Since the magnitude of the saturation vapor pressure depends on the air temperature, with an increase in the latter, the air can absorb more water vapor, while the saturation pressure increases. The increase in saturation pressure does not occur linearly, but along a long curve. This fact is so important for building physics that it should not be overlooked. For example, at a temperature of 0 ° C (273.16 K), the pressure of saturated steam pnas is 610.5 Pa (Pascal), at +10 ° C (283.16 K) it turns out to be equal to 1228.1 Pa, at +20 ° С (293.16 K) 2337.1 Pa, and at +30 ° С (303.16 K) it is equal to 4241.0 Pa. Therefore, with an increase in temperature by 10 ° C (10 K), the saturation vapor pressure will approximately double.
The dependence of the partial pressure of water vapor on temperature changes is shown in fig. 3.
ABSOLUTE HUMIDITY f
Water vapor density, i.e. its content in the air is called the absolute humidity of the air and is measured in g / m.
The maximum water vapor density that is possible at a certain air temperature is called the saturation vapor density, which in turn creates saturation pressure. The density of saturated steam fsat and its pressure psat increase with increasing air temperature. Its increase is also curvilinear, but the course of this curve is not as steep as the course of the rnas curve. Both curves depend on the value 273.16/Tact[K]. Therefore, if the ratio pnas/fus is known, they can be checked against each other.
The absolute humidity of the air in an airtight enclosed space does not depend on the temperature
temperature until the density of saturated steam is reached. The dependence of the absolute humidity of the air on its temperature is shown in Fig. four.
RELATIVE HUMIDITY
The ratio of the actual density of water vapor to the density of saturated steam or the ratio of the absolute humidity of the air to the maximum humidity of the air at a certain temperature is called the relative humidity of the air. It is expressed as a percentage.
When the temperature of an airtight enclosed space decreases, the relative humidity of the air will increase until the value of ϕ becomes equal to 100% and thus the saturation vapor density is reached. With further cooling, the corresponding excess amount of water vapor condenses.
With an increase in the temperature of a closed space, the value of the relative humidity of the air decreases. Rice. 5 illustrates the dependence of the relative humidity of the air on temperature. The relative humidity of the air is measured using a hygrometer or psychrometer. The very reliable Assmann aspiration psychrometer measures the temperature difference between two precision thermometers, one of which is wrapped in damp gauze. Cooling due to the evaporation of water is the greater, the drier the surrounding air. From the ratio of the temperature difference to the actual air temperature, the relative humidity of the ambient air can be determined.
Instead of a thin hair hygrometer, which is sometimes used in high humidity, a lithium-chloride measuring probe is used. He co-
it is made of a metal sleeve with a fiberglass sheath, a separate winding of a heating wire and a resistance thermometer. The fabric sheath is filled with an aqueous lithium chloride solution and is under the action of an alternating voltage between both windings. Water evaporates, salt crystallization occurs and the resistance increases significantly. As a result, the content of water vapor in the surrounding air and the heating power are balanced. According to the temperature difference between the ambient air and the built-in thermometer, using a special measuring circuit, the relative humidity of the air is determined.
The measuring probe reacts to the influence of air humidity on the hygroscopic fiber, which is designed so that a sufficient current arises between the two electrodes. The latter grows as the relative humidity increases in a certain dependence on the air temperature.
A capacitive measuring probe is a condenser with a perforated plate, equipped with a hygroscopic dielectric, the capacitance of which changes with changes in relative humidity, as well as the temperature of the surrounding air. The measuring probe can be used as part of the so-called RC element of the multivibartor circuit. In this case, the humidity of the air is converted to a certain frequency, which can have high values. In this way, an extremely high sensitivity of the instrument is achieved, which makes it possible to record minimal changes in humidity.
PARTIAL PRESSURE OF WATER VAPOR p
Unlike saturation vapor pressure pnas, which denotes the maximum partial pressure of water vapor in air at a certain temperature, the concept of partial pressure of water vapor p denotes the pressure of vapor that is in an unsaturated state, therefore in each case this pressure must be less than rnas.
As the content of water vapor in dry air increases, the value of p approaches the corresponding value of pnas. At the same time, the atmospheric pressure Ptot remains constant. Since the partial pressure of water vapor p is only a fraction of the total pressure of all components of the mixture, its value cannot be determined by direct measurement. On the contrary, the vapor pressure can be determined by first creating a vacuum in the vessel and then introducing water into it. The magnitude of the increase in pressure due to evaporation corresponds to the value of pnas, which refers to the temperature of the space saturated with steam.
With psa known, p can be indirectly measured as follows. The vessel contains a mixture of air and water vapor, first of all of unknown composition. Pressure inside the vessel Ptot = pv + p, i.e. atmospheric pressure of the surrounding air. If you now close the vessel and introduce a certain amount of water into it, then the pressure inside the vessel will increase. After saturation of the water vapor, it will be pv + rnas. The pressure difference pnas - p established with the help of a micromanometer is subtracted from the already known value of the saturated vapor pressure, which corresponds to the temperature in the vessel. The result will correspond to the partial pressure p of the original contents of the vessel, i.e. ambient air.
It is easier to calculate the partial pressure p using data from tables of saturated steam pressure pnas for a certain temperature level. The value of the ratio p / rnas corresponds to the value of the ratio of the density of water vapor f to the density of saturated steam fsat, which is equal to the value of relative humidity
air quality. Thus, we get the equation
nie p = rnas.
As a result, at known air temperature and saturation pressure pnas, it is possible to quickly and clearly determine the value of the partial pressure p. For example, the relative humidity of the air is 60% and the temperature of the air is 10°C. Then, since at this temperature the saturated vapor pressure psa = 1228.1 Pa, the partial pressure p will be equal to 736.9 Pa (Fig. 6).
WATER VAPOR DEW POINT t
The water vapor contained in the air is usually in an unsaturated state and therefore has a certain partial pressure p and a certain relative humidity of the air.<р < 100%.
If the air is in direct contact with solid materials whose surface temperature is lower than its temperature, then with an appropriate temperature difference, the air of the boundary layer cools and its relative humidity increases until its value reaches 100%, i.e. saturated steam density. Even with a slight further cooling, water vapor begins to condense on the surface of a solid material. This will continue until a new equilibrium state of the material surface temperature and saturated vapor density is established. Due to the high density, cooled air sinks, while warmer air rises. The amount of condensate will increase until equilibrium is established and the condensation process stops.
The condensation process is associated with the release of heat, the amount of which corresponds to the heat of vaporization of water. This leads to an increase in the surface temperature of the solids.
The dew point t is the temperature of the surface, the vapor density near which becomes equal to the density of saturated vapor, i.e. the relative humidity of the air reaches 100%. Water vapor condensation begins immediately after its temperature drops below the dew point.
If the air temperature AT and relative humidity are known, the equation p(AT) = rnat(t) = pat can be made. To calculate the required value of pnas, use the saturated vapor pressure table.
Consider an example of such a calculation (Fig. 7). Air temperature vv \u003d 10 ° С, relative humidity \u003d 60%, pnas (+10 ° С) \u003d 1228.1 P pnas (t) \u003d \u003d 0 6 x 1228.1 Pa \u003d 736.9 Pa, dew point \u003d + 2.6°C (table).
The dew point can be determined graphically using the saturation pressure curve. The dew point can only be calculated if, in addition to the air temperature, the relative humidity is also known. Instead of a calculation, you can use a measurement. If you slowly cool the polished surface of a plate (or membrane) made of a heat-conducting material until condensation begins to fall on it, and then measure the temperature of this surface, you can directly find the dew point of the ambient air. this method does not require knowledge of the relative humidity of the air, although it is possible to additionally calculate the value from the air temperature and dew point
On this principle, the operation of the hygrometer for determining the dew point of Daniel and Reynolt, which was developed in the first half of the 19th century, is based. Recently, thanks to the use of electronics, it has been improved so much that it can determine the dew point with very high accuracy. Thus, it is possible to properly calibrate a normal hygrometer and control it with a dew point hygrometer.
Oil and oil products are characterized by a certain saturated vapor pressure, or oil vapor pressure. Saturated vapor pressure is a normalized indicator for aviation and motor gasolines, which indirectly characterizes the volatility of the fuel, its starting qualities, and the tendency to form vapor locks in the engine power system.
For liquids of heterogeneous composition, such as gasolines, the saturation vapor pressure at a given temperature is a complex function of the composition of gasoline and depends on the volume of space in which the vapor phase is located. Therefore, in order to obtain comparable results, practical determinations must be carried out at a standard temperature and a constant ratio of the vapor and liquid phases. In view of the above saturated vapor pressure fuels are called the pressure of the vapor phase of the fuel, which is in dynamic equilibrium with the liquid phase, measured at a standard temperature and a certain ratio of the volumes of the vapor and liquid phases. The temperature at which the saturated vapor pressure becomes equal to the pressure in the system is called the boiling point of the substance. Saturated vapor pressure increases sharply with increasing temperature. At the same temperature, lighter oil products are characterized by a higher saturated vapor pressure.
Currently, there are several ways to determine the DNP of substances, which can be divided into the following groups:
- static method.
- dynamic method.
- Moving gas saturation method.
- Method for studying isotherms.
- Knudsen effusion method.
- chromatographic method.
Static method
Static Method is the most common, because acceptable when measuring the DNP of substances in a wide range of temperatures and pressures. The essence of the method is to measure the pressure of a vapor that is in equilibrium with its liquid at a certain temperature. Pressure can be measured either with manometers (spring, mercury, deadweight, water), or with the help of special sensors (strain gauge, electrical, etc.), which allow conversion to pressure, or by calculation, when the amount of substance in a certain volume is known. The most widely used method using various pressure gauges, the so-called direct static method. In this case, the test substance is poured into a piezometer (or any container), placed in a thermostat that allows maintaining a certain temperature, and using a pressure gauge, it measures the DNP. Moreover, the connection of the pressure gauge can be carried out both in the liquid phase and in the gas phase. When connecting a manometer in the liquid phase, a correction for the hydrostatic liquid column is taken into account. The connection of the measuring device is usually carried out through a separator, which is used as mercury locks, membranes, bellows, etc.
On the basis of the direct static method, a number of experimental setups have been created for studying the DNP of petroleum products.
In oil refining, due to its simplicity, the standard Reid's bomb method(GOST 1756-2000). The bomb consists of two chambers: fuel 1 and air 2 with a volume ratio of 1:4, respectively, connected by a thread. The pressure created by the vapors of the tested fuel is recorded by a pressure gauge 3 attached to the top of the air chamber. The test is carried out at a temperature of 38.8°C and a pressure of 0.1 MPa provided by a special temperature-controlled bath.
The saturated vapor pressure of the test liquid is determined by the formula:
Determining the vapor pressure in a Reid bomb gives approximate results that serve only for a comparative assessment of the quality of motor fuels.
The advantages of the device include the simplicity of design and experimentation, the disadvantages are the constant ratio of the liquid and vapor phases and the roughness of the method (the error in determining the DNP of gasoline reaches 15-20%).
A more accurate option for measuring DNP by a static method is the Sorrel-NATI method. This method can be used to determine the absolute values of saturated vapor pressure even at negative temperatures. The advantage of this method is the ability to measure DNP at various ratios of the liquid and vapor phases, as well as in the presence or absence of air and gases dissolved in the substance. The disadvantages include complexity, applicability only in special laboratories, and a relatively large error in measuring DNP (up to 5%).
The discrepancies between the data obtained using the Reid bomb and the NATI method are 10-20%.
Dynamic method
Dynamic method based on measuring the boiling point of a liquid at a certain pressure. Existing experimental setups based on the dynamic method use ebulliometers in their designs. These are devices based on the principle of spraying a thermometer with a vapor-liquid mixture. The dynamic method was developed to study the DNP of pure substances, for which the boiling point is a fixed value, and was not used to measure the pressure of saturated petroleum products, the boiling point of which changes as the components boil away. It is known that narrow-boiling oil fractions occupy an intermediate position between pure substances and mixtures. The range of pressure measurement by the dynamic method is usually small - up to 0.15-0.2 MPa. Therefore, attempts have recently been made to apply the dynamic method to study the DNP of narrow oil fractions.
Moving gas saturation method
Moving gas saturation method it is used in the case when the DNP of the substance does not exceed a few mm Hg. The disadvantage of the method is the relatively large error in the experimental data and the need to know the molecular weight of the substance under study. The essence of the method is as follows: an inert gas is passed through the liquid and saturated with the vapors of the latter, after which it enters the refrigerator, where the absorbed vapors are condensed. Knowing the amount of gas and absorbed liquid, as well as their molecular weights, it is possible to calculate the saturated vapor pressure of the liquid.
Method for studying isotherms
Method for studying isotherms gives the most accurate results compared to other methods, especially at high temperatures. This method consists in studying the relationship between pressure and volume of saturated steam at a constant temperature. At the saturation point, the isotherm should have a kink, turning into a straight line. It is believed that this method is suitable for measuring the DNP of pure substances and is unsuitable for multicomponent substances, in which the boiling point is an undetermined value. Therefore, it has not received distribution in the measurement of DNP of petroleum products.
Knudsen effusion method
Knudsen effusion method applicable mainly for measuring very low pressures (up to 100 Pa). This method makes it possible to find the rate of vapor effusion by the amount of condensate, provided that the effluent substance is completely condensed. Installations based on this method have the following disadvantages: they are single measurement installations and require depressurization after each measurement, which, in the presence of easily oxidizing and unstable substances, often leads to chemical transformation of the test substance and distortion of the measurement results. An experimental setup has been created that does not have these shortcomings, but the complexity of the design allows it to be used only in specially equipped laboratories. This method is mainly used to measure the DNP of solids.
Knudsen effusion method
Chromatographic method of determination DNP substances began to be developed relatively recently. In this method, the determination of the DNP of petroleum products is based on a complete chromatographic analysis of the liquid and the calculation of the sum of the partial pressures of all components of the mixture. The method for determining the DNP of individual hydrocarbons and fractions of petroleum products is based on the ideas developed by the authors about the physicochemical retention index and the concept of phase specificity. For this purpose, it is necessary to have either a capillary chromatographic column with a high separating power, or literature data on the retention indices of the compounds under study.
However, when analyzing such complex mixtures of hydrocarbons as petroleum products, difficulties arise not only in the separation of hydrocarbons belonging to different classes, but also in the identification of individual components of these mixtures.
Saturated vapor pressure conversion
In technological calculations, it is often necessary to recalculate temperatures from one pressure to another, or pressure with a change in temperature. There are many formulas for this. Ashworth's formula has received the greatest application:
The Ashworth formula refined by V.P. Antonchenkov has the form:
To recalculate temperature and pressure, it is also convenient to use graphical methods.
The most common plot is the Cox plot, which is constructed as follows. The abscissa axis is a logarithmic scale, on which the values of the logarithm of pressure are plotted ( lgP), however, for ease of use, the corresponding values are plotted on the scale R. Temperature values are plotted on the y-axis. A straight line is drawn at an angle of 30° to the abscissa axis, indicated by the index " H 2 0”, which characterizes the dependence of saturated water vapor pressure on temperature. When constructing a graph from a series of points on the abscissa axis, restore perpendiculars to the intersection with a straight line H 2 0 and the resulting points are transferred to the y-axis. On the y-axis, a scale is obtained, built on the boiling points of water, corresponding to various pressures of its saturated vapors. Then, for several well-studied hydrocarbons, a series of points are taken with pre-known boiling points and their corresponding saturation vapor pressures.
It turned out that for alkanes of a normal structure, the graphs constructed according to these coordinates are straight lines that all converge at one point (pole). In the future, it is enough to take any point with the coordinates temperature - hydrocarbon saturated vapor pressure and connect it to the pole in order to obtain the dependence of saturated vapor pressure on temperature for this hydrocarbon.
Despite the fact that the graph is constructed for individual normal alkanes, it is widely used in technological calculations for narrow petroleum fractions, plotting the average boiling point of this fraction on the y-axis.
To recalculate the boiling points of petroleum products from deep vacuum to atmospheric pressure, the UOP nomogram is used, according to which, by connecting two known values on the corresponding scales of the graph with a straight line, the desired value is obtained at the intersection with the third scale R or t. The UOP nomogram is mainly used in laboratory practice.
Saturated vapor pressure of mixtures and solutions, in contrast to individual hydrocarbons, depends not only on temperature, but also on the composition of the liquid and vapor phases. For solutions and mixtures obeying the laws of Raoult and Dalton, the total saturated vapor pressure of the mixture can be calculated using the formulas:
In the region of high pressures, as is known, real gases do not obey the laws of Raoult and Dalton. In such cases, the saturated vapor pressure found by calculation or graphical methods is refined using critical parameters, compressibility factor and fugacity.
| Density |
The saturation vapor pressure of a liquid consisting of strongly interacting molecules is less than the saturation vapor pressure of a liquid consisting of weakly interacting molecules. Tmg 1600 6 0.4 - transformer Tmg tmtorg.ru.
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.
Saturated steam, unlike unsaturated steam, does not obey the laws of an ideal gas.
Thus, the pressure of saturated vapor does not depend on volume, but depends on temperature (it is approximately described by the equation of state of an ideal gas p = nkT). 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.
With increasing temperature, the pressure of saturated vapor increases faster than that of an ideal gas. When a liquid is heated in a closed vessel, the vapor pressure increases not only due to an increase in temperature, but also due to an increase in the concentration of molecules (mass of vapor) due to the evaporation of the liquid. This does not happen with an ideal gas. When all the liquid has evaporated, the vapor, upon further heating, will cease to be saturated and its pressure at constant volume will be directly proportional to the temperature.
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.
AIR HUMIDITY
The concept of air humidity and its dependence on temperature
Determination of relative humidity. Formula. Units.
Dew point
Determination of relative humidity through saturated vapor pressure. Formula
Hygrometers and psychrometers
At the same temperature, the content of water vapor in the air can vary over a wide range: from zero (absolutely dry air) to the maximum possible (saturated steam)
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.
Partial pressure of water vapor (or water vapor pressure)
Atmospheric air is a mixture of various gases and water vapor.
The pressure that water vapor would produce if all other gases were absent is called the partial pressure of water vapor.
Partial water vapor pressure is taken as one of the indicators of air humidity.
Expressed in units of pressure - Pa or mm Hg.
Absolute air humidity
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.
Absolute humidity shows how many grams of water vapor are contained in 1 m3 of air under given conditions.
Designation - ρ
This is the density of water vapor.
Relative humidity
The partial pressure of water vapor cannot be used to judge how close it is to saturation. Namely, the intensity of water evaporation depends on this. Therefore, a value is introduced that shows how close water vapor at a given temperature is to saturation - relative humidity.
Relative humidity φ is the ratio of the partial pressure p of water vapor contained in the air at a given temperature to the pressure p0 of saturated vapor at the same temperature, expressed as a percentage:
Relative humidity - the percentage of the concentration of water vapor in the air and the concentration of saturated vapor at the same temperature
Saturated vapor concentration is the maximum concentration a vapor can have over a liquid. Therefore, the relative humidity can vary from 0 to nn.p
The lower the relative humidity, the drier the air and the more intense the evaporation.
Relative humidity of 25% at +20-25°C is optimal for optimal human heat transfer. At higher temperatures, the optimum humidity is 20%
Since vapor concentration is related to pressure (p = nkT), relative humidity can be expressed as a percentage of vapor pressure in air and saturation vapor pressure at the same temperature:
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%.
If moist air is cooled, then at a certain temperature the vapor in it can be brought to saturation. With further cooling, water vapor will begin to condense in the form of dew. Fog appears, dew falls.
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The pressure (elasticity) of a saturated vapor of an individual substance or mixture of substances is the pressure of the vapor phase that is in equilibrium (i.e., in the limiting, unchanging state) with the liquid phase at a given temperature. In oil refining, the standard method with the Reid bomb according to GOST 1756-2000 is widely used, which has two high-pressure chambers hermetically connected on the thread, the volume of the steam chamber is 4 times the volume of the liquid chamber. The test liquid, for example, gasoline, is poured into the lower chamber, the chambers are connected and heated in a thermostat to a standard temperature of 38 °C. After exposure to achieve equilibrium between the vapor phase (saturated vapor) and the liquid phase, the pressure of the saturated vapor is determined by the pressure gauge on the steam chamber. Such an experimental method is approximate (since, in principle, an infinitely long time is required to achieve an equilibrium state, and air and water vapor are present in the steam chamber before the experiment), but this method is sufficient to assess the conditions of transportation and storage, the magnitude of losses from evaporation, and the commercial characteristics of gasoline , stable gas condensates and liquefied gases. For example, GPP products are ethane, propane, butane, natural gasoline (or mixtures thereof). Natural gasoline is liquefied hydrocarbons extracted from associated petroleum and natural gases. Saturated vapor pressure of commercial gas gasoline should be 0.07-0.23 MPa (0.7-2.4 kg / cm2), propane (liquid) - no more than 1.45 MPa (14.8 kg / cm2), butane (liquid) - no more than 0.48 MPa (4.9 kg / cm2), and motor gasolines and stable gas condensates for shipment in railway tanks - no more than 66.7-93.3 kPa (500-700 mm Hg. ). Thus, the saturation vapor pressure depends on the composition of the initial liquid and temperature. Saturated vapor pressure of hydrocarbons and their mixtures is the most important characteristic for calculating various mass transfer processes (single evaporation of liquid mixtures, single condensation of gas mixtures, absorption of hydrocarbon gases, rectification of liquid multicomponent raw materials, etc.).
Therefore, the literature provides both reference data and numerous empirical formulas for determining the saturation vapor pressure for various temperatures and pressures. The main physical properties of some hydrocarbons and gases are given in table. 2.3 and 2.4.
« Physics - Grade 10 "
What do you think will happen to saturated steam if the volume it occupies is reduced: for example, if you compress 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. The vapor density at the first moment will increase slightly, and more molecules will begin to pass from gas to liquid than from liquid to gas. After all, the number of molecules leaving the liquid per unit time depends only on the temperature, and the compression of the vapor does not change this number. The process continues until the dynamic equilibrium and vapor density are again established, and hence the concentration of its molecules will not take their previous values. Consequently,
the concentration of saturated vapor molecules at a constant temperature does not depend on its volume.
Since the pressure is proportional to the concentration of molecules (p = nkT), it follows from this definition that the pressure of saturated vapor does not depend on the volume it occupies.
pH pressure n pair, in which the liquid is in equilibrium with its vapor, is called saturated steam pressure.
When saturated vapor is compressed, more and more of it passes 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.
Gas laws for saturated steam are unfair (for any volume at a constant temperature, the pressure of saturated steam is the same). At the same time, the state of saturated steam is quite accurately described by the Mendeleev-Clapeyron equation.
unsaturated steam
> If the vapor is gradually compressed at a constant temperature, and its transformation into a liquid does not occur, then such a vapor is called unsaturated.
With a decrease in volume (Fig. 11.1), the pressure of unsaturated vapor increases (section 1-2), just as the pressure changes with a decrease in the volume of an ideal gas. At a certain volume, the vapor becomes saturated, and with further compression, it turns into a liquid (section 2-3). In this case, saturated vapor will already be above the liquid.
As soon as all the vapor turns into a liquid, a further decrease in volume will cause a sharp increase in pressure (liquid is incompressible).
However, vapor does not turn into liquid at any temperature. If the temperature is above a certain value, then no matter how we compress the gas, it will never turn into a liquid.
>The maximum temperature at which a vapor can still turn into a liquid is called critical temperature.
Each substance has its own critical temperature, for helium T cr = 4 K, for nitrogen T cr = 126 K.
The state of matter at a temperature above the critical temperature is called gas; at a temperature below the critical one, when the vapor has the opportunity to turn into a liquid, - ferry.
The properties of saturated and unsaturated steam are different.
Dependence of pressure of saturated vapor on temperature.
The state of saturated steam, as experience shows, is approximately described by the equation of state of an ideal gas (10.4), and its pressure is determined by the formula
r n. n = nkT. (11.1)
As the temperature rises, the pressure rises
Since the pressure of saturated vapor does not depend on volume, therefore, it depends only on temperature.
However, the dependence of the pressure pH. n on temperature T, found experimentally, is not directly proportional, as in an ideal gas at a constant volume. With increasing temperature, the pressure of a real saturated steam increases faster than the pressure of an ideal gas (Fig. 11.2, section of the curve AB). This becomes obvious if we draw the isochores of an ideal gas through points A and B (dashed lines). Why is this happening?
When a liquid is heated in a closed vessel, part of the liquid turns into vapor. As a result, according to formula (11.1), the saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor.
Basically, the increase in pressure with increasing temperature is determined precisely by the increase in concentration. The main difference in the behavior of an ideal gas and saturated steam is that when the temperature of the vapor in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the vapor changes.
Why are tables of saturated vapor pressure versus temperature and no tables of gas pressure versus temperature?
The liquid partially turns into vapor, or, conversely, the vapor partially condenses. Nothing like this happens with an ideal gas.
When all the liquid evaporates, 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. 11.2, section of the BC curve).
Boiling.
As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles form throughout the entire volume of the liquid, which float to the surface.
Boiling- This is the process of vaporization occurring throughout the volume of the liquid at the boiling point.
Under what conditions does boiling begin?
How is the heat supplied to the liquid spent during boiling from the point of view of molecular-kinetic theory?
The boiling point of a liquid remains constant. This is because all the energy supplied to the liquid is spent on turning it into vapor.
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. If the upper layers of the liquid have a lower temperature, then vapor condenses in these layers in the bubbles. The pressure drops rapidly and the bubbles collapse. The collapse is so fast that the walls of the bubble, colliding, produce something like an explosion. Many of these microexplosions create a characteristic noise. When the liquid warms up enough, the bubbles stop collapsing and float to the surface. The liquid will boil.
The dependence of saturation vapor pressure on temperature explains why the boiling point of a liquid depends on the pressure on its surface. A vapor bubble can grow when the pressure of the saturated vapor inside it slightly exceeds the pressure in the liquid, which is the sum of the air pressure on the surface of the liquid (external pressure) and the hydrostatic pressure of the liquid column.
Let us pay attention to the fact that the evaporation of a liquid also occurs at temperatures lower than the boiling point, but only from the surface of the liquid, while at boiling, the formation of vapor occurs throughout the entire volume of the liquid.
Boiling begins at a temperature at which the saturation vapor pressure in the bubbles equalizes and becomes slightly greater than the pressure in the liquid.
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. 11.3), water also boils at elevated pressure. Therefore, the boiling point of the liquid is much higher than 100 °C. Autoclaves are used, for example, to sterilize surgical instruments, speed up cooking (pressure cooker), preserve food, and carry out chemical reactions.
Conversely, by reducing 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. 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 4 10 4 Pa (300 mm Hg). Water boils there at about 70°C. It is impossible to cook meat in these conditions.
Each liquid has its own boiling point, which depends on the properties of the liquid. At the same temperature, the saturation vapor pressure of different liquids is different.
For example, at a temperature of 100 ° C, the pressure of saturated water vapor is 101,325 Pa (760 mm Hg), and mercury vapor is only 117 Pa (0.88 mm Hg). Since boiling occurs at the same temperature at which the saturated vapor pressure is equal to the external pressure, water boils at 100 ° C, but mercury does not. Mercury boils at 357°C at normal pressure.
