Evaporation of liquids. Saturated and unsaturated pairs. Saturated steam pressure. Air humidity.

Evaporation- vaporization occurring at any temperature from the free surface of the liquid. The uneven distribution of the kinetic energy of molecules during thermal motion leads to the fact that at any temperature the kinetic energy of some molecules of a liquid or solid can exceed the potential energy of their connection with other molecules. Molecules with a high speed have greater kinetic energy, and the body temperature depends on the speed of movement of its molecules, therefore, evaporation is accompanied by cooling of the liquid. Evaporation rate depends on: open surface area, temperature, concentration of molecules near the liquid.

Condensation- the process of transition of a substance from a gaseous state to a liquid state.

The evaporation of a liquid in a closed vessel at a constant temperature leads to a gradual increase in the concentration of molecules of the evaporating substance in the gaseous state. Some time after the start of evaporation, the concentration of the substance in the gaseous state will reach such a value at which the number of molecules returning to the liquid becomes equal to the number of molecules leaving the liquid in the same time. A dynamic equilibrium is established between the processes of evaporation and condensation of matter. A substance in a gaseous state that is in dynamic equilibrium with a liquid is called saturated vapor. (Vapor is a collection of molecules that have left the liquid in the process of evaporation.) Steam at a pressure below saturation is called unsaturated.

Due to the constant evaporation of water from the surfaces of reservoirs, soil and vegetation, as well as the respiration of humans and animals, the atmosphere always contains water vapor. Therefore, atmospheric pressure is the sum of the pressure of dry air and the water vapor in it. The water vapor pressure will be maximum when the air is saturated with steam. Saturated steam, unlike unsaturated steam, does not obey the laws of an ideal gas. Thus, the saturation vapor pressure does not depend on volume, but depends on temperature. This dependence cannot be expressed by a simple formula, therefore, on the basis of an experimental study of the dependence of saturated vapor pressure on temperature, tables have been compiled that can be used to determine its pressure at various temperatures.

The pressure of water vapor in air at a given temperature is called absolute humidity, or water vapor pressure. Since vapor pressure is proportional to the concentration of molecules, absolute humidity can be defined as the density of water vapor in the air at a given temperature, expressed in kilograms per cubic meter (p).

Most of the phenomena observed in nature, for example, the rate of evaporation, the drying of various substances, the withering of plants, does not depend on the amount of water vapor in the air, but on how close this amount is to saturation, that is, on relative humidity, which characterizes the degree of saturation air with water vapor. At low temperatures and high humidity, heat transfer increases and the person is exposed to hypothermia. At high temperatures and humidity, heat transfer, on the contrary, is sharply reduced, which leads to overheating of the body. The most favorable for humans in the middle climatic latitudes is a relative humidity of 40-60%. Relative humidity is the ratio of the density of water vapor (or pressure) in the air at a given temperature to the density (or pressure) of water vapor at the same temperature, expressed as a percentage, i.e.

Relative humidity varies widely. Moreover, the diurnal variation of relative humidity is inverse to the diurnal variation of temperature. During the day, with an increase in temperature and, consequently, with an increase in saturation pressure, the relative humidity decreases, and at night it increases. The same amount of water vapor can either saturate or not saturate the air. By lowering the temperature of the air, it is possible to bring the vapor in it to saturation. The dew point is the temperature at which the vapor in the air becomes saturated. When the dew point is reached in the air or on objects with which it comes into contact, water vapor begins to condense. To determine the humidity of the air, devices called hygrometers and psychrometers are used.

The molecular kinetic theory allows not only to understand why a substance can be in gaseous, liquid and solid states, but also to explain the process of transition of a substance from one state to another.

Evaporation and condensation. The amount of water or any other liquid in an open vessel gradually decreases. Evaporation of the liquid occurs, the mechanism of which was described in the course of physics of class VII. During chaotic motion, some molecules acquire such a large kinetic energy that they leave the liquid, overcoming the forces of attraction from the rest of the molecules.

Simultaneously with evaporation, the reverse process occurs - the transition of a part of randomly moving vapor molecules into a liquid. This process is called condensation. If the vessel is open, then the molecules that have left the liquid may not return to

liquid. In these cases, evaporation is not compensated by condensation and the amount of liquid decreases. When the air flow over the vessel carries away the formed vapors, the liquid evaporates faster, since the vapor molecule has less opportunity to return to the liquid again.

Saturated steam. If the vessel with the liquid is tightly closed, then its decline will soon stop. At a constant temperature, the "liquid - vapor" system will come to a state of thermal equilibrium and will remain in it for an arbitrarily long time.

At the first moment, after the liquid is poured into the vessel and closed, it will evaporate and the vapor density above the liquid will increase. However, at the same time, the number of molecules returning to the liquid will increase. The greater the vapor density, the greater the number of vapor molecules returned to the liquid. As a result, in a closed vessel at a constant temperature, a dynamic (moving) equilibrium between liquid and vapor will eventually be established. The number of molecules leaving the surface of the liquid will be equal to the number of vapor molecules returning to the liquid in the same time. Simultaneously with the evaporation process, condensation occurs, and both processes, on average, compensate each other.

Steam in dynamic equilibrium with its liquid is called saturated steam. This name emphasizes that a given volume at a given temperature cannot contain more steam.

If the air from the vessel with the liquid is previously pumped out, then only saturated vapor will be above the surface of the liquid.

Saturated steam pressure. What happens to saturated steam if the volume it occupies is reduced, for example, by compressing the vapor in equilibrium with the liquid in a cylinder under a piston, keeping the temperature of the contents of the cylinder constant?

When the vapor is compressed, the equilibrium will begin to be disturbed. At the first moment, the vapor density slightly increases, and more molecules begin to pass from gas to liquid than from liquid to gas. This continues until the equilibrium and density are again established, and hence the concentration of molecules does not take the same value. The concentration of saturated vapor molecules is therefore independent of volume at constant temperature.

Since the pressure is proportional to the concentration in accordance with the formula, then from the independence of the concentration (or density) of saturated vapors from the volume follows the independence of the pressure of saturated vapor from the volume it occupies.

The volume-independent vapor pressure at which a liquid is in equilibrium with its vapor is called saturation vapor pressure.

When saturated vapor is compressed, more and more of it goes into a liquid state. A liquid of a given mass occupies a smaller volume than a vapor of the same mass. As a result, the volume of vapor at a constant density decreases.

We have used the words "gas" and "steam" many times. There is no fundamental difference between gas and steam, and these words are generally equivalent. But we are accustomed to a certain, relatively small range of ambient temperature. The word "gas" is usually applied to those substances whose saturation vapor pressure at ordinary temperatures is above atmospheric (for example, carbon dioxide). On the contrary, they speak of steam when, at room temperature, the saturated vapor pressure is less than atmospheric pressure and the substance is more stable in the liquid state (for example, water vapor).

The independence of saturated vapor pressure from volume has been established in numerous experiments on the isothermal compression of vapor in equilibrium with its liquid. Let the substance at large volumes be in the gaseous state. As isothermal compression increases, its density and pressure increase (section of the isotherm AB in Figure 51). When the pressure is reached, the steam starts to condense. Further, when saturated vapor is compressed, the pressure does not change until all the vapor turns into a liquid (straight line BC in Figure 51). After that, the pressure during compression begins to increase sharply (a segment of the curve, since liquids are little compressible.

The curve shown in Figure 51 is called the real gas isotherm.

The saturation vapor pressure of a liquid increases sharply with increasing temperature. This can be seen from figure 12, which shows the vapor pressure curves of some liquids, starting at the melting points and ending at the critical points.

Rice. 12. The dependence of the saturation vapor pressure of some liquids on temperature.

The functional dependence of the saturated vapor pressure of a liquid on temperature can be expressed by equation (IV, 5), and far from the critical temperature, by equation (IV, 8).

Assuming the heat of evaporation (sublimation) to be constant in a small temperature range, we can integrate equation (IV, 8)

(IV, 9)

Representing equation (IV, 9) as an indefinite integral, we obtain:

(IV, 10),

where C is the integration constant.

In accordance with these equations, the dependence of the saturated vapor pressure of a liquid (or crystalline substance) on temperature can be expressed by a straight line in coordinates (in this case, the slope of the straight line is ). Such a dependence takes place only in a certain temperature range far from the critical one.

Figure 13 shows the dependence of the saturated vapor pressure of some liquids in the indicated coordinates, which fits satisfactorily into straight lines in the range of 0-100°C.

Rice. 13. Dependence of the logarithm of the saturated vapor pressure of some liquids on the reciprocal temperature.

However, equation (IV, 10) does not cover the dependence of saturated vapor pressure on temperature over the entire temperature range - from the melting temperature to the critical one. On the one hand, the heat of evaporation depends on temperature, and the integration should be carried out taking into account this dependence. On the other hand, saturated steam at high temperatures cannot be considered an ideal gas, because at the same time, its pressure increases significantly. Therefore, the equation covering the dependence P = f(T) over a wide temperature range, inevitably becomes empirical.

supercritical state- the fourth form of the aggregate state of matter, into which many organic and inorganic substances are able to pass.

The supercritical state of matter was first discovered by Cañar de la Tour in 1822. The real interest in the new phenomenon arose in 1869 after the experiments of T. Andrews. Conducting experiments in thick-walled glass tubes, the scientist 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 vapor in equilibrium with it, disappears, while the system becomes homogeneous (homogeneous) and the entire volume takes the form of a milky-white opalescent liquid. With a further increase in temperature, it quickly becomes transparent and mobile, consisting of constantly flowing jets, resembling streams of warm air over a heated surface. A further increase in temperature and pressure did not lead to visible changes.

The point at which such a transition occurs, he called critical, and the state of matter above this point - supercritical. Despite the fact that outwardly this state resembles a liquid, a special term is now used for it - supercritical fluid (from the English word fluid, i.e. "able to flow"). In modern literature, the abbreviation for supercritical fluids is accepted - SCF.

The location of the lines delimiting the regions of the gaseous, liquid and solid state, as well as the position of the triple point, where all three regions converge, is individual for each substance. The supercritical region begins at the critical point (indicated by an asterisk), which is necessarily characterized by two parameters - temperature ( T cr.) and pressure ( R cr.). A decrease in either temperature or pressure below critical values ​​takes the substance out of the supercritical state.

The fact of the existence of a critical point made it possible to understand why some gases, for example, hydrogen, nitrogen and oxygen, could not be obtained in liquid form with increasing pressure for a long time, which is why they were called permanent gases (from the Latin permanentis- "constant"). The above diagram shows that the region of existence of the liquid phase is located to the left of the critical temperature line. Thus, in order to liquefy any gas, it must first be cooled to a temperature below the critical one. CO 2 has a critical temperature above room temperature, so it can be liquefied under these conditions by increasing the pressure. Nitrogen has a much lower critical temperature, -239.9°C, so if you compress nitrogen under normal conditions, you can eventually reach the supercritical region, but liquid nitrogen cannot form. It is necessary to first cool the nitrogen below the critical temperature and then, by increasing the pressure, reach a region where the existence of a liquid is possible. The situation is similar for hydrogen and oxygen (critical temperatures are –118.4° C and –147° C, respectively), therefore, before liquefaction, they are cooled to a temperature below the critical one, and only then the pressure is increased. A supercritical state is possible for most substances, it is only necessary that the substance does not decompose at a critical temperature. In comparison with the indicated substances, the critical point for water is reached with great difficulty: t cr\u003d 374.2 ° C and R cr = 21,4 MPa.

The critical point is recognized as an important physical parameter of a substance, the same as the melting or boiling point. The density of SCF is exceptionally low, for example, water in the SCF state has a density three times lower than under normal conditions. All SCFs have an extremely low viscosity.

Supercritical fluids are a cross between a liquid and a gas. They can compress like gases (ordinary liquids are practically incompressible) and, at the same time, they are able to dissolve many substances in solid and liquid states, which is unusual 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 - stearic acid, paraffin, naphthalene. The properties of supercritical CO 2 as a solvent can be controlled - with an increase in pressure, its dissolving power increases sharply.

Supercritical fluids began to be widely used only in the 1980s, when the general level of industrial development made SFR facilities widely available. From that moment on, the intensive development of supercritical technologies began. SCFs 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 found the widest application, 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, as has a whole range of advantages. It is quite easy to transfer it to the supercritical state ( t cr- 31 ° С, R cr – 73,8 atm.), in addition, it is non-toxic, non-flammable, non-explosive, moreover, it is cheap and available. From the point of view of any technologist, it is an ideal component of any process. It is particularly attractive because it is an integral part of the atmospheric air and, therefore, does not pollute the environment. Supercritical CO 2 can be considered an environmentally absolutely pure solvent.

Now two independent directions of use of supercritical fluids have developed and productively coexist. These two directions differ in the ultimate goals of what is achieved with the help of these supercritical media. In the first case, SCFs are used to extract the necessary substances from various materials, products, or production waste. And there is a huge economic interest in this. In the second case, SCF is used directly for the implementation of valuable, often new chemical transformations. It should be emphasized that the advantages of SCF as extractants are primarily due to the fact that they turned out to be able to extremely effectively dissolve non-polar compounds, including solids. This main advantage is sharply enhanced by the high diffusivity of SCFs, which we have already mentioned, and their exceptionally low viscosity. Both of the latter features lead to the fact that the extraction rate becomes extremely high. Let's just give some examples.

Thus, deasphalting of lubricating oils is carried out using supercritical propane. Crude oil dissolves in supercritical propane at a pressure markedly higher than R cr. In this case, everything passes into the solution, except for heavy asphalt fractions. Due to the huge difference in viscosities between the supercritical fluid and the asphalt fraction, mechanical separation is very easy. Then the supercritical solution enters the expansion tanks, in which the pressure gradually decreases, remaining, however, higher R cr down to the last tank. In these tanks, progressively lighter impurity fractions of oils are sequentially separated from the solution due to a decrease in their solubility with a decrease in pressure. Separation of the phases in each of these containers is again very easy due to the sharp difference in their viscosities. The pressure in the last tank is lower R cr, propane evaporates, as a result, oil purified from unwanted impurities is released.

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

At present, the high solubility of H 2 in supercritical fluids is of great practical importance, since useful hydrogenation processes are very common. For example, an efficient process has been developed for the catalytic hydrogenation of CO 2 in the supercritical state, leading to the formation of formic acid. The process is very fast and clean.

In this lesson, we will analyze the properties of a somewhat specific gas - saturated steam. We will define this gas, point out how it fundamentally differs from the ideal gases we considered earlier, and, more specifically, how the dependence of the pressure of a saturated gas differs. Also in this lesson, such a process as boiling will be considered and described.

To understand the differences between saturated steam and an ideal gas, you need to imagine two experiments.

First, let's take a hermetically sealed vessel with water and start heating it. As the temperature increases, the molecules of the liquid will have an increasing kinetic energy, and an increasing number of molecules will be able to escape from the liquid (see Fig. 2), therefore, the vapor concentration will increase and, consequently, its pressure. So the first position:

Saturated vapor pressure depends on temperature

Rice. 2.

However, this provision is quite expected and not as interesting as the following. If you place a liquid with its saturated vapor under a movable piston and begin to lower this piston, then, undoubtedly, the concentration of saturated vapor will increase due to a decrease in volume. However, after some time, the vapor will move with the liquid to a new dynamic equilibrium by condensing an excess amount of vapor, and the pressure will not change in the end. The second position of the theory of saturated steam:

Saturated vapor pressure does not depend on volume

Now, it should be noted that the saturated vapor pressure, although it depends on temperature, like an ideal gas, but the nature of this dependence is somewhat different. The fact is that, as we know from the basic equation of the MKT, the gas pressure depends on both temperature and gas concentration. And therefore, the pressure of saturated vapor depends on temperature non-linearly until the vapor concentration increases, that is, until all the liquid has evaporated. The graph below (Fig. 3) shows the nature of the dependence of saturated vapor pressure on temperature,

Rice. 3

moreover, the transition from a non-linear section to a linear one just means the point of evaporation of the entire liquid. Since the pressure of a saturated gas depends only on temperature, it is possible to absolutely unambiguously determine what the saturated vapor pressure will be at a given temperature. These ratios (as well as the values ​​of the density of saturated steam) are listed in a special table.

Let us now turn our attention to such an important physical process as boiling. In the eighth grade, boiling was already defined as a process of vaporization more intense than evaporation. Now we will expand this concept somewhat.

Definition. Boiling- the process of vaporization occurring throughout the volume of the liquid. What is the boiling mechanism? The fact is that there is always dissolved air in water, and as a result of an increase in temperature, its solubility decreases, and microbubbles form. Since the bottom and walls of the vessel are not perfectly smooth, these bubbles cling to the irregularities on the inside of the vessel. Now the water-air section exists not only at the surface of the water, but also inside the volume of water, and water molecules begin to pass into the bubbles. Thus, saturated steam appears inside the bubbles. Further, these bubbles begin to float, increasing in volume and taking more water molecules into themselves, and burst near the surface, releasing saturated steam into the environment (Fig. 4).

Rice. 4. Boiling process ()

The condition for the formation and ascent of these bubbles is the following inequality: the saturated vapor pressure must be greater than or equal to atmospheric pressure.

Thus, since the pressure of saturated vapor depends on temperature, the boiling point is determined by the pressure of the environment: the lower it is, the lower the temperature at which the liquid boils, and vice versa.

In the next lesson, we will begin to consider the properties of rigid bodies.

Bibliography

1. Myakishev G.Ya., Sinyakov A.Z. Molecular physics. Thermodynamics. - M.: Bustard, 2010.
2. Gendenstein L.E., Dick Yu.I. Physics grade 10. - M.: Ileksa, 2005.
3. Kasyanov V.A. Physics grade 10. - M.: Bustard, 2010.

1. Physics.ru ().
2. Chemport.ru ().
3. Narod.ru ().

Homework

1. Page 74: No. 546-550. Physics. Task book. 10-11 grades. Rymkevich A.P. - M.: Bustard, 2013. ()
2. Why can't climbers boil eggs at altitude?
3. What are some ways you can cool hot tea? Justify them in terms of physics.
4. Why should the gas pressure on the burner be reduced after boiling water?
5. * How can water be heated above one hundred degrees Celsius?

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Slides captions:

Saturated steam. Dependence of pressure of saturated vapor on temperature. Humidity. Guseva N.P. MOU secondary school No. 41, Saratov

EVAPORATION The process of transition of a substance from a liquid state to a gaseous state is evaporation; the reverse process is called condensation; Evaporation occurs at any temperature other than absolute zero; the rate of evaporation of a liquid depends on the temperature, the area of ​​the evaporated surface, the type of liquid, and the wind.

BOILING - the process of vaporization occurring throughout the volume of a liquid The boiling point is the temperature of a liquid at which the pressure of its saturated vapor is equal to or greater than the external pressure. To maintain boiling, heat must be supplied to the liquid, which is spent on vaporization, because the internal energy of a vapor is greater than the internal energy of a liquid of the same mass. During the boiling process, the temperature of the liquid remains constant.

Steam is a gas formed by evaporated liquid molecules. The equation p \u003d nkT is true for it. The main difference in the behavior of an ideal gas and saturated steam: when the temperature of the steam in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the vapor changes. The liquid partially turns into vapor, or, conversely, the vapor partially condenses. Nothing like this happens with an ideal gas.

The main property of saturated steam is that the vapor pressure at constant temperature does not depend on volume. When all the liquid has evaporated, the vapor, upon further heating, will cease to be saturated and its pressure at constant volume will increase in direct proportion to the absolute temperature (see Fig. 11.1, section of the BC curve). p = nkT

Under what conditions does boiling begin? The liquid always contains dissolved gases that are released on the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are the centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the vapor pressure increases and the bubbles increase in size. Under the action of the buoyant force, they float up. Boiling begins when the pressure of saturated vapor inside the bubbles becomes equal and greater than the external pressure and the hydrostatic pressure of the liquid column.

The greater the external pressure, the higher the boiling point. So, in a steam boiler at a pressure reaching 1.6 10 6 Pa, water does not boil even at a temperature of 200°C. In medical institutions in hermetically sealed vessels - autoclaves (Fig. 11.2), water also boils at elevated pressure. Therefore, the boiling point of the liquid is much higher than 100°C. Autoclaves are used to sterilize surgical instruments, etc.

By lowering the external pressure, we thereby lower the boiling point. By pumping out air and water vapor from the flask, you can make the water boil at room temperature (Fig. 11.3). As you climb mountains, atmospheric pressure decreases, so the boiling point decreases. At an altitude of 7134 m (Lenin Peak in the Pamirs), the pressure is approximately equal (300 mm Hg). Water boils there at about 70°C. It is impossible to cook meat in these conditions.

What process is called evaporation? What factors affect the rate of evaporation of a liquid? What process is called condensation? How to explain evaporation processes from the point of view of MKT? Why is evaporation accompanied by a decrease in liquid temperature?

5. Why does the temperature of a liquid not change during boiling, although the liquid continues to receive energy from the heater? 6. What force raises the bubbles to the surface of the liquid? 7. Is it possible to make water boil at temperatures below 100°C?

AIR HUMIDITY In the Earth's atmosphere there is 13 - 15 thousand km 3 of water in the form of drops, crystals and water vapor. The amount of water vapor in the air is called humidity. Humidity is characterized by: partial pressure (p) - the pressure that water vapor would produce if all other gases were absent; relative humidity (φ) - the ratio of the partial pressure p of water vapor contained in the air at a given temperature to the pressure p of saturated vapor at the same temperature

The weather forecast indicates the value of the relative humidity in percent! Relative humidity indicates how close the water vapor content of the air is to saturation. At a relative humidity of 100%, there is saturated water vapor in the air. Both excessive dry air and high humidity are harmful to human health. The most comfortable air humidity for a person lies in the range of 40-60%.