Density diagram of the phase state of salt water. Water state diagram and phase rule. Phase diagram of water

Application of the Gibbs phase rule to one-component systems. Water and sulfur state diagrams

For a one-component system TO=1 and the phase rule is written as:

C = 3– F

If F= 1, then WITH=2 , they say that the system bivariant;
F= 2, then WITH=1 , system monovariant;
F= 3, then WITH = 0, system invariant.

The relationship between pressure ( R), temperature ( T) and volume ( V) phases can be represented as three-dimensional phase diagram. Each point (called figurative point) on such a diagram depicts some equilibrium state. It is usually more convenient to work with sections of this diagram by a plane R - T(at V = const) or plane P - V(at T = const). In what follows, we will consider only the case of a section by a plane R - T(at V = const).

The state of water has been studied in a wide range of temperatures and pressures. At high pressures, the existence of at least ten crystalline modifications of ice has been established. The most studied is ice I - the only modification of ice found in nature.

The presence of various modifications of the substance - polymorphism leads to the complication of state diagrams.

Phase diagram of water in coordinates R - T shown in Fig.15. It consists of 3 phase fields- areas of various R, T- values ​​at which water exists in the form of a certain phase - ice, liquid water or steam (indicated in the figure by the letters L, W and P, respectively). These phase fields are separated by 3 boundary curves.

Curve AB - evaporation curve, expresses the dependence vapor pressure of liquid water on temperature(or, conversely, represents the dependence of the boiling point of water on external pressure). In other words, this line corresponds to two-phase equilibrium.

Liquid water ↔ steam, and the number of degrees of freedom calculated from the phase rule is WITH= 3 – 2 = 1. Such an equilibrium is called monovariant. This means that for a complete description of the system, it is sufficient to define only one variable- either temperature or pressure, since for a given temperature there is only one equilibrium pressure and for a given pressure - only one equilibrium temperature.

At pressures and temperatures corresponding to points below the line AB, the liquid will evaporate completely, and this region is the vapor region. To describe a system in a given single-phase region, two independent variables are needed: temperature and pressure ( WITH = 3 – 1 = 2).

At pressures and temperatures corresponding to points above line AB, the vapor is completely condensed into a liquid ( WITH= 2). The upper limit of the AB evaporation curve is at point B, which is called the critical point (for water 374.2ºС and 218.5 atm.). Above this temperature, the liquid and vapor phases become indistinguishable (the liquid/vapor interface disappears), so F = 1.

AC line - this ice sublimation curve (sometimes called the sublimation line), reflecting the dependence water vapor pressure above ice on temperature. This line corresponds to the monovariant equilibrium ice ↔ vapor ( WITH= 1). Above the AC line lies the region of ice, below the region of steam.

Line AD - melting curve, expresses the dependence melting temperature of ice on pressure and corresponds to the monovariant equilibrium ice ↔ liquid water. For most substances, the line AD deviates from the vertical to the right, but the behavior of water is anomalous: liquid water occupies a smaller volume than ice. An increase in pressure will cause a shift in equilibrium towards the formation of liquid, i.e., the freezing point will decrease.

Studies pioneered by Bridgman to determine the course of the ice melting curve at high pressures showed that all existing crystalline modifications of ice, with the exception of the first, are denser than water. Thus, the upper limit of the AD line is point D, where ice I (ordinary ice), ice III, and liquid water coexist in equilibrium. This point is at -22ºС and 2450 atm.

Rice. 15. Phase diagram of water

The example of water shows that the phase diagram does not always have such a simple character, as shown in Fig.15. Water can exist in the form of several solid phases, which differ in their crystal structure (see Fig. 16).

Rice. 16. Expanded phase diagram of water in a wide range of pressure values.

The triple point of water (a point reflecting the balance of three phases - liquid, ice and steam) in the absence of air is at 0.01ºС ( T = 273,16K) and 4.58 mmHg. Number of degrees of freedom WITH= 3-3 = 0 and such an equilibrium is called invariant.

In the presence of air, the three phases are in equilibrium at 1 atm. and 0ºС ( T = 273,15K). The decrease in the triple point in air is caused by the following reasons:

1. The solubility of air in liquid water at 1 atm, which leads to a decrease in the triple point by 0.0024ºС;

2. Increasing pressure from 4.58 mmHg. up to 1 atm, which reduces the triple point by another 0.0075ºС.

Water states.

Water can be in three states of aggregation, or phases - solid (ice), liquid (water itself), gaseous (steam). It is very important that with the ranges of atmospheric pressure and temperature that really exist on Earth, water can be simultaneously in different states of aggregation. In this respect, water differs significantly from other physical substances that are in natural conditions mainly either in solid (minerals, metals) or in gaseous (О 2 , N 2 , CO 2 , etc.) state.

Changes in the aggregate state of matter are called phase transitions. In these cases, the properties of the substance (for example, density) change abruptly. Phase transitions are accompanied by the release or absorption of energy, called the heat of the phase transition ("latent heat").

The dependence of the state of aggregation of water on pressure and temperature is expressed by a state diagram of water, or a phase diagram (Fig. 5.1.1.).

Curve BB "O in Fig. 5.1.1. is called the melting curve. When passing through this curve from left to right, melting occurs

Rice. 5.1.1. Water Status Diagram

I - VIII - various modifications of ice

ice, and from right to left - ice formation (crystallization of water). The OK curve is called the vaporization curve. When passing through this curve, water boils from left to right, and water vapor condenses from right to left. The AO curve is called the sublimation curve, or sublimation curve. When crossing it from left to right, evaporation of ice (sublimation) occurs, and from right to left, condensation into a solid phase (or sublimation).

At point O (the so-called triple point, at a pressure of 610 Pa and a temperature of 0.01 ° C or 273.16 K), water is simultaneously in all three states of aggregation.

The temperature at which ice melts (or water crystallizes) is called the temperature or melting point T pl. This temperature can also be called temperature or freezing point T dep.

From the surface of water, as well as ice and snow, a certain number of molecules are constantly torn off and carried into the air, forming water vapor molecules. At the same time, some of the water vapor molecules return to the surface of water, snow and ice. If the first process prevails, then water evaporates, if the second - condensation of water vapor. The regulator of the direction and intensity of these processes is the deficit of humidity - the difference between the elasticity of water vapor that saturates the space at a given air pressure and temperature of the water surface (snow, ice), and the elasticity of the water vapor actually contained in the air, i.e. absolute air humidity. The content of saturated water vapor in the air and its elasticity increase with increasing temperature (at normal pressure) as follows. At a temperature of 0°C, the content and elasticity of saturated water vapor are 4.856 g/m3 and 6.1078 hPa, respectively, at a temperature of 20°C - 30.380 g/m3 and 23.373 hPa, at 40°C - 51.127 g/m3 and 73.777 hPa.

Evaporation from the surface of water (ice, snow), as well as moist soil, occurs at any temperature and is more intense, the greater the moisture deficit. As the temperature rises, the elasticity of the water vapor that saturates the space increases, and evaporation accelerates. An increase in the rate of air movement over the evaporating surface (i.e., wind speed in natural conditions) also leads to an increase in evaporation, which increases the intensity of vertical mass and heat transfer.

When intense evaporation covers not only the free surface of the water, but also its thickness, where evaporation comes from the inner surface of the bubbles formed in this case, the boiling process begins. The temperature at which the pressure of saturated water vapor is equal to the external pressure is called the temperature or boiling point T bale.

At normal atmospheric pressure (1.013 105 Pa \u003d 1.013 bar \u003d 1 atm \u003d 760 mm Hg), the freezing points of water (ice melting) and boiling (condensation) correspond to 0 and 100 ° Celsius.

The freezing point T deputy and the boiling point of water T boil depend on pressure (see Fig. 3.9.2.). In the pressure range from 610 to 1.013 105 Pa (or 1 atm), the freezing point drops slightly (from 0.01 to 0 ° C), then when the pressure rises to approximately 6 107 Pa (600 atm), T zap drops to -5 ° C, with an increase in pressure to 2.2 108 Pa (2,200 atm), T deputy decreases to -22 ° C. With a further increase in pressure, T deputy begins to increase rapidly. At very high pressure, special "modifications" of ice (II-VIII) are formed, which differ in their properties from ordinary ice (ice I).

At actual atmospheric pressure on Earth, fresh water freezes at a temperature of about 0°C. At the maximum depths in the ocean (about 11 km), the pressure exceeds 108 Pa, or 1,000 atm (an increase in depth for every 10 m increases the pressure by about 105 Pa, or 1 atm). At this pressure, the freezing point of fresh water would be about -12°C.

To lower the freezing point of water

influenced by its salinity.

1.4). An increase in salinity for every 10‰ reduces T deput by approximately 0.54 ° C:

T deputy \u003d -0.054 S.

The boiling point decreases with decreasing pressure (see Fig. 3.9.2.). Therefore, at high altitudes in the mountains, water boils at a temperature lower than 100 ° C. With an increase in pressure, T boil increases to the so-called “critical point”, when at p = 2.2 107 Pa and T boil = 374 ° C, water simultaneously has properties of both liquid and gas.

The diagram of the state of water illustrates two "anomalies" of water that have a decisive influence not only on the "behavior" of water on Earth, but also on the natural conditions of the planet as a whole. Compared with substances that are compounds of hydrogen with elements that are in the same row with oxygen in the Periodic Table - tellurium Te, selenium Se and sulfur S, the freezing and boiling points of water are unusually high. Considering the regular relationship between the freezing and boiling points and the mass number of the mentioned substances, one would expect water to have a freezing point of about -90 ° C, and a boiling point of about -70 ° C. Abnormally high values ​​of freezing and boiling temperatures predetermine the possibility of the existence of water on the planet as in solid and liquid states and serve as the determining conditions for the main hydrological and other natural processes on Earth.

Density of water

Density is the most important physical characteristic of any substance. It is the mass of a homogeneous substance per unit of its volume:

where m is mass, V is volume. Density p has the dimension kg/m 3 .

The density of water, like other substances, depends primarily on temperature and pressure (and for natural waters, also on the content of dissolved and finely dispersed suspended substances) and changes abruptly during phase transitions .. With increasing temperature, the density of water, like any other substance , in most of the temperature range decreases, which is associated with an increase in the distance between molecules with increasing temperature. This pattern is violated only when ice melts and when water is heated in the range from 0 to 4 ° (more precisely, 3.98 ° C). Two more very important "anatomies" of water are noted here: 1) the density of water in the solid state (ice) is less than in the liquid state (water), which is not the case for the vast majority of other substances; 2) in the water temperature range from 0 to 4 ° C, the density of water does not decrease with increasing temperature, but increases. Features of the change in water density are associated with the rearrangement of the molecular structure of water. These two "anomalies" of water are of great hydrological significance: ice is lighter than water and therefore "floats" on its surface; reservoirs usually do not freeze to the bottom, since fresh water cooled to a temperature below 4 ° becomes less dense and therefore remains in the surface layer.

The density of ice depends on its structure and temperature. Porous ice may have a density much less than that given in Table 1.1. Even less snow density. Freshly fallen snow has a density of 80-140 kg / m 3, the density of packed snow gradually increases from 140-300 (before melting) to 240-350 (at the beginning of melting) and 300-450 kg / m3 (at the end of melting). Dense wet snow can have a density of up to 600-700 kg/m 3 . Snowflakes during melting have a density of 400-600, avalanche snow 500-650 kg / m 3. The layer of water formed during the melting of ice and snow depends on the thickness of the layer of ice or snow and their density. The storage of water in ice or snow is:

h in = ah l r l / r

where h l is the thickness of the layer of ice or snow, p l is their density, p is the density of water, and is a factor determined by the ratio of the dimensions h in and h l: if the water layer is expressed in mm, and the thickness of ice (snow) in cm, then a=10, with the same dimension a=1.

The density of water also changes depending on the content of dissolved substances in it and increases with increasing salinity (Fig. 1.5). The density of sea water at normal pressure can reach 1025-1033 kg/m 3 .

The combined effect of temperature and salinity on the density of water at atmospheric pressure is expressed using the so-called seawater equation of state. Such an equation in its simplest linear form is written as follows:

p \u003d p o (1 - α 1 T + α 2 S)

where T - water temperature, ° С, S - water salinity, ‰, p o - water density at T \u003d 0 and S \u003d 0, α 1 and α 2 - parameters.

An increase in salinity also leads to a decrease in the temperature of the highest density (°C) according to the formula

T max.pl \u003d 4 - 0.215 S.

Rice. 5.2.1. The dependence of the density of water at normal atmospheric pressure on the temperature and salinity of the water.

An increase in salinity for every 10‰ reduces Tmax by approximately 2°C. The dependence of the highest density temperature and freezing point on water salinity is illustrated by the so-called Helland-Hansen plot (see Fig. 3.10.1.).

The relationship between the temperatures of greatest density and freezing affects the nature of the process of cooling water and vertical convection - mixing due to differences in density. The cooling of water as a result of heat exchange with air leads to an increase in the density of water and, accordingly, to the lowering of more dense water. In its place, warmer and less dense waters rise. A process of vertical density convection takes place. However, for fresh and brackish waters with a salinity of less than 24.7‰, such a process continues only until the water reaches the temperature of the highest density (see Fig. 1.4). Further cooling of water leads to a decrease in its density, and vertical convection stops. Salt waters at S>24.7‰ are subject to vertical convection up to the moment of their freezing.

Thus, in fresh or brackish waters in winter, the bottom water temperature is higher than on the surface, and, according to the Helland-Hansen plot, it is always above the freezing temperature. This circumstance is of great importance for the preservation of life in water bodies at depths. If the water temperatures of the highest density and freezing would coincide, like all other liquids, then the reservoirs could freeze to the bottom, causing the inevitable death of most organisms.

An “anomalous” change in the density of water with a change in temperature entails the same “anomalous” change in the volume of water: with an increase in temperature from 0 to 4 ° C, the volume of chemically pure water decreases, and only with a further increase in temperature does it increase; the volume of ice is always noticeably greater than the volume of the same mass of water (recall how pipes burst when water freezes).

The change in the volume of water with a change in its temperature can be expressed by the formula

V T1 = V T2 (1 + βDT)

where V T1 is the volume of water at temperature T1, V T2 is the volume of water at T2, β is the volumetric expansion coefficient, which takes negative values ​​at temperatures from 0 to 4 ° C and positive values ​​​​at water temperatures above 4 ° C and less than 0 ° C ( ice) (see Table 1.1),

Pressure also has some influence on the density of water. The compressibility of water is very small, but at great depths in the ocean it still affects the density of water. For every 1000 m of depth, the density due to the influence of the pressure of the water column increases by 4.5-4.9 kg/m 3 . Therefore, at maximum ocean depths (about 11 km), the water density will be approximately 48 kg/m 3 higher than on the surface, and at S = 35‰ it will be about 1076 kg/m 3 . If water were completely incompressible, the level of the World Ocean would be 30 m higher than it really is. The low compressibility of water makes it possible to significantly simplify the hydrodynamic analysis of the movement of natural waters.

The influence of small suspended sediments on the physical characteristics of water and, in particular, on its density has not yet been studied enough. It is believed that only very small suspensions can affect the density of water at their extremely high concentration, when water and sediments can no longer be considered in isolation. So, some types of mudflows, containing only 20-30% of water, are essentially a clay solution with an increased density. Another example of the effect of fine sediments on density is the Yellow River water flowing into the Gulf of the Yellow Sea. With a very high content of fine sediments (up to 220 kg / m 3), river muddy waters have a density of 2-2.5 kg / m 3 more than sea waters (their density at actual salinity and temperature is about 1018 kg / m 3). Therefore, they "dive" to the depth and descend along the seabed, forming a "dense" or "turbid" stream.

And here you can already go to the second category. under the word "ice" we are accustomed to understand the solid phase state of water. But besides it, other substances are also subjected to freezing. Thus, ice can be distinguished by the chemical composition of the original substance, for example, carbon dioxide, ammonia, methane ice and others.

Thirdly, there are crystal lattices (modifications) of water ice, the formation of which is due to the thermodynamic factor. That's what we'll talk about a little in this post.

In the article Ice, we dwelled on how the structure of water is restructured with a change in its state of aggregation, and touched upon the crystalline structure of ordinary ice. Thanks to the internal structure of the water molecule itself and hydrogen bonds connecting all molecules into an ordered system, a hexagonal (hexagonal) crystal lattice of ice is formed. The molecules closest to each other (one central and four corners) are arranged in the form of a trihedral pyramid, or tetrahedron, which underlies the hexagonal crystal modification ( ill.1).

By the way, the distance between the smallest particles of matter is measured in nanometers (nm) or angstroms (named after the 19th-century Swedish physicist Anders Jonas Angström; denoted by the symbol Å). 1 Å = 0.1 nm = 10−10 m.

Such a hexagonal structure of ordinary ice extends to its entire volume. You can clearly see this with the naked eye: in winter, during a snowfall, catch a snowflake on a sleeve of clothing or on a glove and take a closer look at its shape - it is six-ray or hexagonal. This is typical for every snowflake, but at the same time, no snowflake ever repeats another (more on this in our article). And even large ice crystals with their external shape correspond to the internal molecular structure ( ill.2).

We have already said that the transition of a substance, in particular water, from one state to another is carried out under certain conditions. Habitual ice forms at temperatures of 0°C and below and at a pressure of 1 atmosphere (normal value). Consequently, for the appearance of other modifications of ice, a change in these values ​​is required, and in most cases, the presence of low temperatures and high pressure, at which the angle of hydrogen bonds changes and the entire crystal lattice is reconstructed.

Each modification of ice belongs to a certain syngony - a group of crystals in which elementary cells have the same symmetry and coordinate system (XYZ axes). In total, seven syngonies are distinguished. The characteristics of each of them are presented on illustrations 3-4. And just below is an image of the main forms of crystals ( ill.5)

All modifications of ice that differ from ordinary ice were obtained in laboratory conditions. The first polymorphic structures of ice became known at the beginning of the 20th century through the efforts of scientists Gustav Heinrich Tammann And Percy Bridgman (Percy Williams Bridgman). The modification diagram compiled by Bridgman was periodically supplemented. New modifications were identified from those obtained earlier. Recent changes to the diagram have been made in our time. Sixteen crystalline types of ice have been obtained so far. Each type has its own name and is indicated by a Roman numeral.

We will not delve deeply into the physical characteristics of each molecular type of water ice, so as not to bore you, dear readers, with scientific details, we will note only the main parameters.

Ordinary ice is called ice Ih (the prefix "h" means hexagonal syngony). On illustrations 7 its crystal structure is presented, consisting of hexagonal bonds (hexamers), which differ in shape - one in the form sun lounger(English) chair-form), another in the form rooks (boat form). These hexamers form a three-dimensional section - two "chaise lounges" are horizontally above and below, and three "rooks" are vertically positioned.

The spatial diagram shows the order in the arrangement of ice hydrogen bonds Ih, but in reality the connections are built randomly. However, scientists do not exclude that hydrogen bonds on the surface of hexagonal ice are more ordered than inside the structure.

The elementary cell of hexagonal ice (i.e. the minimum volume of a crystal, the repeated reproduction of which in three dimensions forms the entire crystal lattice as a whole) includes 4 water molecules. Cell dimensions are 4.51Å on both sides a,b And 7.35Å on side c (side, or axis c in the diagrams has a vertical direction). The angles between the sides, as seen from illustration 4: α=β = 90°, γ = 120°. The distance between adjacent molecules is 2.76Å.

Hexagonal ice crystals form hexagonal plates and columns; the upper and lower faces in them are the base planes, and six identical side faces are called prismatic ( ill.10).

The minimum number of water molecules required to start its crystallization is about 275 (±25). To a large extent, the formation of ice occurs on the surface of the water mass adjacent to the air, rather than inside it. coarse ice crystals Ih form slowly in the direction of the c axis, for example, in stagnant water they grow vertically down from the lamellae, or in conditions where growth to the side is difficult. Fine-grained ice, formed in turbulent water or during its rapid freezing, has an accelerated growth directed from prismatic faces. The temperature of the surrounding water determines the degree of branching of the ice crystal lattice.

Particles of substances dissolved in water, with the exception of helium and hydrogen atoms, whose sizes allow them to fit in the cavities of the structure, are excluded from the crystal lattice at normal atmospheric pressure, being forced out to the surface of the crystal or, as in the case of the amorphous variety (more on this later in the article) forming layers between microcrystals. Sequential freeze-thaw cycles of water can be used to purify it from impurities, such as gases (degassing).

Along with ice Ih there is also ice ic (cubic system), however, in nature, the formation of this type of ice is occasionally possible only in the upper layers of the atmosphere. Artificial ice ic obtained by instant freezing of water, for which steam is condensed on a chilled from minus 80 to minus 110°С metal surface at normal atmospheric pressure. As a result of the experiment, crystals of a cubic shape or in the form of octahedrons fall out on the surface. It will not work to create cubic ice of the first modification from ordinary hexagonal ice by lowering its temperature, but the transition from cubic to hexagonal is possible when ice is heated ic above minus 80°С.

In the molecular structure of ice ic the angle of hydrogen bonds is the same as that of ordinary ice Ih - 109.5°. But the hexagonal ring formed by molecules in the lattice of ice ic present only in the form of a sun lounger.

The ice density Ic is 0.92 g/cm³ at a pressure of 1 atm. The unit cell in a cubic crystal has 8 molecules and dimensions: a=b=c = 6.35 Å, and its angles α=β=γ = 90°.

On a note. Dear readers, in this article we will repeatedly encounter temperature and pressure indicators for one or another type of ice. And if the temperature values ​​expressed in degrees Celsius are clear to everyone, then the perception of pressure values ​​may be difficult for someone. In physics, various units are used to measure it, but in our article we will denote it in atmospheres (atm), rounding the values. Normal atmospheric pressure is 1 atm, which is equal to 760 mmHg, or just over 1 bar, or 0.1 MPa (megapascal).

As you understood, in particular, from the example with ice ic, the existence of crystalline modifications of ice is possible under conditions of thermodynamic equilibrium, i.e. if the balance of temperature and pressure, which determines the presence of any crystalline type of ice, is disturbed, this type disappears, passing into another modification. The range of these thermodynamic values ​​is different, for each species it is different. Let us consider other types of ice, not strictly in nomenclature order, but in connection with these structural transitions.

Ice II belongs to the trigonal syngony. It can be formed from a hexagonal type at a pressure of about 3,000 atm and a temperature of about minus 75°C, or from another modification ( ice V), by a sharp decrease in pressure at a temperature of minus 35°C. Existence II ice type is possible under conditions of minus 170°C and pressure from 1 to 50,000 atm (or 5 gigapascals (GPa)). According to scientists, ice of such a modification can probably be part of the icy satellites of the distant planets of the solar system. Normal atmospheric pressure and temperatures above minus 113°C create conditions for the transition of this type of ice to ordinary hexagonal ice.

On illustrations 13 shows the crystal lattice of ice II. A characteristic feature of the structure is visible - a kind of hollow hexagonal channels formed by molecular bonds. The elementary cell (the area highlighted in the illustration by a rhombus) consists of two bundles that are displaced relative to each other, relatively speaking, "along the height". As a result, a rhombohedral lattice system is formed. Cell sizes a=b=c = 7.78 Å; α=β=γ = 113.1°. There are 12 molecules in a cell. The bond angle between molecules (О–О–О) varies from 80 to 120°.

When heated II modification, you can get ice III, and vice versa, ice cooling III turns it to ice II. Also ice III It is formed when the water temperature is gradually lowered to minus 23 ° C, increasing the pressure to 3,000 atm.
As seen in the phase diagram ( ill. 6), thermodynamic conditions for a stable state of ice III, as well as another modification - ice V, are small.

Ice III And V have four triple points with surrounding modifications (thermodynamic values ​​at which the existence of different states of matter is possible). However, ice II, III And V modifications can exist under conditions of normal atmospheric pressure and temperature of minus 170°C, and heating them to minus 150°C leads to the formation of ice ic.

Compared to other high pressure modifications currently known, ice III has the lowest density - at a pressure of 3,500 atm. it is equal to 1.16 g/cm³.
Ice III is a tetragonal variety of crystallized water, but the structure of the ice lattice itself III has violations. If usually each molecule is surrounded by 4 neighboring ones, then in this case this indicator will have a value of 3.2, and in addition there may be 2 or 3 more molecules nearby that do not have hydrogen bonds.
In the spatial construction, the molecules form right-handed helices.
Unit cell dimensions with 12 molecules at minus 23°C and about 2800 atm: a=b = 6.66, c = 6.93 Å; α=β=γ=90°. The angle of hydrogen bonds in the range from 87 to 141°.

On illustrations 15 the spatial scheme of the molecular structure of ice is conventionally presented III. Molecules (blue dots) located closer to the viewer are shown larger, and hydrogen bonds (red lines) are correspondingly thicker.

And now, as they say, in hot pursuit, let's immediately "jump over" those coming after the ice III in nomenclature order, crystalline modifications, and let's say a few words about ice IX.
This type of ice is, in fact, modified ice. III, subjected to rapid deep cooling from minus 65 to minus 108 ° C in order to avoid its transformation into ice II. Ice IX remains stable at temperatures below 133°C and pressures from 2,000 to 4,000 atm. Its density and structure is identical III mind, but unlike ice III in the structure of ice IX there is order in the arrangement of protons.
Ice heating IX does not return it to the original III modifications, but turns into ice II. Cell dimensions: a=b = 6.69, c = 6.71 Å at minus 108°C and 2800 atm.

By the way, a novel by science fiction writer Kurt Vonnegut (Kurt Vonnegut) 1963 "Cat's Cradle" is built around a substance called ice-nine, which is described as an artificially obtained material that poses a great danger to life, since water crystallizes on contact with it, turning into ice-nine. The ingress of even a small amount of this substance into the natural water area overlooking the world ocean threatens to freeze all the water on the planet, which in turn means the death of all life. In the end, that's how it all happens.

Ice IV is a metastable (weakly stable) trigonal formation of the crystal lattice. Its existence is possible in the phase space of ice III, V And VI modifications. get ice IV can be made from amorphous ice of high density, slowly heating it, starting from minus 130 ° C at a constant pressure of 8,000 atm.
The size of an elementary rhombohedral cell is 7.60 Å, angles α=β=γ = 70.1°. The cell includes 16 molecules; hydrogen bonds between molecules are asymmetric. At a pressure of 1 atm and a temperature of minus 163°C, the density of ice IV is 1.27 g/cm³. O–O–O bond angle: 88–128°.

Similarly IV type of ice formed and ice XII– by heating a high-density amorphous modification (more on this below) from minus 196 to minus 90°C at the same pressure of 8,000 atm, but at a higher rate.
Ice XII also metastable in the phase region V And VI crystalline types. It is a kind of tetragonal syngony.
The unit cell contains 12 molecules, which, due to hydrogen bonds with angles of 84–135°, are located in the crystal lattice, forming a double right-handed helix. The cell has dimensions: a=b = 8.27, c = 4.02 Å; angles α=β=γ = 90º. The density of ice XII is 1.30 g/cm³ at normal atmospheric pressure and a temperature of minus 146°C. Hydrogen bond angles: 67–132°.

Of the modifications of water ice discovered to date, ice has the most complex crystal structure. V. 28 molecules make up its unit cell; hydrogen bonds run through gaps in other molecular compounds, and some molecules form bonds only with certain compounds. The angle of hydrogen bonds between neighboring molecules varies greatly - from 86 to 132 °, therefore, in the crystal lattice of ice V there is a strong tension and a huge supply of energy.
Cell parameters under conditions of normal atmospheric pressure and temperature minus 175°С: a= 9.22, b= 7.54, c= 10.35 Å; α=β = 90°, γ = 109.2°.
Ice V- This is a monoclinic variety formed by cooling water to minus 20 ° C at a pressure of about 5,000 atm. The density of the crystal lattice, taking into account the pressure of 3500 atm, is 1.24 g/cm³.
Spatial diagram of the crystal lattice of ice V type shown in illustrations 18. The region of the elementary cell of the crystal is marked with a gray contour.

Ordered arrangement of protons in the structure of ice V makes it a different kind called ice XIII. This monoclinic modification can be obtained by cooling water below minus 143°C with the addition of hydrochloric acid (HCl) to facilitate the phase transition, creating a pressure of 5,000 atm. Reversible transition from XIII type k V type is possible in the temperature range from minus 193°С to minus 153°С.
Unit cell dimensions of ice XIII slightly different from V modifications: a= 9.24, b= 7.47, c= 10.30 Å; α=β = 90°, γ= 109.7° (at 1 atm, minus 193°C). The number of molecules in the cell is the same - 28. The angle of hydrogen bonds: 82–135°.

In the next part of our article, we will continue to review the modifications of water ice.

See you on our blog pages!

THERMODYNAMIC PROPERTIES OF WATER AND WATER VAPOR

By the term "water" we mean H 2 O in any of its possible phase states.

In nature, water can in three states: tv. (ice, snow), w. (water), g. (steam).

Consider water without energy. interaction with the environment. cf., i.e. in equilibrium.

Steam is always present at the surface of ice or liquid. The contacting phases are in t / d equilibrium: fast molecules fly out of the liquid phase, overcoming surface forces, and slow molecules pass from the vapor phase into the liquid phase. phase.

In a state of equilibrium, each T corresponds to a certain vapor pressure - total (if only vapor is present above the liquid) or partial (if there is a mixture of vapor with air or other gases).

Steam in equilibrium with the phase from which it was formed is saturated, and the corresponding T is saturation T, and the pressure–p saturation.

Non-equilibrium states of water:

a) Let the vapor pressure over the liquid decrease below the saturation pressure. In this case, the equilibrium is disturbed, an uncompensated transition of the substance from the liquid phase to the gaseous phase occurs through the interface due to the fastest molecules.

The process of uncompensated transition of matter from well. phases in - evaporation.

The process of uncompensated transition of a substance from the solid phase to the gas phase is called sublimation or sublimation .

The intensity of evaporation or sublimation increases with the intensive removal of the resulting vapor. In this case, the temperature of the liquid phase decreases due to the escape of molecules with the highest energy from it. This can be achieved without lowering the pressure, simply by blowing the air flow.

b) Let there be a supply of heat to a liquid in an open vessel. In this case, T, and, accordingly, p of saturated vapor above the liquid grows and can reach the total external pressure (P = P n). In the case when P = P n, at the heating surface, T of the liquid rises above T of saturated vapor at the pressure prevailing here those. conditions are created for the formation of steam in the thickness of the liquid.

The process of transition of a substance from the liquid phase to the vapor phase directly inside the liquid is called boiling.

The process of nucleation of vapor bubbles in the bulk of a liquid is complex. For water to boil, it is necessary to have centers of vaporization on the surface of heat supply - depressions, protrusions, irregularities, etc. At the heating surface, during boiling, the difference T of water and saturated steam at the pressure prevailing here depends on the intensity of heat supply and can reach tens of degrees.

The action of the forces of surface tension of the liquid causes overheating of the liquid at the interface when it boils by 0.3-1.5 ° C in relation to the temperature of the saturated vapor above it.

Any process of transition of a substance from a liquid phase to a vapor - vaporization.

The process opposite to vaporization, i.e. uncompensated transition of a substance from the vapor phase to the liquid - condensation.

At constant vapor pressure, condensation occurs (like boiling) at a constant temperature and is the result of heat removal from the system.

The process opposite to sublimation, i.e. the transition of a substance from the vapor phase directly to the solid - desublimation.

The liquid phase of water at its boiling point is called saturated with liquid .

Steam at its boiling (saturation) temperature is called dry saturated steam .

Two-phase mixture "l + p" in a state of saturation - wet saturated steam.

In t / d, this term extends to two-phase systems in which saturated steam can be above the liquid level or represent a mixture of steam with liquid droplets suspended in it. To characterize wet saturated steam, concept degree of dryness X, which is the ratio of the mass of dry saturated steam,m s.n.p., to the total weight of the mixture,m cm = m s.s.p. + m f.s.n., it with the liquid in a state of saturation:

The ratio of the mass of the liquid phase of water in a state of saturation to the mass of the mixture called the degree of humidity (1-x):

The supply of heat to wet saturated steam at a constant p leads to the transition x. the phase of the mixture in p. In this case, the T mixture (saturation) cannot be increased until all the liquid has been converted to vapour. Further heat supply only to the vapor phase in the saturation state leads to an increase in T steam.

Steam above saturation temperature at a given pressure is called superheated steam. Superheated steam temperature difference t and saturated steam of the same pressure t n called degree of steam superheat Dt p \u003d t -t n.

With an increase in the degree of superheating of steam, its volume increases, the concentration of molecules decreases, in terms of its properties it approaches gases.

6.2. Phase diagrams P, t-, P, v- and T, s for H 2 O

To analyze various t/d processes of changing the state of H 2 O, phase diagrams are widely used.

The state diagram (or phase diagram) is a graphic representation of the relationship between the quantities characterizing the state of the system and phase transformations in the system (transition from solid to liquid, from liquid to gaseous, etc.).

Rice. 72. Scheme of the structure of ice.

Rice. 73. Diagram of the state of water in the region of low pressures.

Rice. 74. A cylinder with water in equilibrium with water vapor.

State diagrams are widely used in chemistry. For one-component systems, state diagrams are usually used, showing the dependence of phase transformations on temperature and pressure; they are called P-T state diagrams.

On fig. 73 shows in schematic form (not strictly to scale) a diagram of the state of water. Any point on the diagram corresponds to certain values ​​​​of temperature and pressure.

The diagram shows those states of water that are thermodynamically stable at certain temperatures and pressures. It consists of three curves that delimit all possible temperatures and pressures into three regions corresponding to ice, liquid and vapor.

Let's consider each of the curves in more detail. Let's start with the OA curve (Fig. 73), which separates the region of vapor from the region of the liquid state. Imagine a cylinder from which air is removed, after which a certain amount of pure, free from dissolved substances, including gases, water is introduced into it; the cylinder is equipped with a piston, which is fixed in a certain position (Fig. 74). After some time, some of the water will evaporate and saturated steam will be above its surface. You can measure its pressure and make sure that it does not change over time and does not depend on the position of the piston. If you increase the temperature of the entire system and measure the saturation vapor pressure again, it turns out that it has increased. By repeating such measurements at different temperatures, we find the dependence of the pressure of saturated water vapor on temperature. The OA curve is a graph of this dependence: the points of the curve show those pairs of temperature and pressure values ​​at which liquid water and water vapor are in equilibrium with each other - they coexist. The OA curve is called the liquid-vapor equilibrium curve or the boiling curve. In table. 8 (p. 202) shows saturation vapor pressures at several temperatures.

Let us try to realize in the cylinder a pressure different from the equilibrium one, for example, less than the equilibrium one. To do this, release the piston and raise it. At the first moment, the pressure in the cylinder will indeed drop, but soon the equilibrium will be restored: an additional amount of water will evaporate and the pressure will again reach the equilibrium value. Only when all the water has evaporated can a pressure less than equilibrium be realized. It follows from this that the vapor region corresponds to the points lying on the phase diagram below or to the right of the OA curve.

Table 8. Saturated water vapor pressure at various temperatures

If you try to create a pressure that exceeds the equilibrium, then this can only be achieved by lowering the piston to the surface of the water. In other words, the points of the diagram lying above or to the left of the OA curve correspond to the region of the liquid state.

How long do the regions of the liquid and vapor state extend to the left? Let's outline one point in both areas and we will move from them horizontally to the left. This movement of the points on the diagram corresponds to the cooling of a liquid or vapor at a constant pressure. It is known that if you cool water at normal atmospheric pressure, then when it reaches the water will begin to freeze. Carrying out similar experiments at other pressures, we arrive at the OS curve separating the region of liquid water from the region of ice. This curve - the solid-liquid equilibrium curve, or the melting curve - shows those pairs of temperature and pressure values ​​at which ice and liquid water are in equilibrium.

Moving horizontally to the left in the vapor region (in the lower part of the diagram), we will similarly arrive at the OB curve. This is the equilibrium curve of the solid state - vapor, or sublimation curve. It corresponds to those pairs of temperature and pressure values ​​at which ice and water vapor are in equilibrium.

All three curves intersect at point O. The coordinates of this point are the only pair of temperature and pressure values ​​at which all three phases can be in equilibrium: ice, liquid water and steam. It is called the triple point.

The melting curve has been investigated up to very high pressures. Several modifications of ice were found in this area (not shown in the diagram).

On the right, the boiling curve ends at the critical point. At the temperature corresponding to this point - the critical temperature - the quantities characterizing the physical properties of the liquid and vapor become the same, so that the difference between the liquid and vapor state disappears.

The existence of a critical temperature was established in 1860 by D. I. Mendeleev, studying the properties of liquids. He showed that at temperatures above the critical one, a substance cannot be in a liquid state. In 1869, Andrews, studying the properties of gases, came to a similar conclusion.

The critical temperature and pressure for different substances are different. So, for hydrogen , , for chlorine , , for water , .

One of the features of water that distinguishes it from other substances is the decrease in the melting point of ice with increasing pressure (see § 70). This circumstance is reflected in the diagram. The OC melting curve on the state diagram of water goes up to the left, while for almost all other substances it goes up to the right.

The transformations that occur with water at atmospheric pressure are reflected in the diagram by points or segments located on the horizontal corresponding to . Thus, the melting of ice or the crystallization of water corresponds to point D (Fig. 73), the boiling of water corresponds to point E, the heating or cooling of water corresponds to the segment DE, etc.

State diagrams have been studied for a number of substances of scientific or practical importance. In principle, they are similar to the considered diagram of the state of water. However, the state diagrams of various substances may have features. So, substances are known, the triple point of which lies at a pressure exceeding atmospheric pressure. In this case, heating the crystals at atmospheric pressure does not lead to the melting of this substance, but to its sublimation - the transformation of the solid phase directly into a gaseous one,