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 formed 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 highly 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; c – appearance of the 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 noted 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 heat capacity of 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 (like one water molecule), the tetramers can be connected by these external bonds 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 tetramers described above 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 intertetramer bonds are already 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. that the number of internal bonds in the tetramer is 3 times greater 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-metagermanate, b - pyroxene, c - batisite, g-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. Indeed, in inanimate nature, both right-handed and left-handed molecules are found with equal probability.

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 of D- and L-forms (it is called "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 the 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

Introduction

1. The structure of water molecules

2. The structure of water in its three states of aggregation

3. Varieties of water

4. Anomalous properties of water

5. Phase transformations and state diagram of water

6. Models of the structure of water and ice

7. Aggregate types of ice

Conclusion

Bibliography

Introduction

Water is the most important substance on Earth without which no living organism can exist and no biological, chemical reactions, and technological processes can take place.

Water (hydrogen oxide) is an odorless, tasteless and colorless liquid (bluish in thick layers); H 2 O, they say. m. 18.016, the simplest stable connection. hydrogen with oxygen.

Water is one of the most common substances in nature. It covers about 3/4 of the entire earth's surface, forming the basis of the oceans, seas, lakes, rivers, ground waters and swamps. A large amount of water is also in the atmosphere. Plants and living organisms contain 50-96% water in their composition.

Water molecules have been found in interstellar space. Water is part of comets, most of the planets of the solar system and their satellites. The amount of water on the Earth's surface is estimated at 1.39 * 10 18 tons, most of it is contained in the seas and oceans. The amount of fresh water available for use in rivers, lakes, swamps and reservoirs is 2*10 4 tons. 2.5-3.010 16 tons, which is only 0.0004% of the mass of our entire planet.However, this amount is enough to cover the entire surface of the globe with a 53-meter layer, and if all this mass suddenly melted, turning into water, the level of the World Ocean would rise compared to the current one by about 64 meters.), there is about the same amount of groundwater, and only a small part of it is fresh. The atmosphere contains approx. 1.3 * 10 13 tons of water. Water is part of many minerals and rocks (clay, gypsum, etc.), is present in the soil, and is an essential component of all living organisms.

Density H 2 O \u003d 1 g / cm3 (at 3.98 degrees), t pl. = 0 degrees, and t kip = 100 degrees. The heat capacity of water is 4.18 J / (g / K) Mr (H 2 O) \u003d 18 and corresponds to its simplest formula. However, the molecular weight of liquid water, determined by studying its solutions in other solvents, turns out to be higher. This indicates that in liquid water there is an association of molecules, i.e., their combination into more complex aggregates. Water is the only substance in nature that exists on earth in all three states of aggregation: A lot of water is in a gaseous state in the form of vapors in the atmosphere; in the form of huge masses of snow and ice, it lies all year round on the tops of high mountains and in polar countries. In the bowels of the earth there is also water that soaks the soil and rocks.

Climate depends on water. Geophysicists say that the Earth would have cooled down long ago and turned into a lifeless piece of stone, if not for 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 the Earth is protected from cosmic cold by those water molecules that are scattered in the atmosphere - in clouds and in the form of vapors ... you cannot do without water - this is the most important substance on Earth.

Water is a familiar and unusual substance. famous Soviet scientist

Academician I. V. Petryanov called his popular science book about water "the most extraordinary substance in the world." And "Entertaining Physiology", written by B.F. Sergeev, Doctor of Biological Sciences, begins with a chapter on water - "The substance that created our planet."

1. The structure of the water molecule

Of all common liquids, water is the most versatile solvent, the liquid with the highest values ​​of surface tension, dielectric constant, heat of vaporization and the highest (after ammonia) heat of fusion. Unlike most substances, water expands when it freezes at low pressure.

These specific properties of water are associated with the special structure of its molecule. The chemical formula of water H 2 0 is deceptively simple. In the water molecule, the nuclei of hydrogen atoms are located asymmetrically with respect to the nucleus of the oxygen atom and electrons. If the oxygen atom is in the center of the tetrahedron, the centers of mass of the two hydrogen atoms will be at the corners of the tetrahedron, and the charge centers of the two pairs of electrons will occupy the other two corners (Fig. 1.1). Thus, four electrons are located at the greatest possible distance both from the nucleus of the oxygen atom and from the nuclei of the hydrogen atoms, at which they are still attracted by the nucleus of the oxygen atom. The other six electrons of the water molecule are located as follows: four electrons are in a position that provides a chemical bond between the nuclei of oxygen and hydrogen atoms, and the other two are located near the nucleus of the oxygen atom.

The asymmetric arrangement of the atoms of the water molecule causes an uneven distribution of electric charges in it, which makes the water molecule polar. This structure of the water molecule causes the attraction of water molecules to each other as a result of the formation of hydrogen bonds between them. The arrangement of hydrogen and oxygen atoms inside the formed aggregates of water molecules is similar to the arrangement of silicon and oxygen atoms in quartz. This applies to ice and, to a lesser extent, to liquid water, the aggregates of molecules of which are always in the stage of redistribution. When water is cooled, its molecules are grouped into aggregates, which gradually increase and become more and more stable as they approach a temperature of 4 ° C, when water reaches its maximum density. At this temperature, water does not yet have a rigid structure and, along with long chains of its molecules, there are a large number of individual water molecules. With further cooling, the chains of water molecules grow due to the addition of free molecules to them, as a result of which the density of water decreases. When water turns into ice, all its molecules enter into a more or less rigid structure in the form of open chains that form crystals.

Fig.1.1 The structure of the water molecule

Mutual penetration of hydrogen and oxygen atoms. The nuclei of two hydrogen atoms and two pairs of electrons are located at the corners of the tetrahedron: the nucleus of the oxygen atom is located in the center.

The high values ​​of surface tension and heat of vaporization of water are explained by the fact that a relatively large expenditure of energy is required to separate a water molecule from a group of molecules. The tendency of water molecules to establish hydrogen bonds and their polarity explain the unusually high dissolving power of water. Some compounds, such as sugars and alcohols, are held in solution by hydrogen bonds. Compounds with a high degree of ionization in water, such as sodium chloride, are held in solution due to the fact that ions with opposite charges are neutralized by groups of oriented water molecules.

Another feature of the water molecule is that both hydrogen and oxygen atoms can have different masses for the same nuclear charge. Varieties of a chemical element with different atomic weights are called isotopes of that element. The water molecule is usually formed by hydrogen with an atomic weight of 1 (H 1) and oxygen with an atomic weight of 16 (O 16). More than 99% of water atoms belong to these isotopes. In addition, there are the following isotopes: H 2, H 3, O 14, O 15, O 17 O 18, O 19. Many of them accumulate in water as a result of its partial evaporation and due to their large mass. Isotopes H 3 , O 14 , O 15 , O 19 are radioactive. The most common of these is tritium H 3 , which is formed in the upper atmosphere under the influence of cosmic rays. This isotope has also accumulated as a result of nuclear explosions over the past few years. Based on these and other facts about isotopes, by analyzing the isotopic composition of water, it is possible to partially reveal the history of some natural waters. Thus, the content of heavy isotopes in surface waters indicates a long-term evaporation of water, which occurs, for example, in the Dead Sea, the Great Salt Lake and in other endorheic reservoirs. Elevated levels of tritium in groundwater could mean that these waters are of meteoric origin with a high circulation rate, because the half-life of this isotope is only 12.4 years. Unfortunately, isotope analysis is too expensive and, for this reason, cannot be widely used in studies of natural waters.

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 positive partial 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 unique spatial structure polymers; 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.

Like most substances, water is made up of molecules, and the latter of atoms.

Of the 14 known forms of solid water in nature, we meet only one - ice. The rest are formed under extreme conditions and are not available for observations outside special laboratories. The most intriguing 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 firn on a snowy field, or giant glacial masses.

In the small Japanese city of Kaga, located on the western coast of the island of Honshu, there is an unusual museum. Snow and ice. It was founded by Ukihiro Nakaya - the first person who learned how to grow artificial snowflakes in the laboratory, as beautiful as those that fall from the sky. In this museum, regular hexagons surround visitors from all sides, because it is precisely this - hexagonal - symmetry that is characteristic of ordinary ice crystals (by the way, the Greek word kristallos, in fact, means "ice"). It determines many of its unique properties and causes snowflakes, with all their endless variety, to grow in the form of stars with six, less often three or twelve rays, but never four or five.

Molecules in openwork

The clue to the structure of solid water lies in the structure of its molecule. H2O can be simply imagined as a tetrahedron (a pyramid with a triangular base). In the center is oxygen, in two vertices - by hydrogen, more precisely - by the proton, the electrons of which are involved in the formation of a covalent bond with oxygen. The two remaining vertices are occupied by pairs of valence electrons of oxygen, which do not participate in the formation of intramolecular bonds, which is why they are called lone.

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, oxygen atoms are arranged in an orderly manner, forming regular hexagons, but hydrogen atoms occupy a variety of positions along the bonds. This behavior of atoms is generally atypical - as a rule, in a solid matter, everyone obeys the same law: either all atoms are ordered, and then it is a crystal, or randomly, and then it is an amorphous substance.

The "strangeness" of ice also includes the generation of electromagnetic radiation by its growing crystals. It has long been known that most of the impurities dissolved in water are not transferred to ice when it begins to grow, in other words, it freezes out. Therefore, even on the dirtiest puddle, the ice film is clean and transparent. 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 the rate at which the ice freezes and what kind of ice structure is obtained.

Wrong ice

In the solid state, water has, according to the latest data, 14 structural modifications. There are among them crystalline (they are the majority), there are amorphous ones, but they all differ from each other in the mutual arrangement of water molecules and properties. True, everything, except for the ice that is familiar to us, is formed under exotic conditions - at very low temperatures and high pressures, when the angles of hydrogen bonds in a water molecule change and systems other than hexagonal are formed. For example, at temperatures below -110°C, water vapor precipitates on a metal plate in the form of octahedrons and cubes a few 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.

The last two modifications of ice - XIII and XIV - were discovered by scientists from Oxford quite recently, in 2006. The prediction 40 years ago 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 molecules of ultrapure supercooled water come together in such an amount that a crystal nucleus is formed, difficult. Helped catalyst - hydrochloric acid, which increased the mobility of water molecules at low temperatures. In terrestrial nature, such modifications of ice cannot form, but they can be searched for on the frozen satellites of other planets.

A snowflake is a single crystal of ice, a variation on the theme of a hexagonal crystal, but grown quickly, in non-equilibrium conditions. The most inquisitive minds have been wrestling with the secret of their beauty and endless variety for centuries. Astronomer Johannes Kepler in 1611 wrote a whole treatise "On hexagonal snowflakes". In 1665, Robert Hooke, in a huge volume of sketches of everything he saw with a microscope, published many drawings of snowflakes of various shapes. The first successful photograph of a snowflake under a microscope was taken in 1885 by American farmer Wilson Bentley. Since then, he has not been able to stop. Until the end of his life, for more than forty years, Bentley photographed them. More than five thousand crystals, and none of them are the same.

The most famous followers of the Bentley case are the already mentioned Ukihiro Nakaya and the American physicist Kenneth Libbrecht. Nakaya was the first to suggest that the size and shape of snowflakes depend on air temperature and moisture content, and brilliantly confirmed this hypothesis experimentally, growing ice crystals of various shapes in the laboratory. And Libbrecht at home began to grow snowflakes to order - a predetermined shape.

The life of a snowflake begins with the formation of crystalline ice nuclei in a cloud of water vapor as the temperature drops. The center of crystallization can be dust particles, any solid particles or even ions, but in any case, these pieces of ice less than a tenth of a millimeter in size already have a hexagonal crystal lattice.

Water vapor, condensing on the surface of these nuclei, first forms a tiny hexagonal prism, from the six corners of which identical ice needles begin to grow - lateral processes. They are the same simply because the temperature and humidity around the embryo are also the same. On them, in turn, grow, like on a tree, lateral processes - twigs. Such crystals are called dendrites, that is, similar to a tree.

Moving up and down in the cloud, the snowflake enters conditions with different temperatures and water vapor concentrations. Its shape changes, to the last obeying the laws of hexagonal symmetry. So snowflakes become different. Although theoretically in the same cloud at the same height they can "originate" the same. But each has its own path to the ground, quite a long one - on average, a snowflake falls at a speed of 0.9 km per hour. So, each has its own story and its own final form. The ice that forms a snowflake is transparent, but when there are many of them, sunlight, reflecting and scattering on numerous faces, gives us the impression of a white opaque mass - we call it snow.

The growth of hoarfrost, frost and patterns on glass obeys the same laws. These phenomena, like snowflakes, are formed by condensation, molecule by molecule - on the ground, grass, trees. Patterns on the window appear in frost, when moisture from warm room air condenses on the surface of the glass. But hailstones are obtained when water drops solidify or when, in clouds saturated with water vapor, ice freezes in dense layers on the embryos of snowflakes. Other, already formed snowflakes can freeze on the hailstones, melting with them, thanks to which the hailstones take on the most bizarre shapes.

On Earth, one solid modification of water is enough for us - ordinary ice. It literally permeates all areas of human habitation or stay. Collecting in huge quantities, snow and ice form special structures with fundamentally different properties than individual crystals or snowflakes. Mountain glaciers, ice covers of water areas, permafrost, and just seasonal snow cover significantly affect the climate of large regions and the planet as a whole: even those who have never seen snow feel the breath of its masses accumulated at the poles of the Earth, for example, in the form of long-term fluctuations in the level of the World Ocean. And ice is so important for the appearance of our planet and the comfortable habitation of living beings on it that scientists have assigned a special environment for it - the cryosphere, which extends its possessions high into the atmosphere and deep into the earth's crust.

Olga Maksimenko, Candidate of Chemical Sciences

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:

1. 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.
2. The state of a vapor in which the molecular structure (hydrol) is represented by single molecules between which no hydrogen bonds are formed.
3. 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 the strongest, and therefore their motor activity is low (it ensures the constancy of the ice shape).