each water molecule is a miniature dipole with a high dipole moment of 1.87 debay. Debye is an off-system unit of electric dipole 3.33564·10 30 C·m. Under the influence of water dipoles, interatomic or intermolecular forces on the surface of a substance immersed in it weaken by 80 times. In other words, water has a high dielectric constant, the highest of any compound known to us.
Largely due to this, water manifests itself as a universal solvent. Solids, liquids, and gases are subject to its dissolving action to one degree or another.
The specific heat capacity of water is the highest among all substances. In addition, it is 2 times higher than that of ice, while for most simple substances (for example, metals) the heat capacity practically does not change during melting, and for substances from polyatomic molecules, as a rule, it decreases during melting.
Such an idea of the structure of the molecule makes it possible to explain many properties of water, in particular the structure of ice. In the crystal lattice of ice, each of the molecules is surrounded by four others. In a planar image, this can be represented as follows:
Communication between molecules is carried out through a hydrogen atom. The positively charged hydrogen atom of one water molecule is attracted to the negatively charged oxygen atom of another water molecule. Such a bond is called a hydrogen bond (it is denoted by dots). In terms of strength, a hydrogen bond is about 15–20 times weaker than a covalent bond. Therefore, the hydrogen bond is easily broken, which is observed, for example, during the evaporation of water.
Rice. left - Hydrogen bonds between water molecules
The structure of liquid water resembles that of ice. In liquid water, the molecules are also connected to each other through hydrogen bonds, but the structure of water is less "rigid" than that of ice. Due to the thermal motion of molecules in water, some hydrogen bonds are broken, others are formed.
Rice. Ice crystal lattice. Water molecules H 2 O (black balls) in its nodes are located so that each has four "neighbors".
The polarity of water molecules, the presence of partially uncompensated electric charges in them gives rise to a tendency to group molecules into enlarged "communities" - associates. It turns out that only water in the vapor state fully corresponds to the formula H2O. This was shown by the results of determining the molecular weight of water vapor. In the temperature range from 0 to 100°C, the concentration of individual (monomeric molecules) liquid water does not exceed 1%. All other water molecules are combined into associates of varying degrees of complexity, and their composition is described by the general formula (H 2 O)x.
The immediate reason for the formation of associates is hydrogen bonds between water molecules. They arise between the hydrogen nuclei of some molecules and the electronic "clumps" of the oxygen nuclei of other water molecules. True, these bonds are ten times weaker than "standard" intramolecular chemical bonds, and ordinary molecular movements are enough to destroy them. But under the influence of thermal vibrations, new bonds of this type also easily arise. The emergence and decay of associates can be expressed by the scheme:
x H 2 O↔ (H 2 O) x
Since the electron orbitals in each water molecule form a tetrahedral structure, hydrogen bonds can order the arrangement of water molecules in the form of tetrahedral coordinated associates.
Most researchers explain the anomalously high heat capacity of liquid water by the fact that when ice melts, its crystal structure is not destroyed immediately. In liquid water, hydrogen bonds between molecules are preserved. It remains, as it were, fragments of ice - associates from a large or smaller number of water molecules. However, unlike ice, each associate does not exist for long. Constantly there is a destruction of some and the formation of other associates. At each temperature value in water, its own dynamic equilibrium is established in this process. And when water is heated, part of the heat is spent on breaking hydrogen bonds in associates. In this case, 0.26-0.5 eV is spent on breaking each bond. This explains the anomalously high heat capacity of water compared to melts of other substances that do not form hydrogen bonds. When such melts are heated, energy is spent only on communicating thermal motions to their atoms or molecules. Hydrogen bonds between water molecules are completely broken only when water passes into steam. The correctness of this point of view is also indicated by the fact that the specific heat of water vapor at 100°C practically coincides with the specific heat of ice at 0°C.
Picture below:
The elementary structural element of the associate is the cluster: Rice. A separate hypothetical water cluster. Separate clusters form associates of water molecules (H 2 O) x: Rice. Clusters of water molecules form associates.
There is another point of view on the nature of the anomalously high heat capacity of water. Professor G. N. Zatsepina noticed that the molar heat capacity of water, which is 18 cal/(molgrad), is exactly equal to the theoretical molar heat capacity of a solid body with triatomic crystals. And in accordance with the law of Dulong and Petit, the atomic heat capacities of all chemically simple (monatomic) crystalline bodies at a sufficiently high temperature are the same and equal to 6 calDmol o deg). And for triatomic ones, in the gram of which there are 3 N a crystal lattice sites, - 3 times more. (Here N a is Avogadro's number).
It follows that water is, as it were, a crystalline body consisting of triatomic H 2 0 molecules. This corresponds to the common idea of water as a mixture of crystal-like associates with a small admixture of free H 2 O water molecules between them, the number of which increases with increasing temperature. From this point of view, it is not the high heat capacity of liquid water that is surprising, but the low solid ice. The decrease in the specific heat of water during freezing is explained by the absence of transverse thermal vibrations of atoms in the rigid crystal lattice of ice, where each proton that causes a hydrogen bond has only one degree of freedom for thermal vibrations instead of three.
But due to what and how can such large changes in the heat capacity of water occur without corresponding changes in pressure? To answer this question, let's meet with the hypothesis of the candidate of geological and mineralogical sciences Yu. A. Kolyasnikov about the structure of water.
He points out that even the discoverers of hydrogen bonds J. Bernal and R. Fowler in 1932 compared the structure of liquid water with the crystal structure of quartz, and those associates mentioned above are mainly 4H 2 0 tetramers, in which four molecules waters are connected in a compact tetrahedron with twelve internal hydrogen bonds. As a result, a tetrahedral pyramid is formed - a tetrahedron.
At the same time, hydrogen bonds in these tetramers can form both right-handed and left-handed sequences, just as crystals of widespread quartz (Si0 2), which also have a tetrahedral structure, come in right-handed and left-handed crystalline forms. Since each such water tetramer also has four unused external hydrogen bonds (as in one water molecule), the tetramers can be connected by these external relations into a kind of polymer chains, like a DNA molecule. And since there are only four external bonds, and three times more internal ones, this allows heavy and strong tetramers in liquid water to bend, turn and even break these external hydrogen bonds weakened by thermal vibrations. This is what causes the flow of water.
Water, according to Kolyasnikov, has such a structure only in the liquid state and, possibly, partially in the vapor state. But in ice, the crystal structure of which is well studied, tetrahydrols are interconnected by inflexible equal-strength direct hydrogen bonds into an openwork frame with large voids in it, which makes the density of ice less than the density of water.
Rice. Crystal structure of ice: water molecules are connected in regular hexagons
When the ice melts, some of the hydrogen bonds in it weaken and bend, which leads to a rearrangement of the structure into the above-described tetramers and makes liquid water denser than ice. At 4°C, a state sets in when all hydrogen bonds between tetramers are maximally bent, which determines the maximum density of water at this temperature. Further connections have nowhere to bend.
At temperatures above 4°C, the breaking of individual bonds between tetramers begins, and at 36–37°C, half of the external hydrogen bonds are broken. This determines the minimum on the curve of dependence of the specific heat capacity of water on temperature. At a temperature of 70°C, almost all intertetrameric bonds are broken, and along with free tetramers, only short fragments of "polymeric" chains of them remain in water. Finally, when water boils, the final rupture of now single tetramers into individual molecules of H 2 0 occurs. And the fact that the specific heat of evaporation of water is exactly 3 times greater than the sum of the specific heats of melting ice and subsequent heating of water to 100 ° C, is a confirmation of Kolyasnikov's assumption About. what number internal communications in the tetramer 3 times more than the number of external ones.
Such a tetrahedral-helical structure of water may be due to its ancient rheological relationship with quartz and other silicon-oxygen minerals prevalent in the earth's crust, from the depths of which water once appeared on Earth. Just as a small crystal of salt causes the surrounding solution to crystallize into crystals similar to it, and not into others, so quartz caused the water molecules to line up in tetrahedral structures, which are most energetically favorable. And in our era in the earth's atmosphere, water vapor, condensing into drops, form such a structure because the atmosphere always contains tiny droplets of aerosol water that already has this structure. They are the centers of condensation of water vapor in the atmosphere. Below are possible chain silicate structures based on a tetrahedron, which can also be composed of water tetrahedra.
Rice. Elementary regular silicon-oxygen tetrahedron SiO 4 4- .
Rice. Elementary silicon-oxygen units-ortho groups SiO 4 4- in the structure of Mg-pyroxene enstatite (a) and diortho groups Si 2 O 7 6- in Ca-pyroxenoid wollastonite (b).
Rice. The simplest types of island silicon-oxygen anionic groups: a-SiO 4, b-Si 2 O 7, c-Si 3 O 9, g-Si 4 O 12, e-Si 6 O 18.
Rice. below - The most important types of silicon-oxygen chain anionic groups (according to Belov): a-metahermanate, b - pyroxene, c - batisite, d-wollastonite, d-vlasovite, e-melilitic, g-rhodonite, s-pyroxmangitic, n-metaphosphate, k - fluoroberyllate, l - barylite.
Rice. below - Condensation of pyroxene silicon-oxygen anions into cellular two-row amphibole (a), three-row amphibole-like (b), layered talc and related anions (c).
Rice. below - The most important types of ribbon silicon-oxygen groups (according to Belov): a - sillimanite, amphibole, xonotlite; b-epididymitis; s-orthoclase; g-narsarsukite; d-phenacite prismatic; e-euclase inlaid.
Rice. on the right - A fragment (elementary package) of the layered crystal structure of muscovite KAl 2 (AlSi 3 O 10 XOH) 2 illustrating the interlayering of aluminosilicon-oxygen networks with polyhedral layers of large aluminum and potassium cations, reminiscent of a DNA chain.
Other models of the water structure are also possible. Tetrahedrally bound water molecules form peculiar chains of rather stable composition. Researchers are discovering more and more subtle and complex mechanisms of the "internal organization" of the water mass. In addition to the ice-like structure, liquid water, and monomeric molecules, a third element of the structure, non-tetrahedral, has also been described.
A certain part of water molecules is associated not into three-dimensional frameworks, but into linear ring associations. The rings, when grouped, form even more complex complexes of associates.
Thus, water can theoretically form chains, like a DNA molecule, which will be discussed below. In this hypothesis, it is also interesting that it implies the equiprobability of the existence of right- and left-handed water. But biologists have long noticed that in biological tissues and structures, only either left- or right-handed formations are observed. An example of this is protein molecules built only from left-handed amino acids and twisted only in a left-handed helix. But sugars in wildlife are all right-handed. No one has yet been able to explain why in wildlife there is such a preference for the left in some cases and for the right in others. After all, in inanimate nature with equally likely there are both right-handed and left-handed molecules.
More than a hundred years ago, the famous French naturalist Louis Pasteur discovered that organic compounds in plants and animals are optically asymmetric - they rotate the plane of polarization of the light falling on them. All amino acids that make up animals and plants rotate the plane of polarization to the left, and all sugars to the right. If we synthesize compounds of the same chemical composition, then each of them will have an equal number of left- and right-handed molecules.
As you know, all living organisms are made up of proteins, and they, in turn, are made of amino acids. Connecting to each other in a variety of sequences, amino acids form long peptide chains that spontaneously "twist" into complex protein molecules. Like many other organic compounds, amino acids have chiral symmetry (from the Greek chiros - hand), that is, they can exist in two mirror-symmetrical forms, called "enantiomers". Such molecules are similar to each other, like the left and right hand, so they are called D- and L-molecules (from Latin dexter, laevus - right and left).
Now imagine that the medium with left and right molecules has passed into a state with only left or only right molecules. Experts call such an environment chirally (from the Greek word "heira" - hand) ordered. Self-reproduction of the living (biopoiesis - according to the definition of D. Bernal) could arise and be maintained only in such an environment.
Rice. Mirror symmetry in nature
Another name for enantiomeric molecules - "right-handed" and "left-handed" - comes from their ability to rotate the plane of polarization of light in different directions. If linearly polarized light is passed through a solution of such molecules, its plane of polarization rotates: clockwise if the molecules in the solution are right, and counter-clockwise if they are left. And in a mixture of equal amounts D and L shapes(it's called a "racemate") the light will retain its original linear polarization. This optical property of chiral molecules was first discovered by Louis Pasteur in 1848.
It is curious that almost all natural proteins consist only of left-handed amino acids. This fact is all the more surprising since the synthesis of amino acids under laboratory conditions produces approximately the same number of right and left molecules. It turns out that this feature is possessed not only by amino acids, but also by many other substances important for living systems, and each has a strictly defined sign of mirror symmetry throughout the biosphere. For example, the sugars that make up many nucleotides, as well as DNA and RNA nucleic acids, are represented in the body exclusively by right D-molecules. Although the physical and chemical properties of the "mirror antipodes" coincide, their physiological activity in organisms is different: L-caxara is not absorbed, L-phenylalanine, unlike its harmless D-molecules, causes mental illness, etc.
According to modern ideas about the origin of life on Earth, the choice of a certain type of mirror symmetry by organic molecules served as the main prerequisite for their survival and subsequent self-reproduction. However, the question of how and why the evolutionary selection of one or another mirror antipode occurred is still one of the biggest mysteries of science.
The Soviet scientist L. L. Morozov proved that the transition to chiral ordering could not occur evolutionarily, but only with some specific sharp phase change. Academician V. I. Gol'danskii called this transition, thanks to which life on Earth originated, a chiral catastrophe.
How did the conditions for the phase catastrophe that caused the chiral transition arise?
The most important was that organic compounds melted at 800-1000 0C in the earth's crust, and the upper ones cooled to the temperature of space, that is, absolute zero. The temperature drop reached 1000°C. Under such conditions, the organic molecules melted under the influence of high temperature and even completely destroyed, and the top remained cold, as the organic molecules were frozen. Gases and water vapor that leaked from the earth's crust changed the chemical composition of organic compounds. The gases carried heat with them, causing the melting boundary of the organic layer to move up and down, creating a gradient.
At very low pressures of the atmosphere, water was on the earth's surface only in the form of steam and ice. When the pressure reached the so-called triple point of water (0.006 atmospheres), water for the first time could be in the form of a liquid.
Of course, it is only experimentally possible to prove what exactly caused the chiral transition: terrestrial or cosmic causes. But one way or another, at some point, chirally ordered molecules (namely, left-handed amino acids and right-handed sugars) turned out to be more stable and an unstoppable increase in their number began - a chiral transition.
The chronicle of the planet also tells that at that time there were neither mountains nor depressions on Earth. The semi-molten granite crust was a surface as flat as the level of the modern ocean. However, within this plain there were still depressions due to the uneven distribution of masses inside the Earth. These lowerings have played an extremely important role.
The fact is that flat-bottomed depressions with a diameter of hundreds and even thousands of kilometers and a depth of no more than a hundred meters, probably became the cradle of life. After all, the water that collected on the surface of the planet flowed into them. The water diluted the chiral organic compounds in the ash layer. The chemical composition of the compound gradually changed, and the temperature stabilized. The transition from the inanimate to the living, which began in anhydrous conditions, continued already in the aquatic environment.
Is this the origin of life? Most likely yes. In the Isua geological section (West Greenland), which is 3.8 billion years old, gasoline- and oil-like compounds were found with the C12/C13 isotopic ratio characteristic of photosynthetic carbon.
If the biological nature of carbon compounds from the Isua section is confirmed, it will turn out that the entire period of the origin of life on Earth - from the emergence of chiral organic matter to the appearance of a cell capable of photosynthesis and reproduction - was completed in only a hundred million years. And in this process, water molecules and DNA played a huge role.
The most surprising thing about the structure of water is that water molecules at low negative temperatures and high pressures inside nanotubes can crystallize in the form of a double helix, reminiscent of DNA. This was proven by computer experiments by American scientists led by Xiao Cheng Zeng at the University of Nebraska (USA).
DNA is a double strand twisted into a helix. Each strand consists of "bricks" - of sequentially connected nucleotides. Each DNA nucleotide contains one of the four nitrogenous bases - guanine (G), adenine (A) (purines), thymine (T) and cytosine (C) (pyrimidines), associated with deoxyribose, to the latter, in turn, a phosphate group is attached . Between themselves, adjacent nucleotides are connected in a chain by a phosphodiester bond formed by 3 "-hydroxyl (3"-OH) and 5"-phosphate groups (5"-PO3). This property determines the presence of polarity in DNA, i.e. opposite direction, namely 5 "- and 3"-ends: the 5"-end of one thread corresponds to the 3"-end of the second thread. The sequence of nucleotides allows you to "encode" information about various types of RNA, the most important of which are information or template (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized on the DNA template by copying the DNA sequence into the RNA sequence synthesized during transcription and take part in the most important process of life - the transmission and copying of information (translation).
The primary structure of DNA is the linear sequence of DNA nucleotides in a chain. The sequence of nucleotides in the DNA chain is written in the form of a DNA literal formula: for example - AGTCATGCCAG, the record is from the 5 "to the 3" end of the DNA chain.
The secondary structure of DNA is formed due to the interactions of nucleotides (mostly nitrogenous bases) with each other, hydrogen bonds. A classic example of the secondary structure of DNA is the DNA double helix. The DNA double helix is the most common form of DNA in nature, consisting of two polynucleotide strands of DNA. The construction of each new DNA chain is carried out according to the principle of complementarity, i.e. each nitrogenous base of one strand of DNA corresponds to a strictly defined base of the other strand: in a complementary pair, opposite A is T, and opposite G is C, and so on.
In order for water to form a spiral, like in a simulated experiment, it was "placed" in nanotubes under high pressure, varying in different experiments from 10 to 40,000 atmospheres. After that, the temperature was set, which had a value of -23°C. The reserve compared to the freezing point of water was made due to the fact that with increasing pressure, the melting point of water ice decreases. The diameter of the nanotubes ranged from 1.35 to 1.90 nm.
Rice. General view of the structure of water (image New Scientist)
Water molecules are linked together by hydrogen bonds, the distance between oxygen and hydrogen atoms is 96 pm, and between two hydrogens - 150 pm. In the solid state, the oxygen atom participates in the formation of two hydrogen bonds with neighboring water molecules. In this case, individual H 2 O molecules come into contact with each other with opposite poles. Thus, layers are formed in which each molecule is associated with three molecules of its own layer and one of the neighboring ones. As a result, the crystal structure of ice consists of hexagonal "tubes" interconnected like a honeycomb.
Rice. Inner wall of the water structure (New Scientist image)
Scientists expected to see that water in all cases forms a thin tubular structure. However, the model showed that at a tube diameter of 1.35 nm and a pressure of 40,000 atmospheres, the hydrogen bonds twisted, leading to the formation of a double-walled helix. The inner wall of this structure is a quadruple helix, and the outer wall consists of four double helixes, similar to the structure of a DNA molecule.
The latter fact affects not only the evolution of our ideas about water, but also the evolution of early life and the DNA molecule itself. If we assume that in the era of the origin of life, cryolitic clayey rocks were in the form of nanotubes, the question arises - could the water sorbed in them serve as a structural basis (matrix) for DNA synthesis and information reading? Perhaps that is why the helical structure of DNA repeats the helical structure of water in nanotubes. According to the New Scientist magazine, now our foreign colleagues will have to confirm the existence of such water macromolecules in real experimental conditions using infrared spectroscopy and neutron scattering spectroscopy.
Ph.D. O.V. Mosin
The concept of a molecule (and its derivative ideas about the molecular structure of matter, the structure of the molecule itself) allows us to understand the properties of substances that create the world. Modern, as well as early, physical and chemical research is based on the grandiose discovery of the atomic and molecular structure of matter. A molecule is a single “detail” of all substances, the existence of which was suggested by Democritus. Therefore, it is its structure and relationship with other molecules (forming a certain structure and composition) that determines / explains all the differences between substances, their type and properties.
The molecule itself, being not the smallest component of a substance (which is an atom), has a certain structure and properties. The structure of a molecule is determined by the number of certain atoms entering it and the nature of the bond (covalent) between them. This composition is unchanged, even if the substance is transformed into another state (as, for example, happens with water - this will be discussed later).
The molecular structure of a substance is fixed by a formula that provides information about atoms, their number. In addition, the molecules that make up a substance/body are not static: they themselves are mobile - the atoms rotate, interacting with each other (attract / repel).
Characteristics of water, its condition
The composition of such a substance as water (as well as its chemical formula) is familiar to everyone. Each molecule is made up of three atoms: an oxygen atom, denoted by the letter "O", and hydrogen atoms - the Latin "H", in the amount of 2. The shape of the water molecule is not symmetrical (similar to an isosceles triangle).
Water, as a substance, its constituent molecules, reacts to the external "environment", environmental indicators - temperature, pressure. Depending on the latter, water is able to change the state, of which there are three:
- The most familiar, natural state for water is liquid. A molecular structure (dihydrol) of a peculiar order in which single molecules fill (by hydrogen bonds) voids.
- The state of a vapor in which the molecular structure (hydrol) is represented by single molecules between which no hydrogen bonds are formed.
- The solid state (actually ice) has a molecular structure (trihydrol) with strong and stable hydrogen bonds.
In addition to these differences, naturally, the ways of “transition” of a substance from one state (liquid) to others also differ. These transitions both transform the substance and provoke the transfer of energy (release/absorption). Among them there are direct processes - the transformation of liquid water into steam (evaporation), into ice (freezing) and reverse - into liquid from steam (condensation), from ice (melting). Also, the states of water - vapor and ice - can be transformed into each other: sublimation - ice into steam, sublimation - the reverse process.
Specificity of ice as a state of water
It is widely known that ice freezes (transforms from water) when the temperature crosses the boundary to zero degrees. Although, in this all understandable phenomenon, there are some nuances. For example, the state of ice is ambiguous, its types and modifications are different. They differ primarily in the conditions under which they arise - temperature, pressure. There are fifteen such modifications.
Ice in its various forms has a different molecular structure (molecules are indistinguishable from water molecules). Natural and natural ice, in scientific terminology referred to as ice Ih, is a substance with a crystalline structure. That is, each molecule with four “neighbors” surrounding it (the distance between all is equal) creates a geometric figure of a tetrahedron. Other phases of ice have a more complex structure, such as the highly ordered structure of trigonal, cubic, or monoclinic ice.
The main differences between ice and water at the molecular level
The first and not directly related to the molecular structure of water and ice, the difference between them is an indicator of the density of the substance. The crystalline structure inherent in ice, when formed, contributes to a simultaneous decrease in density (from almost 1000 kg/m³ to 916.7 kg/m³). And this stimulates an increase in volume by 10%. 
The main difference in the molecular structure of these aggregate states of water (liquid and solid) is in the number, type and strength of hydrogen bonds between molecules. In ice (solid state), five molecules are united by them, and the hydrogen bonds themselves are stronger.
The molecules themselves of the substances of water and ice, as mentioned earlier, are the same. But in ice molecules, an oxygen atom (to create a crystalline “lattice” of a substance) forms hydrogen bonds (two) with “neighbor” molecules.
What distinguishes the substance of water in its different states (aggregate) is not only the structure of the arrangement of molecules (molecular structure), but also their movement, the strength of the relationship / attraction between them. Water molecules in the liquid state are attracted rather weakly, ensuring the fluidity of water. In solid ice, the attraction of molecules is strongest, and therefore their motor activity is low (it ensures the constancy of the shape of ice).
O. V. Mosin, I. Ignatov (Bulgaria)
annotation The importance of ice in sustaining life on our planet cannot be underestimated. Ice has a great influence on the living conditions and life of plants and animals and on various types of human economic activity. Covering the water, ice, due to its low density, plays the role of a floating screen in nature, protecting rivers and reservoirs from further freezing and preserving the life of underwater inhabitants. The use of ice for various purposes (snow retention, arrangement of ice crossings and isothermal warehouses, ice laying of storage facilities and mines) is the subject of a number of sections of hydrometeorological and engineering sciences, such as ice technology, snow technology, engineering permafrost, as well as the activities of special services for ice reconnaissance, icebreaking transport and snowplows. Natural ice is used to store and cool food products, biological and medical preparations, for which it is specially produced and harvested, and melt water prepared by melting ice is used in folk medicine to increase metabolism and remove toxins from the body. The article introduces the reader to new little-known properties and modifications of ice.
Ice is a crystalline form of water, which, according to the latest data, has fourteen structural modifications. Among them there are both crystalline (natural ice) and amorphous (cubic ice) and metastable modifications that differ from each other in the mutual arrangement and physical properties of water molecules linked by hydrogen bonds that form the crystal lattice of ice. All of them, except for the familiar natural ice I h, which crystallizes in a hexagonal lattice, are formed under exotic conditions - at very low temperatures of dry ice and liquid nitrogen and high pressures of thousands of atmospheres, when the angles of hydrogen bonds in a water molecule change and crystalline systems are formed that are different from hexagonal. Such conditions are reminiscent of cosmic conditions and are not found on Earth.
In nature, ice is represented mainly by one crystalline variety, crystallizing in a hexagonal lattice resembling a diamond structure, where each water molecule is surrounded by four molecules closest to it, located at the same distance from it, equal to 2.76 angstroms and located at the vertices of a regular tetrahedron. Due to the low coordination number, the structure of ice is a network, which affects its low density, which is 0.931 g/cm 3 .
The most unusual property of ice is the amazing variety of external manifestations. With the same crystal structure, it can look completely different, taking the form of transparent hailstones and icicles, fluffy snow flakes, a dense shiny crust of ice, or giant glacial masses. Ice occurs in nature in the form of continental, floating and underground ice, as well as in the form of snow and hoarfrost. It is widespread in all areas of human habitation. Collecting in large quantities, snow and ice form special structures with fundamentally different properties than individual crystals or snowflakes. Natural ice is formed mainly by ice of sedimentary-metamorphic origin, formed from solid atmospheric precipitation as a result of subsequent compaction and recrystallization. A characteristic feature of natural ice is granularity and banding. Granularity is due to recrystallization processes; every grain glacial ice is an irregularly shaped crystal, closely adjacent to other crystals in the ice mass in such a way that the protrusions of one crystal fit tightly into the recesses of another. Such ice is called polycrystalline. In it, each ice crystal is a layer of the thinnest leaves overlapping each other in the basal plane, perpendicular to the direction of the optical axis of the crystal.
The total reserves of ice on Earth are estimated to be about 30 million tons. km 3(Table 1). Most of the ice is concentrated in Antarctica, where the thickness of its layer reaches 4 km. There is also evidence of the presence of ice on the planets of the solar system and in comets. Ice has so great importance for the climate of our planet and the habitation of living beings on it, that scientists have designated a special environment for ice - the cryosphere, the boundaries of which extend high into the atmosphere and deep into the earth's crust.
Tab. one. Quantity, distribution and lifetime of ice.
Ice crystals are unique in their shape and proportions. Any growing natural crystal, including an ice crystal of ice, always strives to create an ideal regular crystal lattice, since this is beneficial from the point of view of a minimum of its internal energy. Any impurities, as is known, distort the shape of the crystal, therefore, during the crystallization of water, water molecules are first of all built into the lattice, and foreign atoms and molecules of impurities are displaced into the liquid. And only when the impurities have nowhere to go, the ice crystal begins to build them into its structure or leaves them in the form of hollow capsules with a concentrated non-freezing liquid - brine. Therefore, sea ice is fresh and even the dirtiest water bodies are covered with transparent and clean ice. When ice melts, it displaces impurities into the brine. On a planetary scale, the phenomenon of freezing and thawing of water, along with the evaporation and condensation of water, plays the role of a gigantic cleansing process in which water on Earth is constantly purifying itself.
Tab. 2. Some physical properties of ice I.
Property | Meaning | Note |
Heat capacity, cal/(g °C) Melting heat, cal/g Heat of vaporization, cal/g | Decreases strongly with decreasing temperature |
|
Thermal expansion coefficient, 1/°C | 9.1 10 -5 (0 °C) | Polycrystalline ice |
Thermal conductivity, cal/(cm sec °C) | Polycrystalline ice |
|
Refractive index: | Polycrystalline ice |
|
Electrical conductivity, ohm -1 cm -1 | Apparent activation energy 11 kcal/mol |
|
Surface electrical conductivity, ohm -1 | Apparent activation energy 32 kcal/mol |
|
Young's modulus of elasticity, dyne / cm 2 | 9 10 10 (-5 °C) | Polycrystalline ice |
Resistance, MN/m2: crushing | Polycrystalline ice Polycrystalline ice Polycrystalline ice |
|
Dynamic viscosity, poise | Polycrystalline ice |
|
Activation energy during deformation and mechanical relaxation, kcal/mol | Increases linearly by 0.0361 kcal/(mol °C) from 0 to 273.16 K |
1 cal/(g °C)=4.186 kJ/(kg K); 1 ohm -1 cm -1 \u003d 100 sim / m; 1 dyn = 10 -5 N ; 1 N = 1 kg m/s²; 1 dyne/cm=10 -7 N/m; 1 cal / (cm sec ° C) \u003d 418.68 W / (m K); 1 poise \u003d g / cm s \u003d 10 -1 N sec / m 2.
Due to the wide distribution of ice on Earth, the difference in the physical properties of ice (Table 2) from the properties of other substances plays an important role in many natural processes. Ice has many other life-supporting properties and anomalies - anomalies in density, pressure, volume, and thermal conductivity. If there were no hydrogen bonds linking water molecules into a crystal, ice would melt at -90 °C. But this does not happen due to the presence of hydrogen bonds between water molecules. Due to its lower density than that of water, ice forms a floating cover on the surface of the water, which protects rivers and reservoirs from bottom freezing, since its thermal conductivity is much less than that of water. At the same time, the lowest density and volume are observed at +3.98 °C (Fig. 1). Further cooling of water to 0 0 C gradually leads not to a decrease, but to an increase in its volume by almost 10%, when the water turns into ice. This behavior of water indicates the simultaneous existence of two equilibrium phases in water - liquid and quasi-crystalline, by analogy with quasi-crystals, the crystal lattice of which not only has a periodic structure, but also has symmetry axes of different orders, the existence of which previously contradicted the ideas of crystallographers. This theory, first put forward by the well-known domestic theoretical physicist Ya. I. Frenkel, is based on the assumption that some of the liquid molecules form a quasi-crystalline structure, while the rest of the molecules are gas-like, freely moving through the volume. The distribution of molecules in a small neighborhood of any fixed water molecule has a certain order, somewhat reminiscent of a crystalline one, although more loose. For this reason, the structure of water is sometimes called quasi-crystalline or crystal-like, i.e., having symmetry and the presence of order in relative position atoms or molecules.
Rice. one. The dependence of the specific volume of ice and water on temperature
Another property is that the flow rate of ice is directly proportional to the activation energy and inversely proportional to the absolute temperature, so that as the temperature decreases, ice approaches in its properties an absolutely solid body. On average, at a temperature close to melting, the fluidity of ice is 106 times higher than that of rocks. Due to its fluidity, ice does not accumulate in one place, but constantly moves in the form of glaciers. The relationship between flow velocity and stress in polycrystalline ice is hyperbolic; with an approximate description of it by a power equation, the exponent increases as the voltage increases.
Visible light is practically not absorbed by ice, since light rays pass through the ice crystal, but it blocks ultraviolet radiation and most of the infrared radiation from the Sun. In these regions of the spectrum, the ice appears absolutely black, since the absorption coefficient of light in these regions of the spectrum is very high. Unlike ice crystals, White light, falling on the snow, is not absorbed, but is repeatedly refracted in ice crystals and reflected from their faces. That's why snow looks white.
Due to the very high reflectivity of ice (0.45) and snow (up to 0.95), the area covered by them is on average about 72 million hectares per year. km 2 in the high and middle latitudes of both hemispheres, it receives solar heat 65% less than the norm and is a powerful source of cooling of the earth's surface, which largely determines the modern latitudinal climatic zonality. In summer, in the polar regions, solar radiation is greater than in the equatorial belt, nevertheless, the temperature remains low, since a significant part of the absorbed heat is spent on melting ice, which has a very high melting heat.
Other unusual properties of ice include the generation of electromagnetic radiation by its growing crystals. It is known that most of the impurities dissolved in water are not transferred to the ice when it begins to grow; they freeze. Therefore, even on the dirtiest puddle, the ice film is clean and transparent. In this case, impurities accumulate at the boundary of solid and liquid media, in the form of two layers of electric charges of different signs, which cause a significant potential difference. The charged layer of impurities moves along with the lower boundary of the young ice and radiates electromagnetic waves. Thanks to this, the crystallization process can be observed in detail. Thus, a crystal growing in length in the form of a needle radiates differently than one covered with lateral processes, and the radiation of growing grains differs from that which occurs when crystals crack. From the shape, sequence, frequency, and amplitude of the radiation pulses, one can determine how fast the ice freezes and what kind of ice structure is formed.
But the most surprising thing about the structure of ice is that water molecules at low temperatures and high pressures inside carbon nanotubes can crystallize in the form of a double helix, reminiscent of DNA molecules. This has been proven by recent computer experiments by American scientists led by Xiao Cheng Zeng from the University of Nebraska (USA). In order for water to form a spiral in a simulated experiment, it was placed in nanotubes with a diameter of 1.35 to 1.90 nm under high pressure, varying from 10 to 40,000 atmospheres, and a temperature of –23 °C was set. It was expected to see that the water in all cases forms a thin tubular structure. However, the model showed that at a nanotube diameter of 1.35 nm and an external pressure of 40,000 atmospheres, the hydrogen bonds in the ice structure were bent, which led to the formation of a double-walled helix - internal and external. Under these conditions, the inner wall turned out to be twisted into a quadruple helix, and the outer wall consisted of four double helixes similar to a DNA molecule (Fig. 2). This fact can serve as confirmation of the connection between the structure of the vitally important DNA molecule and the structure of water itself and that water served as a matrix for the synthesis of DNA molecules.
Rice. 2. Computer model of the structure of frozen water in nanotubes, resembling a DNA molecule (Photo from New Scientist, 2006)
Another of the most important properties of water discovered and investigated in Lately, lies in the fact that water has the ability to remember information about past impacts. This was first proved by the Japanese researcher Masaru Emoto and our compatriot Stanislav Zenin, who was one of the first to propose a cluster theory of the structure of water, consisting of cyclic associates of a bulk polyhedral structure - clusters general formula(H 2 O) n, where n, according to the latest data, can reach hundreds and even thousands of units. It is due to the presence of clusters in water that water has informational properties. The researchers photographed the processes of water freezing into ice microcrystals, acting on it with various electromagnetic and acoustic fields, melodies, prayer, words or thoughts. It turned out that under the influence of positive information in the form of beautiful melodies and words, the ice froze into symmetrical hexagonal crystals. Where non-rhythmic music sounded, angry and insulting words, water, on the contrary, froze into chaotic and shapeless crystals. This is proof that water has a special structure that is sensitive to external information influences. Presumably, the human brain, which consists of 85-90% of water, has a strong structuring effect on water.
Emoto crystals arouse both interest and insufficiently substantiated criticism. If you look at them carefully, you can see that their structure consists of six tops. But even more careful analysis shows that snowflakes in winter have the same structure, always symmetrical and with six tops. To what extent do crystallized structures contain information about the environment where they were created? The structure of snowflakes can be beautiful or shapeless. This indicates that the control sample (cloud in the atmosphere) where they occur has the same effect on them as the initial conditions. The initial conditions are solar activity, temperature, geophysical fields, humidity, etc. All this means that from the so-called. average ensemble, we can conclude that the structure of water drops, and then snowflakes, is approximately the same. Their mass is almost the same, and they move through the atmosphere at a similar speed. In the atmosphere, they continue to shape their structures and increase in volume. Even if they formed in different parts of the cloud, there are always a certain number of snowflakes in the same group that arose under almost the same conditions. And the answer to the question of what constitutes positive and negative information about snowflakes can be found in Emoto. Under laboratory conditions, negative information (earthquake, sound vibrations unfavorable for humans, etc.) does not form crystals, but positive information, just the opposite. It is very interesting to what extent one factor can form the same or similar structures of snowflakes. The highest density of water is observed at a temperature of 4 °C. It has been scientifically proven that the density of water decreases when hexagonal ice crystals begin to form as the temperature drops below zero. This is the result of the action of hydrogen bonds between water molecules.
What is the reason for this structuring? Crystals are solids, and their constituent atoms, molecules or ions are arranged in a regular, repeating structure, in three spatial dimensions. The structure of water crystals is slightly different. According to Isaac, only 10% of the hydrogen bonds in ice are covalent, i.e. with fairly stable information. Hydrogen bonds between the oxygen of one water molecule and the hydrogen of another are most sensitive to external influences. The spectrum of water during the formation of crystals is relatively different in time. According to the effect of discrete evaporation of a water drop proved by Antonov and Yuskeseliyev and its dependence on the energy states of hydrogen bonds, we can look for an answer about the structuring of crystals. Each part of the spectrum depends on the surface tension of the water droplets. There are six peaks in the spectrum, which indicate the ramifications of the snowflake.
Obviously, in Emoto's experiments, the initial "control" sample has an effect on the appearance of the crystals. This means that after exposure to a certain factor, the formation of such crystals can be expected. It is almost impossible to get identical crystals. When testing the effect of the word "love" on water, Emoto does not clearly indicate whether this experiment was carried out with different samples.
Doubly blind experiments are needed to test whether the Emoto technique differentiates sufficiently. Isaac's proof that 10% of water molecules form covalent bonds after freezing shows us what water uses when it freezes. this information. Emoto's achievement, even without double-blind experiments, remains quite important in relation to the informational properties of water.
Natural snowflake, Wilson Bentley, 1925
Emoto snowflake obtained from natural water
One snowflake is natural, and the other is created by Emoto, indicating that the diversity in the water spectrum is not limitless.
Earthquake, Sofia, 4.0 Richter scale, November 15, 2008,
Dr. Ignatov, 2008©, Prof. Antonov's device ©
This figure indicates the difference between the control sample and those taken on other days. Water molecules break the most energetic hydrogen bonds in water, as well as two peaks in the spectrum during a natural phenomenon. The study was carried out using the Antonov device. The biophysical result shows a decrease in the vitality of the body during an earthquake. During an earthquake, water cannot change its structure in the snowflakes in Emoto's lab. There is evidence of a change in the electrical conductivity of water during an earthquake.
In 1963, Tanzanian schoolboy Erasto Mpemba noticed that hot water freezes faster than cold water. This phenomenon is called the Mpemba effect. Although the unique property of water was noticed much earlier by Aristotle, Francis Bacon and Rene Descartes. The phenomenon has been proven many times over by a number of independent experiments. Water has another strange property. In my opinion, the explanation for this is as follows: the differential nonequilibrium energy spectrum (DNES) of boiled water has a lower average energy of hydrogen bonds between water molecules than a sample taken at room temperature This means that boiled water needs less energy in order to begin to structure crystals and freeze.
The key to the structure of ice and its properties lies in the structure of its crystal. Crystals of all modifications of ice are built from water molecules H 2 O, connected by hydrogen bonds into three-dimensional mesh frames with a certain arrangement of hydrogen bonds. The water molecule can be simply imagined as a tetrahedron (pyramid with a triangular base). At its center is an oxygen atom, which is in a state of sp 3 hybridization, and at two vertices - by a hydrogen atom, one of the 1s electrons of which is involved in the formation of a covalent N-About connection with oxygen. The two remaining vertices are occupied by pairs of unpaired oxygen electrons that do not participate in the formation of intramolecular bonds, therefore they are called lone. The spatial shape of the H 2 O molecule is explained by the mutual repulsion of hydrogen atoms and lone electron pairs of the central oxygen atom.
The hydrogen bond is important in the chemistry of intermolecular interactions and is driven by weak electrostatic forces and donor-acceptor interactions. It occurs when the electron-deficient hydrogen atom of one water molecule interacts with the lone electron pair of the oxygen atom of the neighboring water molecule (О-Н…О). Distinctive feature hydrogen bond is relatively low strength; it is 5-10 times weaker than a chemical covalent bond. Hydrogen bonds are intermediate in energy between chemical bond and van der Waals interactions that keep molecules in the solid or liquid phase. Each water molecule in an ice crystal can simultaneously form four hydrogen bonds with other neighboring molecules at strictly defined angles equal to 109 ° 47 "directed to the vertices of the tetrahedron, which do not allow the formation of a dense structure when water freezes (Fig. 3). In ice structures I, Ic, VII and VIII this tetrahedron is regular. In the structures of ice II, III, V and VI, the tetrahedra are noticeably distorted. In the structures of ice VI, VII and VIII, two mutually crossing systems of hydrogen bonds can be distinguished. This invisible framework of hydrogen bonds arranges water molecules in the form of a grid, the structure resembling a hexagonal honeycomb with hollow internal channels.If the ice is heated, the grid structure is destroyed: water molecules begin to fall into the voids of the grid, leading to a denser structure of the liquid - this explains why water is heavier than ice.
Rice. 3. The formation of a hydrogen bond between four H 2 O molecules (red balls indicate central oxygen atoms, white balls indicate hydrogen atoms)
The specificity of hydrogen bonds and intermolecular interactions, characteristic of the structure of ice, is preserved in melt water, since only 15% of all hydrogen bonds are destroyed during the melting of an ice crystal. Therefore, the bond inherent in ice of each water molecule with four neighboring ones ("short range order") is not violated, although a greater blurring of the oxygen frame lattice is observed. Hydrogen bonds can also be retained when water boils. Hydrogen bonds are absent only in water vapor.
Ice, which forms at atmospheric pressure and melts at 0 ° C, is the most familiar, but still not fully understood substance. Much in its structure and properties looks unusual. At the nodes of the crystal lattice of ice, the oxygen atoms of the tetrahedra of water molecules are arranged in an orderly manner, forming regular hexagons, like a hexagonal honeycomb, and hydrogen atoms occupy various positions on the hydrogen bonds connecting the oxygen atoms (Fig. 4). Therefore, there are six equivalent orientations of water molecules relative to their neighbors. Some of them are excluded, since the presence of two protons on the same hydrogen bond at the same time is unlikely, but there remains a sufficient uncertainty in the orientation of water molecules. This behavior of atoms is atypical, since in a solid matter all atoms obey the same law: either they are atoms arranged in an orderly manner, and then it is a crystal, or randomly, and then it is an amorphous substance. Such an unusual structure can be realized in most modifications of ice - Ih, III, V, VI, and VII (and, apparently, in Ic) (Table 3), and in the structure of ice II, VIII, and IX, water molecules are orientationally ordered. According to J. Bernal, ice is crystalline in relation to oxygen atoms and glassy in relation to hydrogen atoms.
Rice. 4. Structure of ice of natural hexagonal configuration I h
Under other conditions, for example, in space at high pressures and low temperatures, ice crystallizes differently, forming other crystal lattices and modifications (cubic, trigonal, tetragonal, monoclinic, etc.), each of which has its own structure and crystal lattice (Table 3). ). The structures of ice of various modifications were calculated by Russian researchers, Doctor of Chemical Sciences. G.G. Malenkov and Ph.D. E.A. Zheligovskaya from the Institute of Physical Chemistry and Electrochemistry. A.N. Frumkin of the Russian Academy of Sciences. Ice modifications II, III and V remain for a long time at atmospheric pressure if the temperature does not exceed -170 °C (Fig. 5). When cooled to approximately -150 ° C, natural ice turns into cubic ice Ic, consisting of cubes and octahedrons a few nanometers in size. Ice I c sometimes also appears when water freezes in capillaries, which is apparently facilitated by the interaction of water with the wall material and the repetition of its structure. If the temperature is slightly higher than -110 0 C, crystals of denser and heavier glassy amorphous ice with a density of 0.93 g/cm 3 are formed on the metal substrate. Both of these forms of ice can spontaneously transform into hexagonal ice, and the faster, the higher the temperature.
Tab. 3. Some modifications of ice and their physical parameters.
Note. 1 Å = 10 -10 m
Rice. 5. State diagram of crystalline ices of various modifications.
There are also high-pressure ices - II and III of trigonal and tetragonal modifications, formed from hollow acres formed by hexagonal corrugated elements shifted relative to each other by one third (Fig. 6 and Fig. 7). These ices are stabilized in the presence of the noble gases helium and argon. In the structure of ice V of the monoclinic modification, the angles between neighboring oxygen atoms range from 860 to 132°, which is very different from the bond angle in the water molecule, which is 105°47'. Ice VI of the tetragonal modification consists of two frames inserted into each other, between which there are no hydrogen bonds, as a result of which a body-centered crystal lattice is formed (Fig. 8). The structure of ice VI is based on hexamers - blocks of six water molecules. Their configuration exactly repeats the structure of a stable water cluster, which is given by the calculations. Ices VII and VIII of the cubic modification, which are low-temperature ordered forms of ice VII, have a similar structure with frames of ice I inserted into each other. With a subsequent increase in pressure, the distance between the oxygen atoms in the crystal lattice of ices VII and VIII will decrease, as a result, the structure of ice X is formed, in which the oxygen atoms are arranged in a regular lattice, and the protons are ordered.
Rice. 7. Ice of III configuration.
Ice XI is formed by deep cooling of ice I h with the addition of alkali below 72 K at normal pressure. Under these conditions, hydroxyl crystal defects are formed, allowing the growing ice crystal to change its structure. Ice XI has a rhombic crystal lattice with an ordered arrangement of protons and is formed at once in many crystallization centers near the hydroxyl defects of the crystal.
Rice. eight. Ice VI configuration.
Among the ices, there are also metastable forms IV and XII, whose lifetimes are seconds, which have the most beautiful structure (Fig. 9 and Fig. 10). To obtain metastable ice, it is necessary to compress ice I h to a pressure of 1.8 GPa at liquid nitrogen temperature. These ices form much more easily and are especially stable when supercooled heavy water is subjected to pressure. Another metastable modification - ice IX is formed during supercooling Ice III and essentially represents its low-temperature form.
Rice. 9. Ice IV-configuration.
Rice. 10. Ice XII configuration.
The last two modifications of ice - with monoclinic XIII and rhombic configuration XIV were discovered by scientists from Oxford (Great Britain) quite recently - in 2006. The assumption that ice crystals with monoclinic and rhombic lattices should exist was difficult to confirm: the viscosity of water at a temperature of -160 ° C is very high, and it is difficult for molecules of pure supercooled water to come together in such an amount that a crystal nucleus is formed. This was achieved with the help of a catalyst - hydrochloric acid, which increased the mobility of water molecules at low temperatures. On Earth, such modifications of ice cannot form, but they can exist in space on cooled planets and frozen satellites and comets. Thus, the calculation of the density and heat fluxes from the surface of the satellites of Jupiter and Saturn allows us to assert that Ganymede and Callisto should have an ice shell in which ices I, III, V and VI alternate. At Titan, ice forms not a crust, but a mantle, the inner layer of which consists of ice VI, other high-pressure ices and clathrate hydrates, and ice I h is located on top.
Rice. eleven. Variety and shape of snowflakes in nature
High in the Earth's atmosphere at low temperatures, water crystallizes from tetrahedra, forming hexagonal ice I h . The center of formation of ice crystals is solid dust particles that are lifted into the upper atmosphere by the wind. Around this embryonic microcrystal of ice, needles formed by individual water molecules grow in six symmetrical directions, on which lateral processes - dendrites grow. The temperature and humidity of the air around the snowflake are the same, so initially it is symmetrical in shape. As snowflakes form, they gradually sink into the lower layers of the atmosphere, where temperatures are higher. Here melting occurs and their ideal geometric shape is distorted, forming a variety of snowflakes (Fig. 11).
With further melting, the hexagonal structure of ice is destroyed and a mixture of cyclic associates of clusters is formed, as well as from tri-, tetra-, penta-, hexamers of water (Fig. 12) and free water molecules. The study of the structure of the resulting clusters is often significantly difficult, since, according to modern data, water is a mixture of various neutral clusters (H 2 O) n and their charged cluster ions [H 2 O] + n and [H 2 O] - n, which are in dynamic equilibrium between with a lifetime of 10 -11 -10 -12 seconds.
Rice. 12. Possible clusters of water (a-h) composition (H 2 O) n, where n = 5-20.
Clusters are able to interact with each other due to the protruding faces of hydrogen bonds, forming more complex polyhedral structures, such as hexahedron, octahedron, icosahedron, and dodecahedron. Thus, the structure of water is associated with the so-called Platonic solids (tetrahedron, hexahedron, octahedron, icosahedron and dodecahedron), named after the ancient Greek philosopher and geometer Plato who discovered them, the shape of which is determined by the golden ratio (Fig. 13).
Rice. thirteen. Platonic solids, the geometric shape of which is determined by the golden ratio.
The number of vertices (B), faces (G) and edges (P) in any spatial polyhedron is described by the relation:
C + D = P + 2
The ratio of the number of vertices (B) of a regular polyhedron to the number of edges (P) of one of its faces is equal to the ratio of the number of faces (G) of the same polyhedron to the number of edges (P) emerging from one of its vertices. For a tetrahedron, this ratio is 4:3, for a hexahedron (6 faces) and an octahedron (8 faces) - 2:1, and for a dodecahedron (12 faces) and an icosahedron (20 faces) - 4:1.
The structures of polyhedral water clusters calculated by Russian scientists were confirmed using modern methods of analysis: proton magnetic resonance spectroscopy, femtosecond laser spectroscopy, X-ray and neutron diffraction on water crystals. The discovery of water clusters and the ability of water to store information are the two most important discoveries of the 21st millennium. This clearly proves that nature is characterized by symmetry in the form of precise geometric shapes and proportions, characteristic of ice crystals.
LITERATURE.
1. Belyanin V., Romanova E. Life, the water molecule and the golden ratio // Science and Life, 2004, vol. 10, no. 3, p. 23-34.
2. Shumsky P. A., Fundamentals of structural ice science. - Moscow, 1955b p. 113.
3. Mosin O.V., Ignatov I. Awareness of water as a substance of life. // Consciousness and physical reality. 2011, T 16, No. 12, p. 9-22.
4. Petryanov I. V. The most unusual substance in the world. Moscow, Pedagogy, 1981, p. 51-53.
5 Eisenberg D, Kautsman V. Structure and properties of water. - Leningrad, Gidrometeoizdat, 1975, p. 431.
6. Kulsky L. A., Dal V. V., Lenchina L. G. Water is familiar and mysterious. - Kiev, Rodyansk school, 1982, p. 62-64.
7. G. N. Zatsepina, Structure and properties of water. - Moscow, ed. Moscow State University, 1974, p. 125.
8. Antonchenko V. Ya., Davydov N. S., Ilyin V. V. Fundamentals of water physics - Kiev, Naukova Dumka, 1991, p. 167.
9. Simonite T. DNA-like ice "seen" inside carbon nanotubes // New Scientist, V. 12, 2006.
10. Emoto M. Messages of water. Secret codes ice crystals. - Sofia, 2006. p. 96.
11. S. V. Zenin and B. V. Tyaglov, Nature of Hydrophobic Interaction. Occurrence of orientational fields in aqueous solutions // Journal of Physical Chemistry, 1994, V. 68, No. 3, p. 500-503.
12. Pimentel J., McClellan O. Hydrogen connection - Moscow, Nauka, 1964, p. 84-85.
13. Bernal J., Fowler R. Structure of water and ionic solutions // Uspekhi fizicheskikh nauk, 1934, vol. 14, no. 5, p. 587-644.
14. Hobza P., Zahradnik R. Intermolecular complexes: The role of van der Waals systems in physical chemistry and biodisciplines. - Moscow, Mir, 1989, p. 34-36.
15. E. R. Pounder, Physics of Ice, transl. from English. - Moscow, 1967, p. 89.
16. Komarov S. M. Ice patterns of high pressure. // Chemistry and Life, 2007, No. 2, pp. 48-51.
17. E. A. Zheligovskaya and G. G. Malenkov. Crystalline ice // Uspekhi khimii, 2006, No. 75, p. 64.
18. Fletcher N. H. The chemical physics of ice, Cambreage, 1970.
19. Nemukhin A. V. Variety of clusters // Russian Chemical Journal, 1996, vol. 40, no. 2, p. 48-56.
20. Mosin O.V., Ignatov I. Structure of water and physical reality. // Consciousness and physical reality, 2011, vol. 16, no. 9, p. 16-32.
21. Ignatov I. Bioenergetic medicine. The origin of living matter, the memory of water, bioresonance, biophysical fields. - GaiaLibris, Sofia, 2006, p. 93.
Ice- mineral with chem. formula H 2 O is water in the crystalline state.
The chemical composition of ice: H - 11.2%, O - 88.8%. Sometimes contains gaseous and solid mechanical impurities.
In nature, ice is mainly represented by one of several crystalline modifications, stable in the temperature range from 0 to 80°C, with a melting point of 0°C. 10 crystalline modifications of ice and amorphous ice are known. The most studied is ice of the 1st modification - the only modification found in nature. Ice occurs in nature in the form of ice proper (mainland, floating, underground, etc.), as well as in the form of snow, frost, etc.
See also:
STRUCTURE
The crystal structure of ice is similar to the structure: each H 2 0 molecule is surrounded by four molecules closest to it, located at equal distances from it, equal to 2.76Α and located at the vertices of a regular tetrahedron. Due to the low coordination number, the ice structure is openwork, which affects its density (0.917). Ice has a hexagonal spatial lattice and is formed by freezing water at 0°C and atmospheric pressure. The lattice of all crystalline modifications of ice has a tetrahedral structure. Parameters of the unit cell of ice (at t 0°C): a=0.45446 nm, c=0.73670 nm (c is twice the distance between adjacent main planes). As the temperature decreases, they change very little. H 2 0 molecules in the ice lattice are linked by hydrogen bonds. The mobility of hydrogen atoms in the ice lattice is much higher than the mobility of oxygen atoms, due to which the molecules change their neighbors. In the presence of significant vibrational and rotational motions of molecules in the ice lattice, translational jumps of molecules from the site of their spatial connection occur with a violation of further ordering and the formation of dislocations. This explains the manifestation of specific rheological properties in ice, which characterize the relationship between irreversible deformations (flow) of ice and the stresses that caused them (plasticity, viscosity, yield strength, creep, etc.). Due to these circumstances, glaciers flow similarly to highly viscous fluids, and thus natural ice actively participate in the water cycle on Earth. Ice crystals have relatively large sizes(transverse size from fractions of a millimeter to several tens of centimeters). They are characterized by the anisotropy of the viscosity coefficient, the value of which can vary by several orders of magnitude. Crystals are capable of reorientation under the action of loads, which affects their metamorphism and the speed of glacier flow.
PROPERTIES
Ice is colorless. In large clusters, it acquires a bluish tint. Glass luster. Transparent. Has no cleavage. Hardness 1.5. Fragile. Optically positive, refractive index very low (n = 1.310, nm = 1.309). In nature, 14 modifications of ice are known. True, everything, except for the ice that is familiar to us, which crystallizes in the hexagonal syngony and is designated as ice I, is formed under exotic conditions - at very low temperatures (about -110150 0С) and high pressures, when the angles of hydrogen bonds in the water molecule change and systems are formed, other than hexagonal. Such conditions are reminiscent of cosmic conditions and are not found on Earth. For example, at temperatures below -110 ° C, water vapor precipitates on a metal plate in the form of octahedrons and cubes several nanometers in size - this is the so-called cubic ice. If the temperature is slightly above –110 °C, and the vapor concentration is very low, a layer of exceptionally dense amorphous ice forms on the plate.
MORPHOLOGY
Ice is a very common mineral in nature. There are several types of ice in the earth's crust: river, lake, sea, ground, firn and glacier. More often it forms aggregate accumulations of fine-grained grains. Also known are crystalline formations of ice that arise by sublimation, that is, directly from the vapor state. In these cases, the ice has the form of skeletal crystals (snowflakes) and aggregates of skeletal and dendritic growth (cave ice, frost, hoarfrost, and patterns on glass). Large, well-cut crystals are found, but very rarely. N. N. Stulov described ice crystals of the northeastern part of Russia, found at a depth of 55-60 m from the surface, having an isometric and columnar appearance, with the length of the largest crystal being 60 cm and the diameter of its base being 15 cm. forms on ice crystals, only faces of a hexagonal prism (1120), a hexagonal bipyramid (1121), and a pinacoid (0001) were revealed.
Ice stalactites, colloquially called "icicles", are familiar to everyone. With temperature differences of about 0 ° in the autumn-winter seasons, they grow everywhere on the surface of the Earth with slow freezing (crystallization) of flowing and dripping water. They are also common in ice caves.
Ice banks are strips of ice cover from ice crystallizing at the water-air boundary along the edges of reservoirs and fringing the edges of puddles, banks of rivers, lakes, ponds, reservoirs, etc. with the rest of the water area not freezing. With their complete coalescence, a continuous ice cover is formed on the surface of the reservoir.
Ice also forms parallel columnar aggregates in the form of fibrous veinlets in porous soils, and ice antholiths on their surface.
ORIGIN
Ice is formed mainly in water basins when the air temperature drops. At the same time, ice porridge, made up of ice needles, appears on the surface of the water. From below, long ice crystals grow on it, in which the sixth-order symmetry axes are perpendicular to the surface of the crust. The ratios between ice crystals under different conditions of formation are shown in fig. Ice is widespread wherever there is moisture and where the temperature drops below 0 ° C. In some areas, ground ice thaws only to an insignificant depth, below which permafrost begins. These are the so-called permafrost regions; in the areas of distribution of permafrost in the upper layers of the earth's crust, there are so-called underground ice, among which modern and fossil underground ice are distinguished. At least 10% of the entire land area of the Earth is covered by glaciers, the monolithic ice rock that composes them is called glacial ice. Glacial ice is formed mainly from the accumulation of snow as a result of its compaction and transformation. The ice sheet covers about 75% of the area of Greenland and almost all of Antarctica; the largest thickness of glaciers (4330 m) was established near Baird Station (Antarctica). In central Greenland, the thickness of the ice reaches 3200 m.
Ice deposits are well known. In areas with cold long winters and short summers, as well as in high mountainous regions, ice caves with stalactites and stalagmites form, among which the most interesting are Kungurskaya in the Perm region of the Urals, as well as the Dobshine cave in Slovakia.
As a result of freezing sea water sea ice is formed. Characteristic properties of sea ice are salinity and porosity, which determine the range of its density from 0.85 to 0.94 g/cm 3 . Due to such a low density, ice floes rise above the surface of the water by 1/7-1/10 of their thickness. Sea ice begins to melt at temperatures above -2.3°C; it is more elastic and more difficult to break apart than freshwater ice.
APPLICATION
In the late 1980s, the Argonne laboratory developed a technology for the manufacture of ice slurry (Ice Slurry), capable of freely flowing through pipes of various diameters, without gathering into ice buildups, without sticking together and without clogging cooling systems. Salt water suspension consisted of many very small rounded ice crystals. Thanks to this, the mobility of water is preserved and, at the same time, from the point of view of thermal engineering, it is ice, which is 5-7 times more efficient than simple ice. cold water in building cooling systems. In addition, such mixtures are promising for medicine. Animal experiments have shown that the microcrystals of the ice mixture pass perfectly into fairly small blood vessels and do not damage cells. "Icy Blood" lengthens the time during which the victim can be saved. For example, during cardiac arrest, this time lengthens, according to conservative estimates, from 10-15 to 30-45 minutes.
The use of ice as a structural material is widespread in the circumpolar regions for the construction of dwellings - igloos. Ice is part of the Pikerite material proposed by D. Pike, from which it was proposed to make the world's largest aircraft carrier.
Ice (English Ice) - H 2 O
CLASSIFICATION
| Strunz (8th edition) | 4/A.01-10 |
| Nickel-Strunz (10th edition) | 4.AA.05 |
| Dana (8th edition) | 4.1.2.1 |
| Hey's CIM Ref. | 7.1.1 |
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