There are always vapors of this liquid above the free surface of a liquid. If a container with a liquid is not closed, then there will always be vapor molecules that move away from the surface of the liquid and cannot return back to the liquid. In a closed vessel, condensation of steam occurs simultaneously with the evaporation of the liquid. First, the number of molecules flying out of the liquid in 1 s is greater than the number of molecules returning back, and the density, and therefore the vapor pressure, increases. The number of vapor molecules increases until the number of molecules leaving the liquid (evaporated) becomes equal to the number of molecules returning to the liquid (condensed) in the same period of time. This condition is called dynamic balance.

Vapor that is in a state of dynamic equilibrium with its liquid is called saturated steam. For description saturated steam The following values ​​apply: saturated steam pressure p n and saturated vapor densityρ n. At a given temperature, saturated steam has the maximum possible pressure and vapor density.

Steam whose pressure is less than the saturated vapor pressure at a given temperature is called unsaturated. Similarly, it was possible to give a definition in terms of vapor density.

Experience shows that unsaturated vapors obey all gas laws, and the further they are from saturation, the more accurately.

Properties of saturated vapors

For saturated vapors The following properties are characteristic:

Hence, saturated steam does not obey the gas laws of an ideal gas. The values ​​of pressure and density of saturated steam at a given temperature are determined from tables (see table).

Table. Pressure ( R) and density (ρ) of saturated water vapor at different temperatures ( t).

Air humidity

As a result of the evaporation of water from numerous bodies of water (seas, lakes, rivers, etc.), as well as from vegetation in atmospheric air always contains water vapor. The amount of water vapor contained in the air affects the weather, human well-being, the functioning of many of his organs, plant life, as well as the safety of technical objects, architectural structures, works of art. Therefore, it is very important to monitor air humidity and be able to measure it.

Water vapor in the air is usually unsaturated. The movement of air masses, ultimately caused by the radiation of the Sun, leads to the fact that in some places on our planet in this moment evaporation of water predominates over condensation, while in others, on the contrary, condensation predominates.

Absolute humidityρ of air is called a quantity, numerically equal to mass water vapor contained in 1 m 3 of air (i.e., the density of water vapor in the air under given conditions).

The SI unit of absolute humidity is kilogram per cubic meter (kg/m3). Sometimes non-systemic units of grams per cubic meter (g/m3) are used.

Absolute humidity ρ and pressure p water vapor are interconnected by the equation of state

\(~p \cdot V = \dfrac (m \cdot M)(R \cdot T) \Rightarrow p = \dfrac(\rho)(M) \cdot R \cdot T\)

If only absolute humidity is known, it is still impossible to judge how dry or humid the air is. To determine the degree of air humidity, you need to know whether water vapor is close or far from saturation.

Relative humidity air φ is the percentage ratio of absolute humidity to the density ρ 0 of saturated vapor at a given temperature (or pressure ratio p water vapor to pressure p 0 saturated steam at a given temperature):

\(~\varphi = \dfrac(\rho)(\rho_0) \cdot 100\;\%, \;\; ~\varphi = \dfrac(p)(p_0) \cdot 100\;\%.\)

The lower the relative humidity, the further the steam is from saturation, the more intense evaporation occurs. Saturated steam pressure p 0 at a given temperature is a tabular value. Pressure p water vapor (and therefore absolute humidity) is determined by the dew point.

Let at temperature t 1 water vapor pressure p 1 . Steam status on the diagram R, t will be represented by a point A(Fig. 5).

When isobarically cooled to a temperature t p steam becomes saturated and its state is represented by a point IN. Temperature t p at which water vapor becomes saturated is called dew point. When cooling below the dew point, vapor condensation begins: fog appears, dew falls, and windows fog up. The dew point allows you to determine the water vapor pressure p 1 in the air at a temperature t 1 .

Indeed, from Figure 5 we see that the pressure p 1 is equal to the saturated vapor pressure at the dew point p 1 = p 0tp. Therefore, \(~\varphi = \dfrac(p_(0tp))(p_0) \cdot 100 \;\%\)

Psychrometer. Hygrometer

As the temperature decreases, the relative humidity increases. At a certain temperature ( dew point) water vapor becomes saturated. A further decrease in temperature leads to the fact that the resulting excess water vapor begins to condense in the form of droplets of dew or fog.

To determine relative air humidity, you can artificially lower the air temperature in a limited area to the dew point. Absolute humidity and, accordingly, water vapor pressure will remain unchanged. By comparing the water vapor pressure at the dew point with the saturated vapor pressure that could be at the temperature we are interested in, we will thereby find the relative humidity of the air. Rapid cooling can be achieved by intense evaporation of some volatile liquid. This method is used to measure humidity using a condensation hygrometer.

Condensation hygrometer consists of a metal box with two holes (Fig. 6).

Ether is poured into the box. Using a rubber bulb, air is pumped through the box. The ether evaporates very quickly, the temperature of the box and the air near it decreases, and the relative humidity increases. At a certain temperature, which is measured by a thermometer inserted into the hole of the device, the surface of the box is covered with tiny droplets of dew. In order to more accurately record the moment the dew box appears on the surface, this surface is polished to a mirror finish, and a polished metal ring is placed next to the box for control.

In modern condensation hygrometers, a semiconductor element is used to cool the mirror, the operating principle of which is based on the Peltier effect, and the temperature of the mirror is measured by a wire resistance or semiconductor microthermometer built into it.

Action hair hygrometer is based on the property of defatted human hair to change its length when air humidity changes, which makes it possible to measure relative humidity from 30 to 100%. Hair 1 (Fig. 7) is stretched over a metal frame 2. The change in hair length is transmitted to arrow 3, moving along the scale.

Action ceramic hygrometer based on the dependence of the electrical resistance of solid and porous ceramic mass (a mixture of clay, silicon, kaolin and some metal oxides) on air humidity.

Before answering the question posed in the title of the article, let’s figure out what steam is. The images that most people have when hearing this word are: a boiling kettle or pan, a steam room, a hot drink and many more similar pictures. One way or another, in our ideas there is a liquid and a gaseous substance rising above its surface. If you are asked to give an example of steam, you will immediately remember water vapor, alcohol, ether, gasoline, acetone.

There is another word for gaseous states - gas. Here we usually remember oxygen, hydrogen, nitrogen and other gases, without associating them with the corresponding liquids. Moreover, it is well known that they exist in a liquid state. At first glance, the differences are that steam corresponds to natural liquids, and gases must be specially liquefied. However, this is not entirely true. Moreover, the images that arise from the word steam are not steam. To give a more accurate answer, let’s look at how steam arises.

How is steam different from gas?

The state of aggregation of a substance is determined by temperature, or more precisely by the relationship between the energy with which its molecules interact and the energy of their thermal chaotic motion. Approximately, we can assume that if the interaction energy is much greater - a solid state, if the energy is much greater thermal movement- gaseous, if the energies are comparable - liquid.

It turns out that in order for a molecule to break away from the liquid and participate in the formation of vapor, the amount of thermal energy must be greater than the interaction energy. How can this happen? The average speed of thermal movement of molecules is equal to a certain value depending on temperature. However, the individual speeds of molecules are different: most of them have speeds close to the average value, but some have speeds greater than the average, some less.

Faster molecules can have thermal energy greater than the interaction energy, which means that, once on the surface of a liquid, they are able to break away from it, forming vapor. This method of vaporization is called evaporation. Due to the same distribution of speeds, the opposite process also exists - condensation: molecules from vapor pass into liquid. By the way, the images that usually arise when hearing the word steam are not steam, but the result of the opposite process - condensation. The steam cannot be seen.

Under certain conditions, steam can become a liquid, but for this to happen its temperature must not exceed a certain value. This value is called the critical temperature. Steam and gas are gaseous states that differ in the temperature at which they exist. If the temperature does not exceed the critical temperature, it is steam; if it exceeds it, it is gas. If you keep the temperature constant and reduce the volume, the steam liquefies, but the gas does not liquefy.

What is saturated and unsaturated steam

The word “saturated” itself carries certain information; it is difficult to saturate a large area of ​​​​space. This means that in order to obtain saturated steam, you need limit the space in which the liquid is located. The temperature must be less than the critical temperature for a given substance. Now the evaporated molecules remain in the space where the liquid is located. At first, most of the molecular transitions will occur from the liquid, and the vapor density will increase. This in turn will cause larger number reverse transitions of molecules into liquid, which will increase the speed of the condensation process.

Finally, a state is established for which the average number of molecules passing from one phase to another will be equal. This condition is called dynamic equilibrium. This state is characterized by the same change in the magnitude and direction of the rates of evaporation and condensation. This state corresponds to saturated steam. If the state of dynamic equilibrium is not achieved, this corresponds to unsaturated steam.

They begin the study of an object, always with its simplest model. In molecular kinetic theory, this is an ideal gas. The main simplifications here are the neglect of the molecules’ own volume and the energy of their interaction. It turns out that such a model describes unsaturated steam quite satisfactorily. Moreover, the less saturated it is, the more legitimate its use. An ideal gas is a gas; it cannot become either vapor or liquid. Consequently, for saturated steam such a model is not adequate.

The main differences between saturated and unsaturated steam

1. Saturated means that this object has the largest possible value of some parameters. For a couple this is density and pressure. These parameters for unsaturated steam have lower values. The further the steam is from saturation, the smaller these values ​​are. One clarification: the comparison temperature must be constant.
2. For unsaturated steam: Boyle-Mariotte law: if the temperature and mass of the gas are constant, an increase or decrease in volume causes a decrease or increase in pressure by the same amount, pressure and volume are inversely proportional. From the maximum density and pressure at constant temperature It follows that they are independent of the volume of saturated steam; it turns out that for saturated steam, pressure and volume do not depend on each other.
3. For unsaturated steam density does not depend on temperature, and if the volume is maintained, the density value does not change. For saturated steam, while maintaining volume, the density changes if the temperature changes. The dependence in this case is direct. If the temperature increases, the density also increases, if the temperature decreases, the density also changes.
4. If the volume is constant, unsaturated steam behaves in accordance with Charles' law: as the temperature increases, the pressure also increases by the same factor. This dependence is called linear. For saturated steam, as the temperature increases, the pressure increases faster than for unsaturated steam. The dependence is exponential.

To summarize, we can note significant differences in the properties of the compared objects. The main difference is that steam, in a state of saturation, cannot be considered in isolation from its liquid. This is a two-part system to which most gas laws cannot be applied.

During evaporation, simultaneously with the transition of molecules from liquid to vapor, the reverse process also occurs. Moving randomly over the surface of the liquid, some of the molecules that left it return to the liquid again.

Saturated vapor pressure.

When saturated vapor is compressed, the temperature of which is maintained constant, the equilibrium will first begin to be disturbed: the density of the vapor will increase, and as a result, more molecules will pass from gas to liquid than from liquid to gas; this will continue until the vapor concentration in the new volume becomes the same, corresponding to the concentration of saturated vapor at a given temperature (and equilibrium is restored). This is explained by the fact that the number of molecules leaving the liquid per unit time depends only on temperature.

So, the concentration of molecules of saturated steam at a constant temperature does not depend on its volume.

Since the pressure of a gas is proportional to the concentration of its molecules, the pressure of saturated vapor does not depend on the volume it occupies. Pressure p 0, at which the liquid is in equilibrium with its vapor is called saturated steam pressure.

When saturated vapor is compressed, most of it turns into a liquid state. Liquid occupies less volume than vapor of the same mass. As a result, the volume of steam, while its density remains unchanged, decreases.

Dependence of saturated vapor pressure on temperature.

For an ideal gas it is true linear dependence pressure versus temperature at constant volume. As applied to saturated steam with pressure p 0 this dependence is expressed by the equality:

p 0 =nkT.

Since saturated vapor pressure does not depend on volume, it therefore depends only on temperature.

Experimentally determined dependence p0(T) differs from dependence ( p 0 =nkT) for an ideal gas.

With increasing temperature, the pressure of saturated vapor increases faster than the pressure of an ideal gas (section of the curve AB on the image). This becomes especially obvious if we draw an isochore through the point A(dashed line). This happens because when a liquid is heated, part of it turns into steam, and the density of the steam increases. Therefore, according to the formula ( p 0 =nkT), the saturated vapor pressure increases not only as a result of an increase in the temperature of the liquid, but also due to an increase in the concentration of molecules (density) of the vapor. The main difference in the behavior of an ideal gas and saturated vapor is the change in the mass of vapor with a change in temperature at a constant volume (in a closed vessel) or with a change in volume at a constant temperature. Nothing like this can happen with an ideal gas (the molecular kinetic theory of an ideal gas does not provide for the phase transition of gas into liquid).

After all the liquid has evaporated, the behavior of the vapor will correspond to the behavior of an ideal gas (section Sun curve in the figure above).

Unsaturated steam.

If in a space containing vapor of a liquid, further evaporation of this liquid can occur, then the vapor located in this space is unsaturated.

Vapor that is not in equilibrium with its liquid is called unsaturated.

Unsaturated vapor can be converted into liquid by simple compression. Once this transformation has begun, the vapor in equilibrium with the liquid becomes saturated.

Properties of saturated steam

Saturated steam and its properties.

Boiling. critical temperature

If you leave an open glass of water in the room, after a while all the water from it will evaporate. If you cover the glass with a lid, then the water will remain in it indefinitely.

Reader: Is it true that in the second case the water in the glass does not evaporate?

When the glass is open, the evaporation process is more intense than the condensation process, since water molecules that have turned into a gaseous state scatter throughout the room. When the glass is closed, molecules cannot escape from the small space between the surface of the water and the lid. Therefore, soon the number of molecules leaving the water is compared with the number of molecules returning to it. Otherwise: the rate of the evaporation process becomes equal to the rate of the condensation process.

If liquid and vapor are in a closed vessel and neither the amount of liquid nor the amount of vapor changes for a long time, then they say that liquid and vapor are in dynamic equilibrium.

Vapor in a state of dynamic equilibrium with liquid is called saturated.

Properties of saturated steam

The saturated vapor pressure at a given temperature is a constant value. Different liquids have different saturated vapor pressures. Let's consider an experiment that confirms this statement.

Liquid ether is poured into the flask, from which the air has previously been evacuated, through a funnel (Fig. 13.1). Ether vapor creates pressure, which is measured using a column of mercury.

IN starting moment mercury column height h= 760 mm, then as the ether evaporates, it decreases, since the pressure on mercury from the ether vapor increases. As soon as the ether poured into the flask stops evaporating, saturation, and the pressure no longer increases, no matter how much ether is poured into the flask.

Note that the higher the temperature of the flask, the greater the saturated vapor pressure.

The parameters of saturated vapors satisfy the Mendeleev–Clayperon equation

pV = .

Since at this temperature T values ​​m and R are constant for a given gas, then the saturated vapor density for a given substance is a constant value. For example in table. Table 13.1 shows the comparative pressures of saturated vapors of water and mercury at different temperatures.