Due to thermal motion, some of the molecules on the surface of the liquid have velocities high enough to overcome the cohesive forces that hold the molecules in the liquid and leave the liquid. This phenomenon is called evaporation. As a result of the collision, vapor molecules can again find themselves near the surface of the liquid and penetrate deeper.

Thus, individual molecules leave the liquid and return to it again. If more molecules escape than return, the liquid evaporates. If, on the contrary, fewer molecules fly out than return, vapor condensation occurs. In the case when the same number of molecules leave a liquid as they return, equilibrium is established between vapor and liquid. Steam in this case is called rich. Saturated vapor pressure at a constant temperature is a constant value.

Saturated vapor pressure for some substances at T = 20 °C

For a solution, the saturated vapor pressure is the sum of the saturated vapor pressures of the solution components, taking into account their concentrations, and is determined by Raoult’s law.

The value of saturated vapor pressure characterizes the volatility of the liquid. The last characteristic is practically very important for the liquid phase of developers when used in field conditions, especially in the autumn-winter period, when the air temperature drops and the productivity of the control process drops sharply due to the long drying of the developer. In addition, volatility is associated with the environmental safety of the flaw detectorist, as well as with the fire and explosion safety of the entire facility.

Capillary condensation– this is the condensation of steam in capillaries and microcracks of porous bodies, as well as in the spaces between closely adjacent solid particles or bodies. Capillary condensation begins with the adsorption of vapor molecules by the condensation surface and the formation of liquid menisci. Since wetting takes place, the shape of the menisci in the capillaries is concave, and the saturated vapor pressure p above them is lower than the saturated vapor pressure p 0 above the flat surface.

Thus, capillary condensation occurs at pressures lower than p0. The volume of liquid condensed in the pores reaches a limiting value at p = p 0. In this case, the liquid-gas interface has zero curvature (flatness).

Capillary condensation increases the absorption (sorption) of vapors by porous bodies, especially near the point of vapor saturation. Capillary condensation can lead to a significant deterioration in the properties of developers used in capillary flaw detection when they are stored in loosely closed containers, especially in conditions of high humidity.

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

Evaporation- vaporization that occurs at any temperature from the free surface of a 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 may exceed the potential energy of their connection with other molecules. Molecules with greater speed have greater kinetic energy, and the temperature of a body depends on the speed of movement of its molecules, therefore, evaporation is accompanied by cooling of the liquid. The rate of evaporation depends on: the open surface area, temperature, and the 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 a value at which the number of molecules returning to the liquid becomes equal to the number of molecules leaving the liquid during the same time. A dynamic equilibrium is established between the processes of evaporation and condensation of the substance. A substance in a gaseous state that is in dynamic equilibrium with a liquid is called saturated vapor. (Vapor is the collection of molecules that leave the liquid during the process of evaporation.) Vapor at a pressure below saturated 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 contained 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, saturated vapor pressure does not depend on volume, but depends on temperature. This dependence cannot be expressed by a simple formula, therefore, based on an experimental study of the dependence of saturated vapor pressure on temperature, tables have been compiled from which its pressure can be determined at various temperatures.

The pressure of water vapor in the 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 present 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, drying out of various substances, and wilting of plants, depend not on the amount of water vapor in the air, but on how close this amount is to saturation, i.e., on relative humidity, which characterizes the degree of saturation air with water vapor. At low temperatures and high humidity, heat transfer increases and a person becomes hypothermic. 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 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 daily variation of relative humidity is the opposite of the daily variation of temperature. During the day, with increasing temperature and, consequently, with increasing saturation pressure, 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 air temperature, the steam in it can be brought to saturation. The dew point is the temperature at which 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 air humidity, instruments called hygrometers and psychrometers are used.

>>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 the 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 elevated 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). As you climb mountains, the atmospheric pressure decreases, therefore 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 is not an easy question.)

G.Ya.Myakishev, B.B.Bukhovtsev, N.N.Sotsky, Physics 10th grade

If you have corrections or suggestions for this lesson,

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 uneven surfaces 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 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 the saturated vapor pressure depends on temperature, the boiling point is determined by the ambient pressure: the lower it is, the lower the temperature at which the liquid boils, and vice versa.

In the next lesson we will begin to look at the properties of solids.

Bibliography

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

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

Homework

1. Page 74: No. 546-550. Physics. Problem book. 10-11 grades. Rymkevich A.P. - M.: Bustard, 2013. ()
2. Why can't climbers boil eggs at altitude?
3. What ways can you think of to cool hot tea? Justify them from the point of view of physics.
4. Why should you reduce the gas pressure on the burner after the water boils?
5. *How can you achieve water heating above one hundred degrees Celsius?

And 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 density of the vapor 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 a liquid per unit time depends only on temperature, and compression of 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 value. Therefore, 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), then from this definition it follows that saturated vapor pressure does not dependot of the volume it occupies.

Steam pressure, at which a liquid is in equilibrium with its vapor is called saturated vapor pressure.

• Unsaturated steam.

We have used the words many times gas And steam. There is no fundamental difference between gas and steam. But if, at a constant temperature, a gas can be turned into a liquid by simple compression, then we call it steam, or more precisely, unsaturated steam.

• Dependence of saturated vapor pressure on temperature.

The state of saturated steam, as experience says, 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. Because d Pressure saturatedof steam does not depend on volume,it only dependson temperature.

However, this dependence mouth), found experimentally is not directly proportional, like that of an ideal gas at constant volume. With increasing temperature, the pressure of saturated vapor increases faster than the pressure of an ideal gas (Fig. 30, section of the curve AB). This becomes especially obvious if we draw an isochore through the point A(dashed line) Why does this happen?

When a liquid is heated in a closed container, some of the liquid turns into steam. As a result, according to the formula
pressure saturated steam grows not only due to rising temperature liquids, but also due to increased concentration of molecules (density ness) pair . 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. 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. 30, section Sun).