Since saturated steam is one of the components of a thermodynamically equilibrium system of a substance homogeneous in composition but different in phase fractions, understanding the influence of individual physical factors on the amount of pressure it creates makes it possible to use this knowledge in practical activities, for example, when determining the burnout rate of certain liquids in the event of a fire, etc.
Dependence of saturated vapor pressure on temperature
The saturated vapor pressure becomes higher as the temperature increases. In this case, the change in values is not directly proportional, but occurs much faster. This is due to the fact that with increasing temperature, the movement of molecules relative to each other accelerates and it is easier for them to overcome the forces of mutual attraction and move into a different phase, i.e. the number of molecules in the liquid state decreases, and in the gaseous state it increases until all the liquid turns into vapor. This increasing pressure causes the lid to rise in the pan or when the water begins to boil.
Dependence of saturated vapor pressure on other factors
The amount of saturated vapor pressure is also influenced by the number of molecules that have passed into the gaseous state, since their number determines the mass of the vapor formed in a closed vessel. This value is not constant, since with a difference in temperature between the bottom of the vessel and the lid that covers it, two mutually opposite processes constantly occur - vaporization and condensation.
Since for each substance at a certain temperature there are known indicators of the transition of a certain number of molecules from one phase of the state of the substance to another, it is possible to change the value of saturated vapor pressure by changing the volume of the vessel. So, the same volume of water, for example 0.5 liters, will create different pressures in a five-liter canister and a kettle.
The determining factor for determining the reference value of saturated vapor pressure at a constant volume and a gradual increase in temperature is the molecular structure of the liquid itself being heated. Thus, the indicators for acetone, alcohol and ordinary water will differ significantly from each other.
To see the process of boiling of a liquid, it is necessary not only to bring the saturated vapor pressure to certain limits, but also to correlate this value with external atmospheric pressure, since the boiling process is possible only when the pressure outside is higher than the pressure inside the vessel.
In this lesson we will analyze the properties of a somewhat specific gas - saturated steam. We will define this gas, indicate 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, a process such as boiling will be discussed 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 liquid molecules will have more and more kinetic energy, and more and more molecules will be able to escape from the liquid (see Fig. 2), therefore, the vapor concentration and, consequently, its pressure will increase. So, the first point:
Saturated vapor pressure depends on temperature
Rice. 2.
However, this situation is quite expected and not as interesting as the next one. 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 steam will move with the liquid to a new dynamic equilibrium by condensing the excess amount of steam, and the pressure will ultimately not change. The second position of the theory of saturated steam:
Saturated vapor pressure does not depend on volume
Now it should be noted that the pressure of saturated vapor 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 MKT equation, gas pressure depends on both temperature and gas concentration. And therefore, the saturated vapor pressure depends on temperature nonlinearly until the vapor concentration increases, that is, until all the liquid evaporates. The graph below (Fig. 3) shows the nature of the dependence of saturated vapor pressure on temperature,
Rice. 3
Moreover, the transition from a nonlinear section to a linear one precisely means the point of evaporation of all liquid. Since the pressure of a saturated gas depends only on temperature, it is possible to absolutely unambiguously determine what the pressure of saturated vapor will be at a given temperature. These ratios (as well as the values of saturated vapor density) are entered 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 that is more intense than evaporation. Now we will somewhat expand this concept.
Definition. Boiling- the process of vaporization that occurs throughout the entire volume of 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 irregularities inside 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 form bubbles. Thus, saturated steam appears inside the bubbles. Next, these bubbles begin to float, increasing in volume and taking in more water molecules inside themselves, and burst at 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 saturated vapor pressure depends on temperature, the boiling point is determined by pressure environment: the smaller it is, the lower the temperature the liquid boils, and vice versa.
In the next lesson we will begin to look at the properties of solids.
Bibliography
- Myakishev G.Ya., Sinyakov A.Z. Molecular physics. Thermodynamics. - M.: Bustard, 2010.
- Gendenshtein L.E., Dick Yu.I. Physics 10th grade. - M.: Ilexa, 2005.
- Kasyanov V.A. Physics 10th grade. - M.: Bustard, 2010.
- Physics.ru ().
- Chemport.ru ().
- Narod.ru ().
Homework
- Page 74: No. 546-550. Physics. Problem book. 10-11 grades. Rymkevich A.P. - M.: Bustard, 2013. ()
- Why can't climbers boil eggs at altitude?
- What ways can you think of to cool down? hot tea? Justify them from the point of view of physics.
- Why should you reduce the gas pressure on the burner after the water boils?
- *How can you achieve water heating above one hundred degrees Celsius?
Ticket No. 1
Saturated steam.
If a container with liquid is tightly closed, the amount of liquid will first decrease and then remain constant. At a constant temperature, the liquid-vapor system will come to a state of thermal equilibrium and will remain in it for as long as desired. Simultaneously with the evaporation process, condensation also occurs; both processes, on average, compensate each other.
At the first moment, after the liquid is poured into the vessel and closed, the liquid will evaporate and the vapor density above it will increase. However, at the same time, the number of molecules returning to the liquid will increase. The higher the vapor density, the larger number its molecules return to the liquid. As a result, in a closed vessel at a constant temperature, a dynamic (mobile) equilibrium will be established between liquid and vapor, i.e., the number of molecules leaving the surface of the liquid over a certain period of time will be equal on average to the number of vapor molecules returning to the liquid during the same time.
Vapor that is in dynamic equilibrium with its liquid is called saturated vapor. This definition emphasizes that a larger amount of steam cannot exist in a given volume at a given temperature.
Saturated vapor pressure.
What will happen to saturated steam if the volume it occupies is reduced? For example, if you compress steam that is in equilibrium with liquid in a cylinder under a piston, maintaining the temperature of the contents of the cylinder constant.
When the steam is compressed, the equilibrium will begin to be disturbed. At first, the vapor density will increase slightly, and a larger number of molecules will begin to move 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 compression of the vapor does not change this number. The process continues until dynamic equilibrium and vapor density are established again, and therefore the concentration of its molecules takes on its previous values. Consequently, the concentration of saturated vapor molecules at a constant temperature does not depend on its volume.
Since 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.
Pressure p n.p. vapor pressure at which a liquid is in equilibrium with its vapor is called saturated vapor pressure.
Dependence of saturated vapor pressure on temperature
The state of saturated steam, as experience shows, is approximately described by the equation of state of an ideal gas, and its pressure is determined by the formula
As temperature increases, pressure increases. Since saturated vapor pressure does not depend on volume, it therefore depends only on temperature.
However, the dependence of p.n. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. As temperature increases, the pressure of real saturated steam increases faster than the pressure of an ideal gas (Fig. section of curve 12). Why is this happening?
When a liquid is heated in a closed container, some of the liquid turns into steam. As a result, according to the formula P = nkT, 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 steam. 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 vapor 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. The liquid partially turns into vapor, or, conversely, the vapor partially condenses. C Nothing like this happens in an ideal gas.)
When all the liquid has evaporated, the steam will cease to be saturated upon further heating and its pressure at a constant volume will increase in direct proportion to the absolute temperature (see Fig., section of curve 23).
Boiling.
Boiling is an intense transition of a substance from a liquid to a gaseous state, occurring throughout the entire volume of the liquid (and not just from its surface). (Condensation is the reverse process.)
As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles are formed throughout the entire volume of the liquid, which float to the surface. The boiling point of the liquid remains constant. This happens because all the energy supplied to the liquid is spent converting it into vapor.
Under what conditions does boiling begin?
A liquid always contains dissolved gases, released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the saturated vapor pressure increases and the bubbles increase in size. Under the influence of buoyant force they float upward. If the upper layers of liquid have more low temperature, then steam condensation occurs in bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly that the walls of the bubble collide and produce something like an explosion. Many such micro-explosions create a characteristic noise. When the liquid warms up enough, the bubbles will stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before it boils.
The dependence of saturated 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.
Boiling begins at the temperature at which the saturated vapor pressure in the bubbles is equal to the pressure in the liquid.
The greater the external pressure, the higher the boiling point.
And vice versa, by reducing external pressure, we thereby lower the boiling point. By pumping air and water vapor out of the flask, you can make the water boil at room temperature.
Each liquid has its own boiling point (which remains constant until all the liquid has boiled away), which depends on its saturated vapor pressure. The higher the saturated vapor pressure, the lower the boiling point of the liquid.
Specific heat of vaporization.
Boiling occurs with the absorption of heat.
Most of the supplied heat is spent on breaking the bonds between particles of the substance, the rest - on the work done during the expansion of steam.
As a result, the interaction energy between vapor particles becomes greater than between liquid particles, so the internal energy of vapor is greater than the internal energy of liquid at the same temperature.
The amount of heat required to convert liquid into steam during the boiling process can be calculated using the formula:
where m is the mass of the liquid (kg),
L - specific heat of vaporization (J/kg)
The specific heat of vaporization shows how much heat is needed to convert 1 kg of a given substance into steam at the boiling point. Unit specific heat vaporization in the SI system:
[L] = 1 J/kg
Air humidity and its measurement.
There is almost always some amount of water vapor in the air around us. Air humidity depends on the amount of water vapor contained in it.
Damp air contains a higher percentage of water molecules than dry air.
Relative air humidity is of great importance, messages about which are heard every day in weather forecast reports.
ABOUT 
Relative humidity is the ratio of the density of water vapor contained in the air to the density of saturated vapor at a given temperature, expressed as a percentage. (shows how close water vapor in the air is to saturation)
Dew point
The dryness or humidity of the air depends on how close its water vapor is to saturation.
If moist air is cooled, the steam in it can be brought to saturation, and then it will condense.
A sign that the steam has become saturated is the appearance of the first drops of condensed liquid - dew.
The temperature at which vapor in the air becomes saturated is called the dew point.
Dew point also characterizes air humidity.
Examples: dew falling in the morning, fogging up of cold glass if you breathe on it, the formation of a drop of water on a cold water pipe, dampness in the basements of houses.
To measure air humidity, measuring instruments - hygrometers - are used. There are several types of hygrometers, but the main ones are hair and psychrometric. Since it is difficult to directly measure water vapor pressure in the air, relative humidity is measured indirectly.
It is known that the rate of evaporation depends on the relative humidity of the air. The lower the air humidity, the easier it is for moisture to evaporate.
IN 
The psychrometer has two thermometers. One is ordinary, it is called dry. It measures the ambient air temperature. The bulb of another thermometer is wrapped in a fabric wick and placed in a container of water. The second thermometer does not show the temperature of the air, but the temperature of the wet wick, hence the name wet thermometer. The lower the air humidity, the more intensely the moisture evaporates from the wick, the greater the amount of heat per unit time is removed from the moistened thermometer, the lower its readings, therefore, the greater the difference in the readings of the dry and moistened thermometers. saturation = 100 ° C and specific characteristics of the state rich liquid and dry rich pair v"=0.001 v""=1.7 ... wet saturated steam with the degree of dryness We calculate the extensive characteristics of wet rich pair By...
Analysis of industrial hazards during the operation of a recovery system vapors oil when draining from cysts
Abstract >> BiologyFlammability limits (by volume). Pressure saturated vapors at T = -38 °C... exposure to solar radiation, concentration saturation will be determined by either temperature... exposure to solar radiation, concentration saturation will not be determined by temperature...
>>Physics: Dependence of saturated vapor pressure on temperature. Boiling
The liquid not only evaporates. At a certain temperature it boils.
Dependence of saturated vapor pressure on temperature. The state of saturated steam, as experience shows (we talked about this in the previous paragraph), is approximately described by the equation of state of an ideal gas (10.4), and its pressure is determined by the formula
As temperature increases, pressure increases. Because Saturated vapor pressure does not depend on volume, therefore it depends only on temperature.
However, dependence r n.p. from T, found experimentally, is not directly proportional, like that of an ideal gas at constant volume. With increasing temperature, the pressure of real saturated vapor increases faster than the pressure of an ideal gas ( Fig.11.1, part of the curve AB). This becomes obvious if we draw isochores of an ideal gas through the points A And IN(dashed lines). Why is this happening?
When a liquid is heated in a closed container, some of the liquid turns into steam. As a result, according to formula (11.1) 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 steam in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the steam changes. The liquid partially turns into vapor, or, on the contrary, the vapor partially condenses. Nothing like this happens with an ideal gas.
When all the liquid has evaporated, the vapor will cease to be saturated upon further heating and its pressure at a constant volume will increase in direct proportion to the absolute temperature (see. Fig.11.1, part of the curve Sun).
.
As the temperature of the liquid increases, the rate of evaporation increases. Finally, the liquid begins to boil. When boiling, rapidly growing vapor bubbles are formed throughout the entire volume of the liquid, which float to the surface. The boiling point of the liquid remains constant. This happens because all the energy supplied to the liquid is spent converting it into vapor. Under what conditions does boiling begin?
A liquid always contains dissolved gases, released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The liquid vapors inside the bubbles are saturated. As the temperature increases, the saturated vapor pressure increases and the bubbles increase in size. Under the influence of buoyant force they float upward. If the upper layers of the liquid have a lower temperature, then vapor condensation occurs in bubbles in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly that the walls of the bubble collide and produce something like an explosion. Many such micro-explosions create a characteristic noise. When the liquid warms up enough, the bubbles will stop collapsing and float to the surface. The liquid will boil. Watch the kettle on the stove carefully. You will find that it almost stops making noise before it boils.
The dependence of saturated 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 occurs at temperatures below the boiling point, and only from the surface of the liquid; during boiling, vapor formation occurs throughout the entire volume of the liquid.
Boiling begins at the temperature at which the saturated vapor pressure in the bubbles is equal to the pressure in the liquid.
The greater the external pressure, the higher the boiling point. Thus, 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) boiling of water also occurs at high blood pressure. Therefore, the boiling point of the liquid is much higher than 100°C. Autoclaves are used to sterilize surgical instruments, etc.
And vice versa, by reducing external pressure, we thereby lower the boiling point. By pumping air and water vapor out of the flask, you can make the water boil at room temperature ( Fig.11.3). When climbing the mountains Atmosphere 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 under these conditions.
Each liquid has its own boiling point, which depends on its saturated vapor pressure. The higher the saturated vapor pressure, the lower the boiling point of the liquid, since at lower temperatures the saturated vapor pressure becomes equal to atmospheric pressure. For example, at a boiling point of 100°C, the saturated vapor pressure of water is 101,325 Pa (760 mm Hg), and the pressure of mercury vapor is only 117 Pa (0.88 mm Hg). Mercury boils at a temperature of 357°C at normal pressure.
A liquid boils when its saturated vapor pressure becomes equal to the pressure inside the liquid.
???
1. Why does the boiling point increase with increasing pressure?
2. Why is it important for boiling to increase the pressure of saturated vapor in the bubbles, and not to increase the pressure of the air in them?
3. How to make a liquid boil while cooling the vessel? (This question is not easy.)
G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade
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