If an open glass of water is left on for a long time, then eventually the water will completely evaporate. More like it will evaporate. What is evaporation and why does it occur?

2.7.1 Evaporation and condensation

At a given temperature, liquid molecules have different speeds. The velocities of most molecules are close to some average value (characteristic of this temperature). But there are molecules whose velocities differ significantly from the average, both up and down.

On fig. 2.16 shows an approximate graph of the distribution of liquid molecules by velocities. The blue background shows the very majority of molecules whose velocities are grouped around the average value. The red ¾tail¿ of the graph is a small number of ¾fast¿ molecules whose velocities significantly exceed average speed the bulk of the liquid molecules.

Number of molecules

fast molecules

Molecule speed

Rice. 2.16. Velocity distribution of molecules

When such a very fast molecule is on the free surface of the liquid (i.e., at the interface between liquid and air), the kinetic energy of this molecule may be enough to overcome the attractive forces of other molecules and fly out of the liquid. This process and there is evaporation, and the molecules that have left the liquid form vapor.

So, evaporation is the process of converting a liquid into vapor, occurring on the free surface of the liquid7.

It may happen that after some time the vapor molecule will return back to the liquid.

The process of transition of vapor molecules into liquid is called condensation. Vapor condensation is the reverse process of liquid evaporation.

2.7.2 dynamic balance

What happens if a container of liquid is hermetically sealed? The vapor density above the liquid surface will begin to increase; vapor particles will increasingly prevent other liquid molecules from flying out, and the evaporation rate will decrease. Will start at the same time

7 When special conditions the transformation of liquid into vapor can occur throughout the entire volume of the liquid. This process is well known to you, this boiling.

p n = n RT:

the rate of condensation increases, since with an increase in the concentration of vapor, the number of molecules returning to the liquid will become more and more.

Finally, at some point, the rate of condensation will be equal to the rate of evaporation. A dynamic equilibrium will come between the liquid and the vapor: per unit time, as many molecules will fly out of the liquid as they return to it from the vapor. Starting from this moment, the amount of liquid will cease to decrease, and the amount of vapor will increase; the steam will reach ¾saturation¿.

Saturated steam is steam that is in dynamic equilibrium with its liquid. A vapor that has not reached a state of dynamic equilibrium with a liquid is called unsaturated.

The pressure and density of saturated vapor are denoted pn in. Obviously, pn in is the maximum pressure and density that steam can have at a given temperature. In other words, the pressure and density of saturated vapor always exceeds the pressure and density of unsaturated vapor.

2.7.3 Saturated steam properties

It turns out that the state of saturated steam (especially unsaturated steam) can be approximately described by the equation of state of an ideal gas (Mendeleev's Clapeyron equation). In particular, we have an approximate relationship between saturated vapor pressure and its density:

This is very amazing fact, confirmed by experiment. Indeed, in its properties, saturated steam differs significantly from an ideal gas. We list the most important of these differences.

1. At a constant temperature, the density of saturated vapor does not depend on its volume.

If, for example, saturated vapor is isothermally compressed, then its density will increase at the first moment, the rate of condensation will exceed the rate of evaporation, and part of the vapor will condense into liquid until dynamic equilibrium is reached again, in which the vapor density returns to its previous value.

Similarly, during isothermal expansion of saturated vapor, its density will decrease at the first moment (the vapor will become unsaturated), the evaporation rate will exceed the condensation rate, and the liquid will additionally evaporate until dynamic equilibrium is again established, i.e., until the vapor becomes saturated again with the same density.

2. Saturated vapor pressure does not depend on its volume.

This follows from the fact that the density of saturated vapor does not depend on the volume, and the pressure is uniquely related to the density by equation (2.6).

As you can see, Boyle Mariotte's law, which is valid for ideal gases, does not hold for saturated steam. This is not surprising because it is derived from Mendeleev's Clapeyron equation under the assumption that the mass of the gas remains constant.

3. At a constant volume, the density of saturated vapor increases with increasing temperature and decreases with decreasing temperature.

Indeed, as the temperature increases, the rate of evaporation of the liquid increases. The dynamic balance is disturbed at the first moment, and an additional

evaporation of some liquid. The pair will be added until the dynamic equilibrium is restored again.

In the same way, as the temperature decreases, the rate of evaporation of the liquid becomes less, and part of the vapor condenses until dynamic equilibrium is restored, but with less vapor.

Thus, during isochoric heating or cooling of saturated steam, its mass changes, so Charles's law does not work in this case. The dependence of saturation vapor pressure on temperature will no longer be a linear function.

4. Saturated vapor pressure increases with temperature faster than linearly.

Indeed, with increasing temperature, the density of saturated vapor increases, and according to equation (2.6), the pressure is proportional to the product of density and temperature.

The dependence of saturated vapor pressure on temperature is exponential (Fig. 2.17). It is represented by section 1–2 of the graph. This dependence cannot be derived from the laws of an ideal gas.

isochore steam

Rice. 2.17. Steam pressure versus temperature

At point 2, all liquid evaporates; with a further increase in temperature, the vapor becomes unsaturated, and its pressure grows linearly according to the Charles law (segment 2–3).

Recall that the linear increase in the pressure of an ideal gas is caused by an increase in the intensity of impacts of molecules on the walls of the vessel. In the case of heating a saturated vapor, the molecules begin to beat not only stronger, but more often, because the vapor becomes larger. The simultaneous action of these two factors caused an exponential increase in the saturation vapor pressure.

2.7.4 Air humidity

Absolute humidity is the partial pressure of water vapor in the air (i.e., the pressure that water vapor would exert on its own, in the absence of other gases). Sometimes absolute humidity is also called the density of water vapor in the air.

Relative humidity "is the ratio of the partial pressure of water vapor in it to the pressure of saturated water vapor at the same temperature. As a rule, this is

ratio is expressed as a percentage:

" = p 100%: pn

From the Mendeleev-Clapeyron equation (2.6) it follows that the ratio of vapor pressures is equal to the ratio of densities. Since the equation (2.6), we recall, describes saturated steam only approximately, we have an approximate relationship:

" = 100%:n

A psychrometer is one of the instruments that measures air humidity. It includes two thermometers, the reservoir of one of which is wrapped in a wet cloth. The lower the humidity, the more intense the evaporation of water from the fabric, the more the reservoir of the “wet” thermometer is cooled, and the greater the difference between its readings and the readings of the dry thermometer. According to this difference, using a special psychrometric table, the humidity of the air is determined.

Evaporation - this is vaporization that occurs only from the free surface of a liquid adjoining a gaseous medium or vacuum.

Uneven distribution of kinetic energy thermal motion molecules leads to the fact that at any temperature the kinetic energy of some molecules of a liquid or solid can exceed the potential energy of their connection with the rest of the molecules.

Evaporation- This is a process in which molecules fly out from the surface of a liquid or solid, the kinetic energy of which exceeds the potential energy of the interaction of molecules. Evaporation is accompanied by cooling of the liquid.

Let us consider the evaporation process from the point of view of molecular-kinetic theory. To leave the liquid, the molecules must do work by reducing their kinetic energy. Among the randomly moving molecules of a liquid in its surface layer, there will always be molecules that tend to fly out of the liquid. When such a molecule leaves the surface layer, a force arises that draws the molecule back into the liquid. Therefore, only those molecules fly out of the liquid, in which the kinetic energy is greater than the work necessary to overcome the counteraction of molecular forces.

The rate of evaporation depends on:

a) the type of liquid;

b) on the area of ​​its free surface. The larger this area, the faster the liquid evaporates.

c) the lower the vapor density of a liquid above its surface, the more speed evaporation. Therefore, pumping vapors (wind) from the surface will accelerate its evaporation.

d) with increasing temperature, the rate of evaporation of the liquid increases.

vaporization is the transition of a substance from a liquid state to a gaseous state.

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

During vaporization internal energy matter increases, and when condensed - decreases.

Heat of vaporization – is the amount of heat Q required to turn a liquid into vapor at a constant temperature.

Specific heat of vaporization L is measured by the amount of heat required to turn a unit mass of liquid into steam at a constant temperature

Saturated and unsaturated steam. 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 the evaporation process, the concentration of a substance in the gaseous state reaches a value at which the number of molecules returning to the liquid per unit time becomes equal to the number of molecules leaving the surface of the liquid in the same time. A dynamic equilibrium is established between the processes of evaporation and condensation of matter.

dynamic balance- this is when the process of liquid evaporation is fully compensated with vapor condensation, i.e. how many molecules leave the liquid, the same number returns to it.

Saturated steam Vapor is in dynamic equilibrium with its liquid. The pressure and density of a saturated vapor are uniquely determined by its temperature.

Unsaturated steam - it is the vapor that is above the surface of the liquid when evaporation predominates over condensation, and the vapor in the absence of liquid. Its pressure is below the saturation vapor pressure .

When saturated vapor is compressed, the concentration of vapor molecules increases, the equilibrium between the processes of evaporation and condensation is disturbed, and part of the vapor turns into a liquid. When saturated vapor expands, the concentration of its molecules decreases and part of the liquid turns into vapor. Thus, the concentration of saturated vapor remains constant regardless of the volume. Since the pressure of a gas is proportional to concentration and temperature, the pressure of saturated vapor at a constant temperature does not depend on volume.

The intensity of the evaporation process increases with increasing liquid temperature. Therefore, the dynamic equilibrium between evaporation and condensation with increasing temperature is established at high concentrations of gas molecules.

DEFINITION

Evaporation is the process of converting liquid into vapor.

In a liquid (or solid) at any temperature, there is a certain number of "fast" molecules, the kinetic energy of which is greater than the potential energy of their interaction with the rest of the particles of the substance. If such molecules are near the surface, then they can overcome the attraction of other molecules and fly out of the liquid, forming vapor above it. Evaporation of solids is also often referred to as sublimation or sublimation.

Evaporation occurs at any temperature at which a given substance can be in a liquid or solid state. However, the rate of evaporation depends on the temperature. As the temperature rises, the number of "fast" molecules increases, and, consequently, the intensity of evaporation increases. The rate of evaporation also depends on the free surface area of ​​the liquid and the type of substance. So, for example, water poured into a saucer will evaporate faster than water poured into a glass. Alcohol evaporates faster than water, etc.

Condensation

The amount of liquid in an open vessel decreases continuously due to evaporation. But in a tightly closed vessel, this does not happen. This is explained by the fact that, simultaneously with evaporation in a liquid (or solid), the reverse process occurs. Vapor molecules move randomly above the liquid, so some of them, under the influence of the attraction of the molecules of the free surface, fall back into the liquid. The process of turning a vapor into a liquid is called condensation. The process of turning vapor into a solid is commonly referred to as crystallization from vapor.

After we pour the liquid into the vessel and close it tightly, the liquid will begin to evaporate, and the vapor density above the free surface of the liquid will increase. However, at the same time, the number of molecules returning back to the liquid will increase. In an open vessel, the situation is different: the molecules that have left the liquid may not return to the liquid. In a closed vessel, an equilibrium state is established over time: the number of molecules leaving the surface of the liquid becomes equal to the number of vapor molecules returning to the liquid. Such a state is called state of dynamic equilibrium(Fig. 1). In a state of dynamic equilibrium between liquid and vapor, both evaporation and condensation occur simultaneously, and both processes compensate each other.

Fig.1. Fluid in dynamic equilibrium

Saturated and unsaturated steam

DEFINITION

Saturated steam Vapor is in dynamic equilibrium with its liquid.

The name "saturated" emphasizes that a given volume at a given temperature cannot contain more steam. Saturated steam has a maximum density at a given temperature, and therefore exerts maximum pressure on the walls of the vessel.

DEFINITION

unsaturated steam- steam that has not reached the state of dynamic equilibrium.

For different liquids, vapor saturation occurs at different densities, which is due to the difference in the molecular structure, i.e. the difference in the forces of intermolecular interaction. In liquids in which the interaction forces of molecules are high (for example, in mercury), the state of dynamic equilibrium is achieved at low vapor densities, since the number of molecules that can leave the surface of the liquid is small. On the contrary, in volatile liquids with low forces of attraction of molecules, at the same temperatures, a significant number of molecules fly out of the liquid and saturation of the vapor is achieved at a high density. Examples of such liquids are ethanol, ether, etc.

Since the intensity of the vapor condensation process is proportional to the concentration of vapor molecules, and the intensity of the evaporation process depends only on temperature and increases sharply with its growth, the concentration of molecules in saturated vapor depends only on the temperature of the liquid. That's why Saturated vapor pressure depends only on temperature and does not depend on volume. Moreover, with increasing temperature, the concentration of saturated vapor molecules and, consequently, the density and pressure of saturated vapor rapidly increase. Specific dependences of pressure and density of saturated vapor on temperature are different for different substances and can be found from reference tables. It turns out that saturated steam, as a rule, is well described by the Claiperon-Mendeleev equation. However, when compressed or heated, the mass of saturated vapor changes.

Unsaturated steam obeys the laws of an ideal gas with a reasonable degree of accuracy.

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Ticket number 1

Saturated steam.

If the vessel with liquid is tightly closed, then the amount of liquid will first decrease, and then will remain constant. At a constant temperature, the liquid - vapor system will come to a state of thermal equilibrium and will remain in it for an arbitrarily long time. 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 also increase. The greater the vapor density, the greater the number of its molecules returned to the liquid. As a result, in a closed vessel at constant temperature a dynamic (mobile) equilibrium will be established between the 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 in the same time.

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

Saturated steam pressure.

What will happen to saturated steam if the volume occupied by it is reduced? For example, if you compress vapor that is in equilibrium with a 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.

Pressure p n.p. the vapor at which the liquid is in equilibrium with its vapor is called the saturation vapor pressure.

Saturated vapor pressure versus 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 the temperature rises, the pressure rises. Since the saturation vapor pressure does not depend on volume, it therefore depends only on temperature.

However, the dependence of рn.p. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. With increasing temperature, 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 vessel, part of the liquid turns into vapor. As a result, according to the formula Р = nкТ, the saturated vapor pressure increases not only due to an increase in the temperature of the liquid, but 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 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 vapor will cease to be saturated upon further heating, and its pressure at constant volume will increase in direct proportion to the absolute temperature (see Fig., curve section 23).

Boiling.

Boiling is an intense transition of a substance from a liquid state 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 form throughout the volume of the liquid, which float to the surface. The boiling point of a liquid remains constant. This is because all the energy supplied to the liquid is spent on turning it into steam.

Under what conditions does boiling begin?

The liquid always contains dissolved gases that are released on the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are the centers of vaporization. The liquid vapors inside the bubbles are saturated. With increasing temperature, the pressure saturated vapors 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. Watch the kettle on the stove carefully. You will find that it almost stops making noise before boiling.

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.

Boiling begins at a temperature at which the saturation vapor pressure in the bubbles is equal to the pressure in the liquid.

The greater the external pressure, the higher the boiling point.

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.

Each liquid has its own boiling point (which remains constant until the entire liquid boils away), which depends on its saturated vapor pressure. The higher the saturation vapor pressure, the lower the boiling point of the liquid.

Specific heat of vaporization.

Boiling occurs with the absorption of heat.

Most of the heat supplied is spent on breaking the bonds between the particles of the substance, the rest - on the work done during the expansion of the steam.

As a result, the interaction energy between vapor particles becomes greater than between liquid particles, so the internal energy of the vapor is greater than the internal energy of the liquid at the same temperature.

The amount of heat required to transfer liquid to vapor during the boiling process can be calculated using the formula:

where m is the mass of liquid (kg),

L - specific heat of vaporization (J / kg)

The specific heat of vaporization shows how much heat is needed to turn 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.

The air around us almost always contains some amount of water vapor. The humidity of the air depends on the amount of water vapor it contains.

Raw air contains higher percentage water molecules than dry.

Of great importance is the relative humidity of the air, reports of which are heard every day in weather forecast reports.

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, then the vapor in it can be brought to saturation, and then it will condense.

A sign that the steam is saturated is the appearance of the first drops of condensed liquid - dew.

The temperature at which the vapor in the air becomes saturated is called the dew point.

The dew point also characterizes the humidity of the air.

Examples: dew in the morning, fogging 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.

Hygrometers are used to measure air humidity. There are several types of hygrometers, but the main ones are hair and psychrometric. Since it is difficult to directly measure the pressure of water vapor in air, relative humidity air 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..

The psychrometer has two thermometers. One is ordinary, it is called dry. It measures the temperature of the surrounding air. The flask of another thermometer is wrapped in a fabric wick and lowered into 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 bulb. The lower the air humidity, the more intense the moisture evaporates from the wick, the more heat per unit time is removed from the humidified thermometer, the lower its readings, therefore, the greater the difference between the readings of dry and wetted 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 Calculate the extensive characteristics of wet rich pair By...

Above the free surface of a liquid there are always vapors of this liquid. If the vessel with the 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, at the same time as the liquid evaporates, the vapor condenses. First, the number of molecules emitted from the liquid in 1 s, more number molecules returning back, and the density, and hence the vapor pressure, increases. The number of vapor molecules increases until the number of molecules that have left the liquid (evaporated) becomes equal to the number of molecules that have returned to the liquid (condensed) in the same period of time. Such a state is called dynamic balance.

A vapor that is in dynamic equilibrium with its liquid is called saturated steam. The following quantities are used to describe saturated steam: saturated steam pressure p n and saturated steam densityρ n. At a given temperature, saturated steam has the highest possible vapor pressure and density.

A vapor 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 more accurate they are, the farther they are from saturation.

Saturated vapor properties

Saturated vapors have the following properties:

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 the tables (see table).

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

Air humidity

As a result of the evaporation of water from numerous water bodies (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, the well-being of a person, the functioning of many of his organs, the life of plants, as well as the safety of technical objects, architectural structures, and works of art. Therefore, it is very important to monitor the humidity of the air, to be able to measure it.

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

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

The SI unit for absolute humidity is the kilogram per cubic meter (kg/m3). Sometimes off-system units of grams per cubic meter (g / m 3) 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 the absolute humidity is known, it is still impossible to judge how dry or humid the air is. To determine the degree of air humidity, it is necessary to know whether the water vapor is close or far from saturation.

relative humidity air φ is called the ratio of absolute humidity expressed as a percentage to the density ρ 0 of saturated steam at a given temperature (or the ratio of pressure 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 from saturation, the more intense the evaporation. Saturated steam pressure p 0 at a given temperature - tabular value. Pressure p water vapor (and hence absolute humidity) is determined by the dew point.

Let at a temperature t 1 steam pressure p 1 . Steam state on the diagram R, t represented by a dot A(Fig. 5).

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

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

Psychrometer. Hygrometer

As the temperature drops, the relative humidity of the air increases. At some 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 dew drops or fog.

To determine the relative humidity of the air, it is possible to artificially lower the air temperature in some limited area to the dew point. Absolute humidity and, accordingly, the pressure of water vapor will remain unchanged. Comparing the pressure of water vapor at the dew point with the pressure of saturated vapor, which could be at the temperature of interest to us, we thereby find the relative humidity of the air. Rapid cooling can be achieved by intensive 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. With the help of a rubber pear, 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 rises. At a certain temperature, which is measured with a thermometer inserted into the hole of the device, the surface of the box is covered with tiny dew drops. In order to more accurately fix the moment when the dew box appears on the surface, this surface is polished to a mirror finish, and a polished metal ring is located next to the box for control.

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

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

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