Strictly speaking, in this material we will briefly consider not only chemical and physical properties of liquid water, but also the properties inherent in it in general as such.

You can find out more about the properties of water in the solid state in the article - PROPERTIES OF WATER IN THE SOLID STATE (read →).

Water is a super-important substance for our planet. Without it, life on Earth is impossible; without it, not a single geological process takes place. The great scientist and thinker Vladimir Ivanovich Vernadsky wrote in his works that there is no such component whose significance could “be compared with it in its influence on the course of the main, most formidable geological processes.” Water is present not only in the body of all living creatures on our planet, but also in all substances on Earth - in minerals, in rocks... The study of the unique properties of water constantly reveals to us more and more new secrets, asks us new riddles and poses new challenges.

Anomalous properties of water

Many physical and chemical properties of water surprise and fall out of general rules and patterns and are anomalous, for example:

• In accordance with the laws established by the principle of similarity, within the framework of sciences such as chemistry and physics, we could expect that:
  • water will boil at minus 70°C and freeze at minus 90°C;
  • the water will not drip from the tip of the tap, but will flow in a thin stream;
  • the ice will sink rather than float on the surface;
  • more than a few grains of sugar would not dissolve in a glass of water.
• The surface of water has a negative electrical potential;
• When heated from 0°C to 4°C (3.98°C to be exact), water contracts;
• The high heat capacity of liquid water is surprising;

As noted above, in this material we will list the main physical and chemical properties of water and make brief comments on some of them.

Physical properties of water

PHYSICAL PROPERTIES are properties that appear outside of chemical reactions.

Water purity

The purity of water depends on the presence of impurities, bacteria, salts of heavy metals in it..., to familiarize yourself with the interpretation of the term PURE WATER according to our website, you need to read the article PURE WATER (read →).

Water color

The color of water depends on the chemical composition and mechanical impurities

As an example, let us give the definition of “Color of the Sea” given by the Great Soviet Encyclopedia.

The color of the sea. The color perceived by the eye when an observer looks at the surface of the sea. The color of the sea depends on the color of sea water, the color of the sky, the number and nature of clouds, the height of the Sun above the horizon, and other reasons.

The concept of the color of the sea should be distinguished from the concept of the color of sea water. Seawater color refers to the color perceived by the eye when viewing seawater vertically above a white background. Only a small part of the light rays incident on it is reflected from the surface of the sea, the rest of them penetrates into the depths, where they are absorbed and scattered by water molecules, particles of suspended substances and tiny gas bubbles. The scattered rays reflected and emerging from the sea create the color spectrum. Water molecules scatter blue and green rays the most. Suspended particles scatter all rays almost equally. Therefore, sea water with a small amount of suspended matter appears blue-green (the color of the open parts of the oceans), and with a significant amount of suspended matter it appears yellowish-green (for example, the Baltic Sea). The theoretical side of the doctrine of central mathematics was developed by V. V. Shuleikin and C. V. Raman.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978

The smell of water

Odor of water – clean water usually has no odor.

Water clarity

The transparency of water depends on the minerals dissolved in it and the content of mechanical impurities, organic substances and colloids:

WATER TRANSPARENCY is the ability of water to transmit light. Usually measured by a Secchi disk. Depends mainly on the concentration of suspended and dissolved organic and inorganic substances in water. It can sharply decrease as a result of anthropogenic pollution and eutrophication of water bodies.

Ecological encyclopedic dictionary. - Chisinau I.I. Dedu. 1989

WATER TRANSPARENCY is the ability of water to transmit light rays. It depends on the thickness of the layer of water traversed by the rays, the presence of suspended impurities, dissolved substances, etc. In water, red and yellow rays are absorbed more strongly, and violet rays penetrate deeper. According to the degree of transparency, in order of decreasing it, waters are distinguished:

• transparent;
• slightly opalescent;
• opalescent;
• slightly cloudy;
• cloudy;
• very cloudy.

Dictionary of hydrogeology and engineering geology. - M.: Gostoptekhizdat. 1961

Taste of water

The taste of water depends on the composition of the substances dissolved in it.

Dictionary of hydrogeology and engineering geology

The taste of water is a property of water that depends on the salts and gases dissolved in it. There are tables of the palatable concentration of salts dissolved in water (in mg/l), for example the following table (according to Staff).

Water temperature

Melting point of water:

MELTING POINT - the temperature at which a substance changes from SOLID to liquid. The melting point of a solid is equal to the freezing point of a liquid, for example, the melting point of ice, O °C, is equal to the freezing point of water.

Boiling point of water : 99.974°C

Scientific and technical encyclopedic dictionary

BOILING POINT, the temperature at which a substance passes from one state (phase) to another, that is, from liquid to vapor or gas. The boiling point increases with increasing external pressure and decreases with decreasing pressure. It is usually measured at a standard pressure of 1 atmosphere (760 mm Hg). The boiling point of water at standard pressure is 100 °C.

Scientific and technical encyclopedic dictionary.

Triple point of water

Triple point of water: 0.01 °C, 611.73 Pa;

Scientific and technical encyclopedic dictionary

TRIPLE POINT, temperature and pressure at which all three states of matter (solid, liquid, gas) can exist simultaneously. For water, the triple point is at a temperature of 273.16 K and a pressure of 610 Pa.

Scientific and technical encyclopedic dictionary.

Surface tension of water

Surface tension of water - determines the strength of adhesion of water molecules to each other, for example, how this or that water is absorbed by the human body depends on this parameter.

Hardness of water

Marine dictionary

WATER HARDNESS (Stiffness of Water) - a property of water that is exsanguinated by the content of alkaline earth metal salts dissolved in it, ch. arr. calcium and magnesium (in the form of bicarbonate salts - bicarbonates), and salts of strong mineral acids - sulfuric and hydrochloric. L.V. is measured in special units, the so-called. degrees of hardness. The degree of hardness is the weight content of calcium oxide (CaO), equal to 0.01 g in 1 liter of water. Hard water is unsuitable for feeding boilers, as it promotes strong scale formation on their walls, which can cause burnout of the boiler tubes. Boilers of high power and especially high pressure must be fed with completely purified water (condensate from steam engines and turbines, purified by filters from oil impurities, as well as distillate prepared in special evaporator apparatus).

Samoilov K.I. Marine dictionary. — M.-L.: State Naval Publishing House of the NKVMF of the USSR, 1941

Scientific and technical encyclopedic dictionary

WATER HARDNESS, the inability of water to form foam with soap due to salts dissolved in it, mainly calcium and magnesium.

Scale in boilers and pipes is formed due to the presence of dissolved calcium carbonate in the water, which enters the water upon contact with limestone. In hot or boiling water, calcium carbonate precipitates as hard limescale deposits on surfaces inside boilers. Calcium carbonate also prevents soap from foaming. The ion exchange container (3) is filled with granules coated with sodium-containing materials. with which water comes into contact. Sodium ions, being more active, replace calcium ions. Since sodium salts remain soluble even when boiled, scale does not form.

Scientific and technical encyclopedic dictionary.

Water structure

Water mineralization

Water mineralization :

Ecological encyclopedic dictionary

WATER MINERALIZATION - saturation of water with inorganic. (mineral) substances found in it in the form of ions and colloids; the total amount of inorganic salts contained mainly in fresh water, the degree of mineralization is usually expressed in mg/l or g/l (sometimes in g/kg).

Ecological encyclopedic dictionary. - Chisinau: Main editorial office of the Moldavian Soviet Encyclopedia. I.I. Dedu. 1989

Water viscosity

Water viscosity characterizes the internal resistance of liquid particles to its movement:

Geological dictionary

The viscosity of water (liquid) is a property of a liquid that causes the occurrence of friction force during movement. It is a factor that transfers motion from layers of water moving at high speed to layers at lower speed. V. in. depends on the temperature and concentration of the solution. Physically, it is estimated by coefficient. viscosity, which is included in a number of formulas for the movement of water.

Geological Dictionary: in 2 volumes. - M.: Nedra. Edited by K. N. Paffengoltz et al. 1978

There are two types of water viscosity:

• Dynamic viscosity of water is 0.00101 Pa s (at 20°C).

• Kinematic viscosity of water is 0.01012 cm 2 /s (at 20°C).

Critical point of water

The critical point of water is its state at a certain ratio of pressure and temperature, when its properties are the same in the gaseous and liquid states (gaseous and liquid phases).

Critical point of water: 374°C, 22.064 MPa.

Dielectric constant of water

Dielectric constant, in general, is a coefficient indicating how much the force of interaction between two charges in a vacuum is greater than in a certain environment.

In the case of water, this figure is unusually high and for static electric fields it is 81.

Heat capacity of water

Heat capacity of water - water has a surprisingly high heat capacity:

Ecological dictionary

Heat capacity is the property of substances to absorb heat. It is expressed as the amount of heat absorbed by a substance when it is heated by 1°C. The heat capacity of water is about 1 cal/g, or 4.2 J/g. The heat capacity of the soil (at 14.5-15.5°C) ranges (from sandy to peat soils) from 0.5 to 0.6 cal (or 2.1-2.5 J) per unit volume and from 0.2 up to 0.5 cal (or 0.8-2.1 J) per unit mass (g).

Ecological Dictionary. - Alma-Ata: “Science”. B.A. Bykov. 1983

Scientific and technical encyclopedic dictionary

SPECIFIC HEAT CAPACITY (symbol c), the heat required to raise the temperature of 1 kg of a substance by 1K. It is measured in J/K.kg (where J is JOUL). Substances with a high specific heat, such as water, require more energy to raise their temperature than substances with a low specific heat.

Scientific and technical encyclopedic dictionary.

Thermal conductivity of water

Thermal conductivity of a substance implies its ability to conduct heat from its hotter parts to its colder parts.

Heat transfer in water occurs either at the molecular level, i.e., transferred by water molecules, or due to the movement / displacement of any volumes of water - turbulent thermal conductivity.

The thermal conductivity of water depends on temperature and pressure.

Fluidity of water

The fluidity of substances is understood as their ability to change their shape under the influence of constant stress or constant pressure.

The fluidity of liquids is also determined by the mobility of their particles, which at rest are unable to perceive shear stress.

Water inductance

Inductance determines the magnetic properties of closed electric current circuits. Water, with the exception of some cases, conducts electric current, and therefore has a certain inductance.

Density of water

The density of water is determined by the ratio of its mass to volume at a certain temperature. Read more in our material - WHAT IS WATER DENSITY(read →).

Compressibility of water

The compressibility of water is insignificant and depends on the salinity of the water and pressure. For example, for distilled water it is 0.0000490.

Electrical conductivity of water

The electrical conductivity of water largely depends on the amount of salts dissolved in it.

Radioactivity of water

The radioactivity of water depends on the content of radon in it, the emanation of radium.

Physico-chemical properties of water

Dictionary of hydrogeology and engineering geology

PHYSICAL AND CHEMICAL PROPERTIES OF WATER - parameters that determine the physical and chemical characteristics of natural waters. These include indicators of the concentration of hydrogen ions (pH) and oxidation-reduction potential (Eh).

Dictionary of hydrogeology and engineering geology. - M.: Gostoptekhizdat. Compiled by A. A. Makkaveev, editor O. K. Lange. 1961

Acid-base balance of water

Redox potential of water

The oxidation-reduction potential of water (ORP) is the ability of water to enter into biochemical reactions.

Chemical properties of water

CHEMICAL PROPERTIES OF A SUBSTANCE are properties that appear as a result of chemical reactions.

Below are the chemical properties of water according to the textbook “Fundamentals of Chemistry. Internet textbook” by A. V. Manuilova, V. I. Rodionov.

Interaction of water with metals

When water interacts with most metals, a reaction occurs that releases hydrogen:

• 2Na + 2H2O = H2 + 2NaOH (boisterous);
• 2K + 2H2O = H2 + 2KOH (boisterous);
• 3Fe + 4H2O = 4H2 + Fe3O4 (only when heated).

Not all, but only sufficiently active metals can participate in redox reactions of this type. Alkali and alkaline earth metals of groups I and II react most easily.

Interaction of water with non-metals

Of the non-metals, for example, carbon and its hydrogen compound (methane) react with water. These substances are much less active than metals, but are still capable of reacting with water at high temperatures:

• C + H2O = H2 + CO (high heat);
• CH4 + 2H2O = 4H2 + CO2 (at high heat).

Interaction of water with electric current

When exposed to electric current, water decomposes into hydrogen and oxygen. This is also a redox reaction, where water is both an oxidizing agent and a reducing agent.

Interaction of water with non-metal oxides

Water reacts with many non-metal oxides and some metal oxides. These are not redox reactions, but coupling reactions:

SO2 + H2O = H2SO3 (sulfurous acid)

SO3 + H2O = H2SO4 (sulfuric acid)

CO2 + H2O = H2CO3 (carbonic acid)

Interaction of water with metal oxides

Some metal oxides can also react with water. We have already seen examples of such reactions:

CaO + H2O = Ca(OH)2 (calcium hydroxide (slaked lime)

Not all metal oxides are capable of reacting with water. Some of them are practically insoluble in water and therefore do not react with water. For example: ZnO, TiO2, Cr2O3, from which, for example, water-resistant paints are prepared. Iron oxides are also insoluble in water and do not react with it.

Hydrates and crystalline hydrates

Water forms compounds, hydrates and crystalline hydrates, in which the water molecule is completely preserved.

For example:

• CuSO4 + 5 H2O = CuSO4.5H2O;
• CuSO4 is a white substance (anhydrous copper sulfate);
• CuSO4.5H2O - crystalline hydrate (copper sulfate), blue crystals.

Other examples of hydrate formation:

• H2SO4 + H2O = H2SO4.H2O (sulfuric acid hydrate);
• NaOH + H2O = NaOH.H2O (caustic soda hydrate).

Compounds that bind water into hydrates and crystalline hydrates are used as desiccants. With their help, for example, water vapor is removed from humid atmospheric air.

Biosynthesis

Water participates in bio-synthesis as a result of which oxygen is formed:

6n CO 2 + 5n H 2 O = (C 6 H 10 O 5) n + 6n O 2 (under the influence of light)

We see that the properties of water are diverse and cover almost all aspects of life on Earth. As one of the scientists formulated ... it is necessary to study water comprehensively, and not in the context of its individual manifestations.

When preparing the material, information was used from the books - Yu. P. Rassadkin “Ordinary and Extraordinary Water”, Yu. Ya. Fialkov “Unusual Properties of Ordinary Solutions”, Textbook “Fundamentals of Chemistry. Internet textbook” by A. V. Manuilova, V. I. Rodionov and others.

As follows from the P–V phase diagram (Fig. 3.3), as pressure increases, the difference between the specific volumes of boiling liquid (V") and dry saturated steam (V") gradually decreases and at point K becomes equal to zero. This state is called critical, and point K is the critical point of the substance.

P k, T k, V k, S k – critical thermodynamic parameters of the substance.

For example, for water:

P k = 22.129 MPa;

Tc = 374.14 0 C;

V k = 0.00326 m 3 /kg

At the critical point, the properties of the liquid and gaseous phases are the same.

As follows from the T – S phase diagram (Figure 3.4), at the critical point the heat of vaporization, depicted as the area under the horizontal phase transition line (C" - C""), from boiling liquid to dry saturated steam, is zero.

Point K for the Tk isotherm in the P–V phase diagram (Fig. 3.3) is an inflection point.

The isotherm Tk passing through point K is ultimate isotherm of the two-phase region, i.e. separates the liquid phase region from the gaseous region.

At temperatures above Tk, the isotherms no longer have straight sections indicating phase transitions, nor the inflection point characteristic of the Tk isotherm, but gradually take the form of smooth curves, close in shape to the isotherms of an ideal gas.

The concepts of “liquid” and “gas” (steam) are to a certain extent arbitrary, because the interactions of molecules in liquid and gas have common patterns, differing only quantitatively. This thesis can be illustrated by Figure 3.6, where the transition from point E of the gaseous phase to point L of the liquid phase is made bypassing the critical point K along the EFL trajectory.

Fig.3.6. Two phase transition options

from gaseous to liquid phase

When passing along the AD line at point C, the substance separates into two phases and then the substance gradually passes from the gaseous (vapor) phase into the liquid phase.

At point C, the properties of the substance change abruptly (in the P–V phase diagram, point C of the phase transition turns into a line of phase transition (C" - C")).

When moving along the EFL line, the transformation of gas into liquid occurs continuously, since the EFL line does not intersect the TC vaporization curve anywhere, where the substance simultaneously exists in the form of two phases: liquid and gaseous. Consequently, when moving along the EFL line, the substance will not disintegrate into two phases and will remain single-phase.

Critical temperature T To is the limiting temperature for the equilibrium coexistence of two phases.

With regard to thermodynamic processes in complex systems, this classic laconic definition of Tc can be expanded as follows:

Critical temperature T To - this is the lower temperature limit of the region of thermodynamic processes in which the appearance of a two-phase state of a substance “gas - liquid” is impossible under any changes in pressure and temperature. This definition is illustrated in Figures 3.7 and 3.8. From these figures it follows that this region limited by the critical temperature covers only the gaseous state of the substance (gas phase). The gaseous state of the substance, called steam, is not included in this region.

Rice. 3.7. To the definition of critical Fig. 3.8. To the definition of critical

temperature

From these figures it follows that this shaded area, limited by the critical temperature, covers only the gaseous state of the substance (gas phase). The gaseous state of the substance, called steam, is not included in this region.

Using the concept of a critical point, it is possible to distinguish the concept of “vapor” from the general concept of “gaseous state of matter”.

Steam – this is the gaseous phase of a substance in the temperature range below the critical one.

In thermodynamic processes, when the process line intersects either the vaporization curve TC or the sublimation curve 3, the gaseous phase is always initially vapor.

Critical pressure P To - this is the pressure above which the separation of a substance into two simultaneous and equilibrium coexisting phases: liquid and gas is impossible at any temperature.

This classic definition of P k, in relation to thermodynamic processes in complex systems, can be formulated in more detail:

Critical pressure P To - this is the lower pressure boundary of the region of thermodynamic processes in which the appearance of a two-phase state of a substance “gas - liquid” is impossible under any changes in pressure and temperature. This definition of critical pressure is illustrated in Fig. 3.9. and 3.10. From these figures it follows that this region, limited by the critical pressure, covers not only the part of the gaseous phase located above the Pk isobar, but also the part of the liquid phase located below the Tk isotherm.

For the supercritical region, the critical isotherm is conventionally taken as the probable (conditional) liquid-gas boundary.

Fig. 3.9. Towards the definition of critical - Fig. 3.10. Toward the definition of critical

who is the pressure of the pressure

If the transition pressure is much greater than the pressure at the critical point, then the substance will go from the solid (crystalline) state directly to the gaseous state, bypassing the liquid state.

This is not obvious from the P-T phase diagrams of the anomalous substance (Figures 3.6, 3.7, 3.9), because they do not show that part of the diagram where a substance, which at high pressures has several crystalline modifications (and, accordingly, several triple points), again acquires normal properties.

On the phase P – T diagram of normal matter, Fig. 3.11 this transition from the solid phase directly to the gaseous phase is shown in the form of process A "D".

Rice. 3.11. Transition to normal

substances from the solid phase immediately into

gaseous at P>Ptr

The transition of a substance from the solid phase to the vapor phase, bypassing the liquid phase, is assigned only at P<Р тр. Примером такого перехода, называемого сублимацией, является процесс АDна рис 3.11.

The critical temperature has a very simple molecular-kinetic interpretation.

The combination of freely moving molecules into a drop of liquid during gas liquefaction occurs solely under the influence of mutual attraction forces. At T>T k, the kinetic energy of the relative motion of two molecules is greater than the energy of attraction of these molecules, so the formation of liquid droplets (i.e., the coexistence of two phases) is impossible.

Only vaporization curves have critical points, since they correspond to the equilibrium coexistence of two isotropic phases: liquid and gaseous. Melting and sublimation lines do not have critical points, because they correspond to such two-phase states of matter when one of the phases (solid) is anisotropic.

If a certain amount of liquid is placed in a closed vessel, then part of the liquid will evaporate and saturated steam will exist above the liquid. The pressure, and therefore the density of this vapor, depends on the temperature. The density of vapor is usually much less than the density of liquid at the same temperature. If you increase the temperature, the density of the liquid will decrease (§ 198), while the pressure and density of the saturated vapor will increase. In table Figure 22 shows the density values ​​of water and saturated water vapor for different temperatures (and therefore for the corresponding pressures). In Fig. 497 the same data is presented in graph form. The top part of the graph shows the change in the density of a liquid depending on its temperature. As the temperature increases, the density of the liquid decreases. The lower part of the graph shows the dependence of saturated vapor density on temperature. The vapor density increases. At the temperature corresponding to point , the densities of the liquid and saturated vapor coincide.

Rice. 497. Dependence of the density of water and its saturated vapor on temperature

Table 22. Properties of water and its saturated steam at different temperatures

The table shows that the higher the temperature, the smaller the difference between the density of the liquid and the density of its saturated vapor. At a certain temperature (at water) these densities coincide. The temperature at which the densities of the liquid and its saturated vapor coincide is called the critical temperature of the substance. In Fig. 497 corresponds to the dot. The pressure corresponding to point is called critical pressure. The critical temperatures of different substances vary greatly. Some of them are given in table. 23.

Table 23. Critical temperature and critical pressure of some substances

What does the existence of a critical temperature indicate? What happens at even higher temperatures?

Experience shows that at temperatures higher than critical, a substance can only be in a gaseous state. If we reduce the volume occupied by steam at a temperature above the critical temperature, then the pressure of the steam increases, but it does not become saturated and continues to remain homogeneous: no matter how high the pressure, we will not find two states separated by a sharp boundary, as is always observed at lower temperatures due to steam condensation. So, if the temperature of a substance is above the critical temperature, then equilibrium of the substance in the form of a liquid and the vapor in contact with it is impossible at any pressure.

The critical state of a substance can be observed using the device shown in Fig. 498. It consists of an iron box with windows, which can be heated higher (“air bath”), and a glass ampoule with ether located inside the bath. When the bath is heated, the meniscus in the ampoule rises, becomes flatter and finally disappears, which indicates a transition through a critical state. As the bath cools, the ampoule suddenly becomes cloudy due to the formation of many tiny droplets of ether, after which the ether collects at the bottom of the ampoule.

Rice. 498. Device for observing the critical state of the ether

As can be seen from table. 22, as the critical point is approached, the specific heat of vaporization becomes less and less. This is explained by the fact that as the temperature increases, the difference in the internal energies of a substance in the liquid and vapor states decreases. In fact, the adhesive forces of molecules depend on the distances between molecules. If the densities of liquid and vapor differ little, then the average distances between molecules differ little. Consequently, the values ​​of the potential energy of interaction between molecules will differ little. The second term of the heat of vaporization - work against external pressure - also decreases as the critical temperature is approached. This follows from the fact that the smaller the difference in the densities of vapor and liquid, the smaller the expansion that occurs during evaporation, and, therefore, the less work done during evaporation.

The existence of a critical temperature was first pointed out in 1860. Dmitry Ivanovich Mendeleev (1834-1907), Russian chemist who discovered the fundamental law of modern chemistry - the periodic law of chemical elements. Great achievements in the study of critical temperature belong to the English chemist Thomas Andrews, who carried out a detailed study of the behavior of carbon dioxide during an isothermal change in the volume it occupies. Andrews showed that at lower temperatures in a closed vessel the coexistence of carbon dioxide in liquid and gaseous states is possible; at higher temperatures such coexistence is impossible and the entire vessel is filled only with gas, no matter how much its volume is reduced.

After the discovery of the critical temperature, it became clear why gases such as oxygen or hydrogen could not be converted into liquid for a long time. Their critical temperature is very low (Table 23). To turn these gases into liquid, they must be cooled below a critical temperature. Without this, all attempts to liquefy them are doomed to failure.

The supercritical state of matter was first discovered by Cagnard de la Tour in 1822 by heating various liquids in a tightly closed metal ball (the spherical shape was chosen so that the vessel could withstand the maximum possible pressure). Inside the ball, in addition to the liquid, he placed a simple sensor - a small pebble. By shaking the ball during the heating process, Cagniard de la Tour found that the sound emitted by the pebble when it collides with the wall of the ball changes sharply at a certain moment - it becomes dull and weaker. For each liquid, this happened at a strictly defined temperature, which became known as the Cañara de la Tour point. Real interest in the new phenomenon arose in 1869 after the experiments of T. Andrews. Conducting experiments in thick-walled glass tubes, he investigated the properties of CO 2, which easily liquefies with increasing pressure. As a result, he found that at 31 ° C and 7.2 MPa, the meniscus, the boundary separating the liquid and the space filled with gas, disappears and the entire volume is uniformly filled with a milky-white opalescent liquid. With a further increase in temperature, it quickly becomes transparent and mobile, consisting of constantly flowing jets, reminiscent of flows of warm air over a heated surface. Further increases in temperature and pressure did not lead to visible changes.

He called the point at which such a transition occurs critical, and the state of the substance located above this point - supercritical. Despite the fact that outwardly it resembles a liquid, a special term is now used to apply it to it - supercritical fluid (from the English word fluid, that is, “capable of flowing”). In modern literature, the abbreviated designation for supercritical fluids is SCF.

Critical point.

When temperature or pressure changes, mutual transitions occur: solid - liquid - gas, for example, when heated, a solid turns into a liquid; when the temperature increases or the pressure decreases, the liquid turns into gas. All these transitions are usually reversible. In general they are presented in the figure:

The location of the lines delimiting the regions of gaseous, liquid and solid states, as well as the position of the triple point where these three regions converge, are different for each substance. The supercritical region begins at the critical point (indicated by an asterisk), which is certainly characterized by two parameters - temperature and pressure (the same as the boiling point). A decrease in either temperature or pressure below the critical level removes the substance from the supercritical state.

The fact of the existence of a critical point made it possible to understand why some gases, for example, hydrogen, nitrogen, oxygen, for a long time could not be obtained in liquid form using increased pressure, which is why they were previously called permanent gases (lat. permanentis - permanent). From the above figure it can be seen that the region of existence of the liquid phase is located to the left of the critical temperature line. Thus, to liquefy any gas, it must first be cooled to a temperature below the critical temperature. Gases such as CO 2 or Cl 2 have a critical temperature above room temperature (31 ° C and 144 ° C, respectively), so they can be liquefied at room temperature only by increasing the pressure. Nitrogen has a critical temperature much lower than room temperature: -239.9 ° C, therefore, if you compress nitrogen under normal conditions (the yellow starting point in the figure below), you can ultimately reach the supercritical region, but liquid nitrogen will cannot be formed. It is necessary to first cool the nitrogen below the critical temperature (green dot) and then, increasing the pressure, to reach the region where the existence of a liquid is possible - red dot (the solid state of nitrogen is possible only at very high pressures, so the corresponding region is not shown in the figure):

The situation is similar for hydrogen and oxygen (critical temperatures are –118.4° C, –147° C, respectively), so before liquefaction they are first cooled to a temperature below the critical temperature, and only then the pressure is increased.

Supercritical state

possible for most liquid and gaseous substances, it is only necessary that the substance does not decompose at a critical temperature. Substances for which such a state is most easily achievable (i.e., relatively low temperature and pressure are needed) are shown in the diagram:

In comparison with the indicated substances, the critical point for water is reached with great difficulty: t cr = 374.2 ° C and p cr = 21.4 MPa.

Since the mid-1880s, the critical point has been universally recognized as an important physical parameter of a substance, just like the melting or boiling point. The density of SCF is exceptionally low, for example, water in the form of SCF has a density three times lower than under normal conditions. All SCF have extremely low viscosity.

Supercritical fluids are a cross between a liquid and a gas. They can be compressed like gases (ordinary liquids are practically incompressible) and, at the same time, are capable of dissolving solids, which is not typical for gases. Supercritical ethanol (at temperatures above 234° C) very easily dissolves some inorganic salts (CoCl 2, KBr, KI). Carbon dioxide, nitrous oxide, ethylene and some other gases in the SCF state acquire the ability to dissolve many organic substances - camphor, stearic acid, paraffin and naphthalene. The properties of supercritical CO 2 as a solvent can be adjusted - with increasing pressure, its dissolving ability increases sharply:

The experiments carried out to visually observe the supercritical state were dangerous, since not every glass ampoule is capable of withstanding pressure of tens of MPa. Later, in order to establish the moment when a substance becomes a fluid, instead of visual observations in glass tubes, they returned to a technique close to that used by Cagniard de la Tour. Using special equipment, they began to measure the speed of sound in the medium being studied; at the moment the critical point is reached, the speed of propagation of sound waves drops sharply.

Application of SCF.

By the mid-1980s, reference books contained information on the critical parameters of hundreds of inorganic and organic substances, but the unusual properties of SCF were still not used.

Supercritical fluids began to be widely used only in the 1980s, when the general level of industry development allowed SCF production facilities to become widely available. From that moment on, the intensive development of supercritical technologies began. Researchers have primarily focused on the high solubility of SCF. Compared to traditional methods, the use of supercritical fluids has proven to be very effective. SCF are not only good solvents, but also substances with a high diffusion coefficient, i.e. they easily penetrate into the deep layers of various solids and materials. Supercritical CO 2 began to be used most widely, which turned out to be a solvent for a wide range of organic compounds. Carbon dioxide has become a leader in the world of supercritical technologies because it has a whole range of advantages. It is quite easy to transform it into a supercritical state (t cr - 31 ° C, p cr - 73.8 atm), in addition, it is non-toxic, non-flammable, non-explosive and, moreover, cheap and accessible. From the point of view of any technologist, it is an ideal component of any process. What makes it particularly attractive is that it is an integral part of atmospheric air and, therefore, does not pollute the environment. Supercritical CO 2 can be considered an environmentally friendly solvent.

The pharmaceutical industry was one of the first to turn to the new technology, since SCFs allow the most complete isolation of biologically active substances from plant raw materials, while maintaining their composition unchanged. The new technology fully complies with modern sanitary and hygienic standards for the production of medicines. In addition, the stage of distilling off the extraction solvent and its subsequent purification for repeated cycles is eliminated. Currently, the production of some vitamins, steroids and other drugs using this technology has been organized.

Caffeine, a drug used to improve the functioning of the cardiovascular system, is obtained from coffee beans even without first grinding them. Complete extraction is achieved due to the high penetrating ability of SCF. The grains are placed in an autoclave - a container that can withstand high pressure, then gaseous CO 2 is fed into it, and then the necessary pressure is created (>73 atm), as a result of which the CO 2 goes into a supercritical state. All contents are mixed, after which the fluid along with dissolved caffeine is poured into an open container. Carbon dioxide, once under atmospheric pressure, turns into gas and flies into the atmosphere, and the extracted caffeine remains in an open container in its pure form:

In the production of cosmetics and perfumes, SCF technologies are used to extract essential oils, vitamins, and phytoncides from plant and animal products. The extracted substances contain no traces of solvent, and the gentle extraction method allows them to preserve their biological activity.

In the food industry, new technology makes it possible to delicately extract various flavoring and aromatic components from plant materials that are added to food products.

Radiochemistry uses new technology to solve environmental problems. Many radioactive elements in a supercritical environment easily form complexes with added organic compounds - ligands. The resulting complex, unlike the initial compound of the radioactive element, is soluble in fluid, and therefore is easily separated from the bulk of the substance. In this way, it is possible to extract the remains of radioactive elements from waste ores, as well as to decontaminate soil contaminated with radioactive waste.

Removing contaminants using SC solvent is especially effective. There are projects of installations for removing contaminants from clothes (supercritical dry cleaning), as well as for cleaning various electronic circuits during their production.

In addition to the mentioned advantages, the new technology in most cases turns out to be cheaper than the traditional one.

The main disadvantage of supercritical solvents is that containers filled with SCF operate in a periodic process mode: loading raw materials into the apparatus - unloading finished products - loading a fresh portion of raw materials. It is not always possible to increase the productivity of an installation by increasing the volume of devices, since creating large containers that can withstand pressures close to 10 MPa is a difficult technical task.

For some chemical technology processes, it has been possible to develop continuous processes - a constant supply of raw materials and a continuous output of the resulting product. Productivity increases because that there is no need to waste time loading and unloading. In this case, the volume of the devices can be significantly reduced.

Hydrogen gas is highly soluble in supercritical CO 2 , allowing continuous hydrogenation of organic compounds in a fluid environment. The reactor containing the hydrogenation catalyst is continuously supplied with reagents (organic matter and hydrogen), as well as fluid. The products are discharged through a special valve, and the fluid simply evaporates and can be returned to the reactor. Using the described method, it is possible to hydrogenate almost a kilogram of the original compound in two minutes, and a reactor with such productivity literally fits in the palm of your hand. It is much easier to manufacture such a small reactor that can withstand high pressures than a large apparatus.

Such a reactor has been tested in the processes of hydrogenation of cyclohexene to cyclohexane (used as a solvent for essential oils and some rubbers), as well as isophorone to trimethylcyclohexanone (used in organic synthesis):

In polymer chemistry, supercritical CO 2 is rarely used as a polymerization medium. Most of the monomers are soluble in it, but during the polymerization process the growing molecule loses solubility long before it has time to grow noticeably. This disadvantage was turned into an advantage. Conventionally produced polymers are then effectively purified of impurities by recovering unreacted monomer and polymerization initiator using SCF. Due to exceptionally high diffusion properties, the fluid easily penetrates into the polymer mass. The process is technologically advanced - it does not require huge amounts of organic solvents, which, by the way, are difficult to remove from the polymer mass.

In addition, polymers easily swell when saturated with fluid, absorbing up to 30%. After swelling, the rubber ring almost doubles its thickness:

With a slow decrease in pressure, the previous size is restored. If you take a hard material, not an elastic one, and sharply release the pressure after swelling, then CO 2 quickly flies away, leaving the polymer in the form of a microporous material. This is essentially a new technology for producing porous plastics.

SC fluid is indispensable for introducing dyes, stabilizers, and various modifiers into the polymer mass. For example, copper complexes are introduced into the polyarylate, which upon subsequent reduction form metallic copper. As a result, a composition with increased wear resistance emerges from the polymer and evenly distributed metal.

Some polymers (polysiloxanes and fluorinated polyhydrocarbons) dissolve in SC-CO 2 at a temperature close to 100 0 C and a pressure of 300 atm. This fact allows the use of SCF as a medium for the polymerization of conventional monomers. Soluble fluorinated polyhydrocarbons are added to the polymerizing acrylate, with the growing molecule and the fluorinated “additive” holding each other together through polar interactions. Thus, the fluorinated groups of the added polymer play the role of “floaters” that maintain the entire system in solution. As a result, the growing polyacrylate molecule does not precipitate out of solution and manages to grow to a significant size:

In polymer chemistry, the previously mentioned property of fluids is used - to change the dissolving ability with increasing pressure ( cm. naphthalene dissolution graph). The polymer is placed in a fluid environment and, gradually increasing the pressure, portions of the solution are taken. In this way, it is possible to fairly finely divide the polymer into its constituent fractions, that is, sort the molecules by size.

Substances used as fluids. Prospects.

Now 90% of all SCF technologies are focused on supercritical CO 2. In addition to carbon dioxide, other substances are gradually beginning to come into use. Supercritical xenon (t cr - 16.6 ° C, p cr - 58 atm.) is an absolutely inert solvent, and therefore chemists use it as a reaction medium to produce unstable compounds (most often organometallic), for which CO 2 is a potential reagent. Widespread use of this fluid is not expected, since xenon is an expensive gas.

For the extraction of animal fats and vegetable oils from natural raw materials, supercritical propane (t cr - 96.8, p cr - 42 atm.) is more suitable, since it dissolves these compounds better than CO 2.

One of the most common and environmentally friendly substances is water, but it is quite difficult to transform it into a supercritical state, since the parameters of the critical point are very high: t cr - 374 ° C, p cr - 220 atm. Modern technologies make it possible to create installations that meet such requirements, but working in this range of temperatures and pressures is technically difficult. Supercritical water dissolves almost all organic compounds that do not decompose at high temperatures. Such water, when oxygen is added to it, becomes a powerful oxidizing medium, converting any organic compounds into H 2 O and CO 2 in a few minutes. Currently, they are considering the possibility of recycling household waste in this way, primarily plastic containers (such containers cannot be burned, since this creates toxic volatile substances).

Mikhail Levitsky

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Critical point- a combination of temperature and pressure values ​​(or, equivalently, molar volume) at which the difference in the properties of the liquid and gaseous phases of a substance disappears.

Critical phase transition temperature- temperature value at the critical point. Above the critical temperature, the gas cannot condense at any pressure.

Physical meaning

At the critical point, the density of the liquid and its saturated vapor become equal, and the surface tension of the liquid drops to zero, so the liquid-vapor phase boundary disappears.

For a mixture of substances, the critical temperature is not a constant value and can be represented by a spatial curve (depending on the proportion of the constituent components), the extreme points of which are the critical temperatures of the pure substances - the components of the mixture in question.

The critical point on the state diagram of a substance corresponds to the limiting points on the phase equilibrium curves; in the vicinity of the point, the phase equilibrium is disrupted, and a loss of thermodynamic stability in the density of the substance occurs. On one side of the critical point the substance is homogeneous (usually at), and on the other it separates into liquid and vapor.

In the vicinity of the point, critical phenomena are observed: due to an increase in the characteristic sizes of density fluctuations, the scattering of light sharply increases when passing through a substance - when the fluctuation sizes reach the order of hundreds of nanometers, i.e., the wavelengths of light, the substance becomes opaque - its critical opalescence is observed. An increase in fluctuations also leads to increased absorption of sound and an increase in its dispersion, a change in the nature of Brownian motion, anomalies in viscosity, thermal conductivity, a slowdown in the establishment of thermal equilibrium, etc.

In this typical phase diagram, the boundary between the liquid and gaseous phases is depicted as a curve starting at the triple point and ending at the critical point.

Story

The phenomenon of a critical state of matter was first discovered in 1822 by Charles Cagniard de La Tour, and in 1860 it was rediscovered by D.I. Mendeleev. Systematic research began with the work of Thomas Andrews. In practice, the phenomenon of the critical point can be observed when heating a liquid that partially fills a sealed tube. As the meniscus heats up, it gradually loses its curvature, becoming more and more flat, and when a critical temperature is reached, it ceases to be distinguishable.

Critical points exist not only for pure substances, but also, in some cases, for their mixtures and determine the parameters of the loss of stability of the mixture (with phase separation) - solution (one phase). An example of such a mixture is a phenol-water mixture.

Simple gases at the critical point, according to some data, have the property of being compressed to ultra-high densities without increasing pressure, subject to strict maintenance of a temperature equal to the critical point and a high degree of purity (foreign gas molecules become nuclei of transition to the gaseous phase, which leads to an avalanche-like increase in pressure). In other words, the substance is compressed like a gas, but maintains a pressure equal to that of a liquid. The implementation of this effect in practice will allow ultra-dense gas storage.

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