Type of ice crystal lattice. Hexagonal tyranny

It is not individual atoms or molecules that enter into chemical interactions, but substances.

Our task is to get acquainted with the structure of matter.

At low temperatures, substances are in a stable solid state.

☼ The hardest substance in nature is diamond. He is considered the king of all gems and precious stones. And its name itself means “indestructible” in Greek. Diamonds have long been looked upon as miraculous stones. It was believed that a person wearing diamonds does not know stomach diseases, is not affected by poison, retains his memory and a cheerful mood until old age, and enjoys royal favor.

☼ A diamond that has been subjected to jewelry processing - cutting, polishing - is called a diamond.

When melting as a result of thermal vibrations, the order of the particles is disrupted, they become mobile, while the nature of the chemical bond is not disrupted. Thus, there are no fundamental differences between solid and liquid states.

The liquid acquires fluidity (i.e., the ability to take the shape of a vessel).

Liquid crystals

Liquid crystals were discovered at the end of the 19th century, but have been studied in the last 20-25 years. Many display devices of modern technology, for example, some electronic watches and mini-computers, operate on liquid crystals.

In general, the words “liquid crystals” sound no less unusual than “hot ice”. However, in reality, ice can also be hot, because... at a pressure of more than 10,000 atm. water ice melts at temperatures above 200 0 C. The unusualness of the combination “liquid crystals” is that the liquid state indicates the mobility of the structure, and the crystal implies strict ordering.

If a substance consists of polyatomic molecules of an elongated or lamellar shape and having an asymmetrical structure, then when it melts, these molecules are oriented in a certain way relative to each other (their long axes are parallel). In this case, the molecules can move freely parallel to themselves, i.e. the system acquires the property of fluidity characteristic of a liquid. At the same time, the system retains an ordered structure, which determines the properties characteristic of crystals.

The high mobility of such a structure makes it possible to control it through very weak influences (thermal, electrical, etc.), i.e. purposefully change the properties of a substance, including optical ones, with very little energy expenditure, which is what is used in modern technology.

Types of crystal lattices

Any chemical substance is formed by a large number of identical particles that are interconnected.

At low temperatures, when thermal movement is difficult, the particles are strictly oriented in space and form crystal lattice.

Crystal cell - This structure with a geometrically correct arrangement of particles in space.

In the crystal lattice itself, nodes and internodal space are distinguished.

The same substance depending on the conditions (p, t,...)exists in various crystalline forms (i.e. they have different crystal lattices) - allotropic modifications that differ in properties.

For example, four modifications of carbon are known: graphite, diamond, carbyne and lonsdaleite.

☼ The fourth variety of crystalline carbon, “lonsdaleite,” is little known. It was discovered in meteorites and obtained artificially, and its structure is still being studied.

☼ Soot, coke, and charcoal were classified as amorphous carbon polymers. However, it has now become known that these are also crystalline substances.

☼ By the way, shiny black particles were found in the soot, which were called “mirror carbon.” Mirror carbon is chemically inert, heat-resistant, impervious to gases and liquids, has a smooth surface and is absolutely compatible with living tissues.

☼ The name graphite comes from the Italian “graffito” - I write, I draw. Graphite is dark gray crystals with a weak metallic luster and has a layered lattice. Individual layers of atoms in a graphite crystal, connected to each other relatively weakly, are easily separated from each other.

TYPES OF CRYSTAL LATTICES

	ionic			metal
What is in the nodes of the crystal lattice, structural unit	ions	atoms	molecules	atoms and cations
Type of chemical bond between particles of the node	ionic		covalent: polar and non-polar	metal
Interaction forces between crystal particles	electrostatic logical	covalent	intermolecular- new	electrostatic logical
Physical properties due to the crystal lattice	· the attractive forces between ions are strong, · T pl. (refractory), · easily dissolves in water, · melt and solution conducts electric current, non-volatile (no odor)	· covalent bonds between atoms are large, · T pl. and T kip is very, · do not dissolve in water, · the melt does not conduct electric current	· the forces of attraction between molecules are small, · T pl. ↓, some are soluble in water, · have a volatile odor	· interaction forces are large, · T pl. , High heat and electrical conductivity
Aggregate state of a substance under normal conditions	hard	hard	hard, gaseous liquid	hard, liquid(N g)
Examples	most salts, alkalis, typical metal oxides	C (diamond, graphite), Si, Ge, B, SiO 2, CaC 2, SiC (carborundum), BN, Fe 3 C, TaC (t pl. =3800 0 C) Red and black phosphorus. Oxides of some metals.	all gases, liquids, most non-metals: inert gases, halogens, H 2, N 2, O 2, O 3, P 4 (white), S 8. Hydrogen compounds of non-metals, oxides of non-metals: H 2 O, CO 2 "dry ice". Most organic compounds.	Metals, alloys

If the rate of crystal growth is low upon cooling, a glassy state (amorphous) is formed.

The relationship between the position of an element in the Periodic Table and the crystal lattice of its simple substance.

There is a close relationship between the position of an element in the periodic table and the crystal lattice of its corresponding elemental substance.

		group
			III			VII	VIII
P e R And O d						H 2
				N 2	O2	F 2
	III			P 4	S 8	Cl2
						BR 2
						I 2
Type crystal lattice		metal			atomic	molecular

The simple substances of the remaining elements have a metallic crystal lattice.

FIXING

Study the lecture material and answer the following questions in writing in your notebook:

What is a crystal lattice?
What types of crystal lattices exist?
Characterize each type of crystal lattice according to the plan: What is in the nodes of the crystal lattice, structural unit → Type of chemical bond between the particles of the node → Interaction forces between the particles of the crystal → Physical properties due to the crystal lattice → Aggregate state of the substance under normal conditions → Examples

Complete tasks on this topic:

What type of crystal lattice does the following substances widely used in everyday life have: water, acetic acid (CH 3 COOH), sugar (C 12 H 22 O 11), potassium fertilizer (KCl), river sand (SiO 2) - melting point 1710 0 C , ammonia (NH 3), table salt? Make a general conclusion: by what properties of a substance can one determine the type of its crystal lattice?
Using the formulas of the given substances: SiC, CS 2, NaBr, C 2 H 2 - determine the type of crystal lattice (ionic, molecular) of each compound and, based on this, describe the physical properties of each of the four substances.
Trainer No. 1. "Crystal lattices"
Trainer No. 2. "Test tasks"
Test (self-control):

1) Substances that have a molecular crystal lattice, as a rule:

a). refractory and highly soluble in water
b). fusible and volatile
V). Solid and electrically conductive
G). Thermally conductive and plastic

2) The concept of “molecule” not applicable in relation to the structural unit of a substance:

a). water

b). oxygen

V). diamond

G). ozone

3) The atomic crystal lattice is characteristic of:

a). aluminum and graphite

b). sulfur and iodine

V). silicon oxide and sodium chloride

G). diamond and boron

4) If a substance is highly soluble in water, has a high melting point, and is electrically conductive, then its crystal lattice is:

A). molecular

b). atomic

V). ionic

G). metal

Water is a familiar and unusual substance. Almost 3/4 of the surface of our planet is occupied by oceans and seas. Hard water - snow and ice - covers 20% of the land. The climate of the planet 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. It has a very high heat capacity. When heated, it absorbs heat; cooling down, he gives it away. Earth's water both absorbs and returns a lot of heat and thereby “evens out” the climate. And what protects the Earth from cosmic cold are those water molecules that are scattered in the atmosphere - in clouds and in the form of vapor.

Water is the most mysterious substance in nature after DNA, possessing unique properties that not only have not yet been fully explained, but are far from all known. The longer it is studied, the more new anomalies and mysteries are found in it. Most of these anomalies that make life possible 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.

A water molecule (H 2 16 O) consists of two hydrogen atoms (H) and one oxygen atom (16 O). It turns out that almost the entire variety of properties of water and the unusualness of their manifestation are determined, ultimately, by the physical nature of these atoms, the way they are combined into a molecule and the grouping of the resulting molecules.

Rice. Structure of a water molecule . Geometric diagram (a), flat model (b) and spatial electronic structure (c) of the H2O monomer. Two of the four electrons in the outer shell of the oxygen atom are involved in creating 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 the oxygen, the electrical charges in it are dispersed. 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. This chemical bond is called a hydrogen bond. It unites H 2 O molecules into unique associates of 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 abnormally high temperatures of its melting and boiling. Additional energy must be supplied to loosen and then destroy hydrogen bonds. And this energy is very significant. This is why the heat capacity of water is so high.

A water molecule contains two polar covalent bonds H–O. 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, a 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 - non-bonding. They form an electron cloud. The cloud is heterogeneous - individual concentrations and rarefactions can be distinguished in it.

A water molecule has four pole charges: two positive and two negative. Positive charges are concentrated on hydrogen atoms, since oxygen is more electronegative than hydrogen. The two negative poles come from two non-bonding electron pairs of oxygen.

An excess electron density is created at the oxygen core. The internal electron pair of oxygen evenly frames the nucleus: schematically it is represented by a circle with the center - the O 2- nucleus. The four outer electrons are grouped into two electron pairs that gravitate towards the nucleus, but are partially not compensated. Schematically, the total electron orbitals of these pairs are shown in the form of ellipses elongated from a common center - the O 2- nucleus. Each of the remaining two electrons in oxygen pairs with one electron in hydrogen. These vapors also gravitate towards the oxygen core. Therefore, hydrogen nuclei - protons - turn out to be somewhat bare, and a lack of electron density is observed here.

Thus, in a water molecule there are four charge poles: two negative (excess electron density in the region of the oxygen nucleus) and two positive (lack of electron density in the two hydrogen nuclei). For greater clarity, we can imagine that the poles occupy the vertices of a deformed tetrahedron, in the center of which there is an oxygen nucleus.

Rice. Structure of a water molecule: a – angle between O-H bonds; b – location of charge poles; c – appearance of the electron cloud of a water molecule.

The almost spherical water molecule has a noticeably pronounced polarity, since the electrical charges in it are located asymmetrically. Each water molecule is a miniature dipole with a high dipole moment of 1.87 deBy. 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 are weakened by 80 times. In other words, water has a high dielectric constant, the highest of all compounds 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 of 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 the melting process, and for substances made of polyatomic molecules it, as a rule, decreases during melting.

Such an understanding of the structure of the molecule makes it possible to explain many properties of water, in particular the structure of ice. In the ice crystal lattice, each molecule is surrounded by four others. In a planar image, this can be represented as follows:

The connection 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. This bond is called a hydrogen bond (it is designated by dots). The strength of a hydrogen bond is approximately 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, 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 movement of molecules in water, some hydrogen bonds are broken and others are formed.

Rice. Crystal lattice of ice. The water molecules H 2 O (black balls) in its nodes are located so that each has four “neighbors”.

The polarity of water molecules and the presence of partially uncompensated electrical charges in them gives rise to a tendency to group molecules into large “communities” - associates. It turns out that only water in a vapor state fully corresponds to the formula H2O. This was shown by the results of determining the molecular mass of water vapor. In the temperature range from 0 to 100°C, the concentration of individual (monomeric molecules) of 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 direct cause of the formation of associates is hydrogen bonds between water molecules. They arise between the hydrogen nuclei of some molecules and the electron “condensations” of the oxygen nuclei of other water molecules. True, these bonds are tens of times weaker than “standard” intramolecular chemical bonds, and ordinary molecular movements are enough to destroy them. But under the influence of thermal vibrations, new connections of this type just as easily arise. The emergence and decay of associates can be expressed by the following diagram:

x·H 2 O↔ (H 2 O) x

Since the electron orbitals in each water molecule form a tetrahedral structure, hydrogen bonds can arrange the arrangement of water molecules into tetrahedral coordinated associates.

Most researchers explain the anomalously high heat capacity of liquid water by the fact that when ice melts, its crystalline structure does not immediately collapse. In liquid water, hydrogen bonds between molecules are preserved. What remains in it are fragments of ice - associates of a large or smaller number of water molecules. However, unlike ice, each associate does not exist for long. The destruction of some and the formation of other associates constantly occurs. 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 heating such melts, energy is spent only on imparting thermal movements to their atoms or molecules. Hydrogen bonds between water molecules are completely broken only when water turns into steam. The correctness of this point of view is also indicated by the fact that the specific heat capacity of water vapor at 100°C practically coincides with the specific heat capacity of ice at 0°C.

Picture below:

The elementary structural element of an associate is a cluster: Rice. A separate hypothetical water cluster. Individual 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, amounting to 18 cal/(molgrad), is exactly equal to the theoretical molar heat capacity of a solid 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, the grammol of which contains 3 N a crystal lattice sites, it is 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, what is surprising is not the high heat capacity of liquid water, but the low heat capacity of solid ice. The decrease in the specific heat capacity 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 the discoverers of hydrogen bonds, J. Bernal and R. Fowler, in 1932 compared the structure of liquid water with the crystalline structure of quartz, and those associates mentioned above are mainly 4H 2 0 tetramers, in which there are four molecules waters are connected into a compact tetrahedron with twelve internal hydrogen bonds. As a result, a tetrahedron is formed.

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- and left-handed rotational crystal 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 3 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 determines the fluidity of water.

Water, according to Kolyasnikov, has this structure only in the liquid state and, possibly, partially in the vapor state. But in ice, the crystal structure of which has been well studied, tetrahydrols are connected to each other by inflexible, equally strong 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 ice melts, some of the hydrogen bonds in it weaken and bend, which leads to a restructuring of the structure into the above-described tetramers and makes liquid water more dense than ice. At 4°C, a state occurs when all hydrogen bonds between tetramers are maximally bent, which determines the maximum density of water at this temperature. There is nowhere for connections to go any further.

At temperatures above 4°C, individual bonds between tetramers begin to break, and at 36-37°C, half of the external hydrogen bonds are broken. This determines the minimum on the curve of the specific heat capacity of water versus temperature. At a temperature of 70°C, almost all intertetramer bonds are broken, and along with free tetramers, only short fragments of “polymer” chains of them remain in water. Finally, when water boils, the final rupture of now single tetramers into individual H 2 0 molecules 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 confirms Kolyasnikov’s assumption About. that the number of internal bonds in a tetramer is 3 times greater than the number of external ones.

This tetrahedral-helical structure of water may be due to its ancient rheological connection with quartz and other silicon-oxygen minerals that predominate in the earth's crust, from the depths of which water once appeared on Earth. Just as a small crystal of salt causes the solution surrounding it to crystallize into similar crystals, and not into others, so quartz caused water molecules to line up in tetrahedral structures, which are energetically most favorable. And in our era, in the earth’s atmosphere, water vapor, condensing into droplets, forms such a structure because the atmosphere always contains tiny droplets of aerosol water that already have this structure. They are centers of condensation of water vapor in the atmosphere. Below are possible chain silicate structures based on the tetrahedron, which can also be composed of water tetrahedra.

Rice. Elementary regular silicon-oxygen tetrahedron SiO 4 4-.

Rice. Elementary silicon-oxygen units-orthogroups SiO 4 4- in the structure of Mg-pyroxene enstatite (a) and diortho groups Si 2 O 7 6- in the 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, d-Si 4 O 12, d-Si 6 O 18.

Rice. below - The most important types of silicon-oxygen chain anionic groups (according to Belov): a-metagermanate, b - pyroxene, c - bathysite, d-wollastonite, d-vlasovite, e-melilite, f-rhodonite, z-pyroxmangite, i-metaphosphate, k - fluoroberyllate, l - barylite.

Rice. below - Condensation of pyroxene silicon-oxygen anions into honeycomb two-row amphibole (a), three-row amphibole-like (b), layered talc and related anions (c).

Rice. below - The most important types of banded silicon-oxygen groups (according to Belov): a - sillimanite, amphibole, xonotlite; b-epididymitis; β-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 aluminum-silicon-oxygen networks with polyhedral layers of large aluminum and potassium cations, reminiscent of a DNA chain.

Other models of water structure are also possible. Tetrahedrally bound water molecules form peculiar chains of fairly stable composition. Researchers are uncovering increasingly subtle and complex mechanisms of the “internal organization” of the water mass. In addition to the ice-like structure, liquid water and monomer molecules, a third element of the structure is also described - non-tetrahedral.

A certain part of water molecules are associated not in three-dimensional frameworks, but in linear ring associations. The rings, when grouped, form even more complex complexes of associates.

Thus, water can theoretically form chains, like a DNA molecule, as will be discussed below. Another interesting thing about this hypothesis is that it implies the equal probability 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 spiral. But sugars in nature are all right-handed. No one has yet been able to explain why in living nature 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 asymmetrical - they rotate the plane of polarization of the light incident on them. All amino acids that make up animals and plants rotate the plane of polarization to the left, and all sugars rotate to the right. If we synthesize compounds with the same chemical composition, then each of them will contain an equal number of left- and right-handed molecules.

As you know, all living organisms consist of proteins, and they, in turn, are made of amino acids. By combining with 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 one another, like the left and right hands, so they are called D- and L-molecules (from the Latin dexter, laevus - right and left).

Now let us imagine that a 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 “cheira” - hand) ordered. Self-reproduction of living things (biopoiesis - as defined by D. Bernal) could arise and be maintained only in such an environment.

Rice. Mirror symmetry in nature

Another name for enantiomer molecules - "dextrorotatory" and "levorotatory" - 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, the plane of its polarization rotates: clockwise if the molecules in the solution are right-handed, and counterclockwise if the molecules in the solution are left-handed. And in a mixture of equal amounts of D- and L-forms (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 in laboratory conditions produces approximately the same number of right- and left-handed molecules. It turns out that not only amino acids have this feature, but also many other substances important for living systems, and each has a strictly defined sign of mirror symmetry throughout the biosphere. For example, sugars that are part of many nucleotides, as well as nucleic acids DNA and RNA, are represented in the body exclusively by right-handed D-molecules. Although the physical and chemical properties of the “mirror antipodes” are the same, their physiological activity in organisms is different: L-caxara are 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 still remains one of the biggest mysteries of science.

The Soviet scientist L.L. Morozov proved that the transition to chiral order could not occur evolutionarily, but only with some specific sharp phase change. Academician V.I. Goldansky called this transition, thanks to which life on Earth originated, a chiral catastrophe.

How did the conditions arise for the phase catastrophe that caused the chiral transition?

The most important thing 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 difference reached 1000 °C. Under such conditions, organic molecules melted under the influence of high temperature and were 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 point of the organic layer to move up and down, creating a gradient.

At very low atmospheric pressures, 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 was able to exist in the form of a liquid for the first time.

Of course, only experimentally can one prove what exactly caused the chiral transition: terrestrial or cosmic reasons. But one way or another, at some point, chirally ordered molecules (namely, levorotatory amino acids and dextrorotatory 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 no mountains or depressions on Earth. The semi-molten granitic crust presented a surface as smooth as the level of the modern ocean. However, within this plain there were still depressions due to the uneven distribution of masses within the Earth. These reductions played an extremely important role.

The fact is that flat-bottomed depressions hundreds and even thousands of kilometers across and no more than a hundred meters deep probably became the cradle of life. After all, 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 lifeless to living, which began in anhydrous conditions, continued in an aquatic environment.

Is this the plot of the origin of life? Most likely yes. In the geological section of Isua (Western Greenland), which is 3.8 billion years old, gasoline- and oil-like compounds were found with the C12/C13 isotope ratio characteristic of carbon of photosynthetic origin.

If the biological nature of carbon compounds from the Isua section is confirmed, then it turns 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 one hundred million years. And water molecules and DNA played a huge role in this process.

The most amazing thing about the structure of water is that water molecules at low negative temperatures and high pressures inside nanotubes can crystallize into a double helix shape, reminiscent of DNA. This was proven by computer experiments of American scientists led by Xiao Cheng Zeng at the University of Nebraska (USA).

DNA is a double strand twisted into a spiral. Each thread consists of “bricks” - nucleotides connected in series. Each nucleotide of DNA contains one of 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 . Neighboring nucleotides are connected to each other 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 directions, 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 messenger or template (mRNA), ribosomal (rRNA) and transport (tRNA). All these types of RNA are synthesized on a DNA template by copying a DNA sequence into an RNA sequence synthesized during transcription and take part in the most important process of life - the transfer 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 a DNA chain is written in the form of a letter DNA formula: for example - AGTCATGCCAG, the entry is made 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 DNA secondary structure is the DNA double helix. DNA double helix is the most common form of DNA in nature, consisting of two polynucleotide chains 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 DNA chain corresponds to a strictly defined base of another chain: in a complementary pair, opposite A is T, and opposite G is C, etc.

In order for water to form a spiral, like this, in a simulated experiment it was “placed” in nanotubes under high pressure, varying in different experiments from 10 to 40,000 atmospheres. After this, the temperature was set, which had a value of -23°C. The margin 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 by New Scientist)

Water molecules are connected to each other through 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 layer and one from the neighboring one. As a result, the crystal structure of ice consists of hexagonal “tubes” interconnected like a honeycomb.

Rice. Inner wall of a water structure (New Scientist image)

Scientists expected to see that the 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 were bent, 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 helices, similar to the structure of the DNA molecule.

The latter fact leaves an imprint not only on the evolution of our ideas about water, but also on the evolution of early life and the DNA molecule itself. If we assume that in the era of the origin of life, cryolite clay rocks had the shape of nanotubes, the question arises: could the water sorbed in them serve as a structural basis (matrix) for DNA synthesis and information reading? Perhaps this is why the helical structure of DNA repeats the helical structure of water in nanotubes. As New Scientist magazine reports, now our foreign colleagues will have to confirm the existence of such water macromolecules under real experimental conditions using infrared spectroscopy and neutron scattering spectroscopy.

Ph.D. O.V. Mosin

Ice- mineral with chemical formula H 2 O, represents water in a crystalline state.
Chemical composition of ice: H - 11.2%, O - 88.8%. Sometimes it contains gaseous and solid mechanical impurities.
In nature, ice is represented mainly by one of several crystalline modifications, stable in the temperature range from 0 to 80°C, with a melting point of 0°C. There are 10 known crystalline modifications of ice and amorphous ice. The most studied is ice of the 1st modification - the only modification found in nature. Ice is found in nature in the form of ice itself (continental, floating, underground, etc.), as well as in the form of snow, frost, etc.

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STRUCTURE

The crystal structure of ice is similar to the structure: each H 2 0 molecule is surrounded by the 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 an ice unit cell (at t 0°C): a=0.45446 nm, c=0.73670 nm (c is double the distance between adjacent main planes). When the temperature drops, they change very little. H 2 0 molecules in the ice lattice are connected to each other 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 movements of molecules in the ice lattice, translational jumps of molecules from the site of their spatial connection occur, disrupting further order and forming 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 stress, creep, etc.). Due to these circumstances, glaciers flow similarly to highly viscous liquids, and thus natural ice actively participates in the water cycle on Earth. Ice crystals are relatively large in size (transverse size from fractions of a millimeter to several tens of centimeters). They are characterized by anisotropy of the viscosity coefficient, the value of which can vary by several orders of magnitude. Crystals are capable of reorientation under the influence of loads, which affects their metamorphization and the flow rate of glaciers.

PROPERTIES

Ice is colorless. In large clusters it takes on a bluish tint. Glass shine. Transparent. Has no cleavage. Hardness 1.5. Fragile. Optically positive, refractive index very low (n = 1.310, nm = 1.309). There are 14 known modifications of ice in nature. True, everything except the familiar ice, which crystallizes in the hexagonal system and is designated as ice I, is formed under exotic conditions - at very low temperatures (about -110150 0C) and high pressures, when the angles of hydrogen bonds in the water molecule change and systems are formed, different from hexagonal. Such conditions resemble those in space and do not occur on Earth. For example, at temperatures below –110 °C, water vapor precipitates on a metal plate in the form of octahedra 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 extremely 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 clusters of fine-crystalline grains. Crystalline ice formations are also known that arise by sublimation, that is, directly from the vapor state. In these cases, the ice appears as skeletal crystals (snowflakes) and aggregates of skeletal and dendritic growth (cave ice, hoarfrost, hoarfrost, and patterns on glass). Large well-cut crystals are found, but very rarely. N. N. Stulov described ice crystals in the northeastern part of Russia, found at a depth of 55-60 m from the surface, having an isometric and columnar appearance, and the length of the largest crystal was 60 cm, and the diameter of its base was 15 cm. From simple forms on ice crystals, only the faces of the hexagonal prism (1120), hexagonal bipyramid (1121) and pinacoid (0001) were identified.
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 the slow freezing (crystallization) of flowing and dripping water. They are also common in ice caves.
Ice banks are strips of ice cover made of ice that crystallizes at the water-air boundary along the edges of reservoirs and bordering the edges of puddles, the banks of rivers, lakes, ponds, reservoirs, etc. with the rest of the water space not freezing. When they completely grow together, a continuous ice cover is formed on the surface of the reservoir.
Ice also forms parallel columnar aggregates in the form of fibrous veins in porous soils, and ice antholites on their surface.

ORIGIN

Ice forms mainly in water basins when the air temperature drops. At the same time, an ice porridge composed of ice needles appears on the surface of the water. From below, long ice crystals grow on it, whose sixth-order symmetry axes are located perpendicular to the surface of the crust. The relationships between ice crystals under different formation conditions are shown in Fig. Ice is common wherever there is moisture and where the temperature drops below 0° C. In some areas, ground ice thaws only to a shallow depth, below which permafrost begins. These are the so-called permafrost areas; In areas of permafrost distribution in the upper layers of the earth's crust, so-called underground ice is found, among which modern and fossil underground ice are distinguished. At least 10% of the Earth's total land area is covered by glaciers; the monolithic ice rock that composes them is called glacial ice. Glacial ice is formed primarily from the accumulation of snow as a result of its compaction and transformation. The ice sheet covers about 75% of Greenland and almost all of Antarctica; the largest thickness of glaciers (4330 m) is located near the Byrd station (Antarctica). In central Greenland the ice thickness 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 are formed, among which the most interesting are Kungurskaya in the Perm region of the Urals, as well as the Dobshine cave in Slovakia.
When sea water freezes, sea ice is formed. The 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 . Because of such 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 into pieces than freshwater ice.

APPLICATION

In the late 1980s, the Argonne laboratory developed a technology for making ice slurry that can flow freely through pipes of various diameters without collecting in ice build-ups, sticking together, or clogging cooling systems. The salty water suspension consisted of many very small round-shaped ice crystals. Thanks to this, the mobility of water is maintained and, at the same time, from the point of view of thermal engineering, it represents ice, which is 5-7 times more effective than simple cold water in cooling systems of buildings. In addition, such mixtures are promising for medicine. Experiments on animals have shown that microcrystals of the ice mixture pass perfectly into fairly small blood vessels and do not damage cells. “Icy Blood” extends the time during which the victim can be saved. Let's say, in case of 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 polar regions for the construction of dwellings - igloos. Ice is part of the Pikerit material proposed by D. Pike, from which it was proposed to make the world's largest aircraft carrier.

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

Crystal structure of ice: water molecules are connected in regular hexagons Crystal lattice of ice: Water molecules H 2 O (black balls) in its nodes are arranged so that each has four neighbors. The water molecule (center) is bonded to its four nearest neighboring molecules by hydrogen bonds. Ice is a crystalline modification of water. According to the latest data, ice has 14 structural modifications. Among them there are both crystalline (the majority of them) and amorphous modifications, but they all differ from each other in the relative arrangement of water molecules and properties. True, everything except the familiar ice, which crystallizes in the hexagonal system, is formed under exotic conditions at very low temperatures and high pressures, when the angles of hydrogen bonds in the water molecule change and systems other than hexagonal are formed. Such conditions resemble those in space and do not occur on Earth. For example, at temperatures below –110 °C, water vapor precipitates on a metal plate in the form of octahedra and cubes several nanometers in size—the so-called cubic ice. If the temperature is slightly above –110 °C and the vapor concentration is very low, a layer of extremely dense amorphous ice forms on the plate. The most unusual property of ice is its amazing variety of external manifestations. With the same crystalline structure, it can look completely different, taking the form of transparent hailstones and icicles, flakes of fluffy snow, a dense shiny crust of ice or giant glacial masses.

A snowflake is a single crystal of ice - a type of hexagonal crystal, but one that grew quickly under non-equilibrium conditions. Scientists have been struggling with the secret of their beauty and endless diversity for centuries. 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 it begins grow identical ice needles, lateral shoots, because the temperature and humidity around the embryo are also the same. On them, in turn, lateral shoots of branches grow, like on a tree. Such crystals are called dendrites, that is, similar to wood. Moving up and down in a cloud, a snowflake encounters conditions with different temperatures and concentrations of water vapor. Its shape changes, obeying the laws of hexagonal symmetry to the last. This is how snowflakes become different. Until now, it has not been possible to find two identical snowflakes.

The color of ice depends on its age and can be used to assess its strength. Ocean ice is white in the first year of its life because it is saturated with air bubbles, from the walls of which light is reflected immediately, without having time to be absorbed. In summer, the surface of the ice melts, loses its strength, and under the weight of new layers lying on top, air bubbles shrink and disappear completely. The light inside the ice travels a longer path than before and emerges as a bluish-green hue. Blue ice is older, denser and stronger than white “foamy” ice saturated with air. Polar researchers know this and choose reliable blue and green ice floes for their floating bases, research stations and ice airfields. There are black icebergs. The first press report about them appeared in 1773. The black color of icebergs is caused by the activity of volcanoes - the ice is covered with a thick layer of volcanic dust, which is not washed off even by sea water. Ice is not equally cold. There is very cold ice, with a temperature of about minus 60 degrees, this is the ice of some Antarctic glaciers. The ice of the Greenland glaciers is much warmer. Its temperature is approximately minus 28 degrees. Very “warm ice” (with a temperature of about 0 degrees) lies on the tops of the Alps and Scandinavian mountains.

The density of water is maximum at +4 C and is equal to 1 g/ml; it decreases with decreasing temperature. When water crystallizes, the density decreases sharply; for ice it is equal to 0.91 g/cm3. Due to this, ice is lighter than water and when reservoirs freeze, ice accumulates on top, and at the bottom of reservoirs there is more dense water with a temperature of 4 ̊ C. Poor thermal conductivity of ice and The snow cover covering it protects reservoirs from freezing to the bottom and thereby creates conditions for the life of the inhabitants of reservoirs in winter.

Glaciers, ice sheets, permafrost, and seasonal snow cover significantly influence 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 Earth’s poles, for example, in the form of long-term fluctuations in level World ocean. Ice is so important for the appearance of our planet and the comfortable habitat of living creatures on it that scientists have allocated a special environment for it - the cryosphere, which extends its domain high into the atmosphere and deep into the earth's crust. Natural ice is usually much cleaner than water, because... the solubility of substances (except NH4F) in ice is extremely low. The total ice reserves on Earth are about 30 million km 3. Most of the ice is concentrated in Antarctica, where the thickness of its layer reaches 4 km.

Today we will talk about the properties of snow and ice. It is worth clarifying that ice is formed not only from water. In addition to water ice, there is ammonia and methane ice. Not long ago, scientists invented dry ice. Its properties are unique, we will consider them a little later. It is formed when carbon dioxide freezes. Dry ice got its name due to the fact that when it melts it does not leave puddles. The carbon dioxide contained in it immediately evaporates into the air from its frozen state.

Ice definition

First of all, let's take a closer look at ice, which is obtained from water. There is a regular crystal lattice inside it. Ice is a common natural mineral produced when water freezes. One molecule of this liquid binds to four nearby ones. Scientists have noticed that such an internal structure is inherent in various precious stones and even minerals. For example, diamond, tourmaline, quartz, corundum, beryl and others have this structure. The molecules are held at a distance by a crystal lattice. These properties of water and ice indicate that the density of such ice will be less than the density of the water due to which it was formed. Therefore, ice floats on the surface of the water and does not sink in it.

Millions of square kilometers of ice

Do you know how much ice there is on our planet? According to recent research by scientists, there are approximately 30 million square kilometers of frozen water on planet Earth. As you may have guessed, the bulk of this natural mineral is found on the polar ice caps. In some places the thickness of the ice cover reaches 4 km.

How to get ice

Making ice is not difficult at all. This process is not difficult and does not require any special skills. This requires low water temperature. This is the only constant condition for the ice formation process. Water will freeze when your thermometer shows a temperature below 0 degrees Celsius. The crystallization process begins in water due to low temperatures. Its molecules are built into an interesting ordered structure. This process is called the formation of a crystal lattice. It is the same in the ocean, in a puddle, and even in the freezer.

Research into the freezing process

Conducting research on the topic of water freezing, scientists came to the conclusion that the crystal lattice is built in the upper layers of water. Microscopic ice sticks begin to form on the surface. A little later they freeze together. Thanks to this, a thin film is formed on the surface of the water. Large bodies of water take much longer to freeze compared to still water. This is due to the fact that the wind ripples and ripples the surface of a lake, pond or river.

Ice pancakes

Scientists made another observation. If excitement continues at low temperatures, then the thinnest films are collected into pancakes with a diameter of about 30 cm. Then they freeze into one layer, the thickness of which is at least 10 cm. A new layer of ice freezes on top and bottom of the ice pancakes. This creates a thick and durable ice cover. Its strength depends on the type: the most transparent ice will be several times stronger than white ice. Environmentalists have noticed that 5-centimeter ice can support the weight of an adult. A layer of 10 cm can withstand a passenger car, but it should be remembered that going out on the ice in autumn and spring is very dangerous.

Properties of snow and ice

Physicists and chemists have long studied the properties of ice and water. The most famous and also important property of ice for humans is its ability to easily melt even at zero temperature. But other physical properties of ice are also important for science:

ice is transparent, so it transmits sunlight well;
colorlessness - ice has no color, but it can be easily colored using color additives;
hardness - ice masses perfectly retain their shape without any outer shells;
fluidity is a particular property of ice, inherent in the mineral only in some cases;
fragility - a piece of ice can be easily split without much effort;
cleavage - ice breaks easily in those places where it is fused along a crystallographic line.

Ice: displacement and purity properties

Ice has a high degree of purity in its composition, since the crystal lattice does not leave free space for various foreign molecules. When water freezes, it displaces various impurities that were once dissolved in it. In the same way, you can get purified water at home.

But some substances can slow down the freezing process of water. For example, salt in sea water. Ice in the sea only forms at very low temperatures. Surprisingly, the process of freezing water every year is capable of maintaining self-purification of various impurities for many millions of years in a row.

The secrets of dry ice

The peculiarity of this ice is that it contains carbon in its composition. Such ice forms only at a temperature of -78 degrees, but it melts already at -50 degrees. Dry ice, the properties of which allow you to skip the stage of liquids, immediately produces steam when heated. Dry ice, like its counterpart water ice, has no odor.

Do you know where dry ice is used? Due to its properties, this mineral is used when transporting food and medicine over long distances. And the granules of this ice can extinguish the fire of gasoline. Also, when dry ice melts, it forms a thick fog, which is why it is used on film sets to create special effects. In addition to all of the above, you can take dry ice with you on hikes and in the forest. After all, when it melts, it repels mosquitoes, various pests and rodents.

As for the properties of snow, we can observe this amazing beauty every winter. After all, every snowflake has the shape of a hexagon - this is unchanged. But besides the hexagonal shape, snowflakes can look different. The formation of each of them is influenced by air humidity, atmospheric pressure and other natural factors.

The properties of water, snow, and ice are amazing. It is important to know a few more properties of water. For example, it is able to take the shape of the vessel into which it is poured. When water freezes, it expands and also has memory. It is able to remember the surrounding energy, and when it freezes, it “resets” the information that it has absorbed.

We looked at the natural mineral - ice: properties and its qualities. Continue to study science, it is very important and useful!