How to tell if steam is saturated or not. Saturated and unsaturated steam

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 in the figure). 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.

As you know, liquids evaporate, that is, they turn into steam. For example, puddles dry up after rain. The evaporation of a liquid is due to the fact that some of its molecules, thanks to the shocks of their “neighbors,” acquire kinetic energy sufficient to escape from the liquid.
As a result of evaporation, there is always vapor above the surface of the liquid. This is the gaseous state of the substance. Water vapor is invisible, just like air. What is often called steam is a collection of tiny water droplets formed by the condensation of steam.

Condensation is the transformation of steam into liquid, that is, the process opposite to evaporation. Due to the condensation of water vapor contained in the air, clouds (Fig. 44.1) and fog (Fig. 44.2) are formed. Cold glass fogs up when it comes into contact with warm air (Fig. 44.3). This is also the result of condensation of water vapor.

Dynamic balance

If a jar of water is tightly closed, the water level in it remains unchanged for many months.

Does this mean that in a closed container the liquid does not evaporate?

No, of course: it always contains fairly fast molecules that constantly fly out of the liquid. However, condensation occurs simultaneously with evaporation: molecules from the vapor fly back into the liquid.

If the liquid level does not change over time, this means that the processes of evaporation and condensation occur with the same intensity. In this case, the liquid and vapor are said to be in dynamic equilibrium.

2. Saturated and unsaturated steam

Saturated steam

Figure 44.4 schematically depicts the processes of evaporation and condensation in a tightly closed vessel when liquid and vapor are in dynamic equilibrium.

Vapor that is in dynamic equilibrium with its liquid is called saturated.

Unsaturated steam

If a container with liquid is opened, steam will begin to escape from the container. As a result, the vapor concentration in the vessel will decrease, and vapor molecules will be less likely to collide with the surface of the liquid and fly into it. Therefore, the intensity of condensation will decrease.

But the intensity of evaporation remains the same. Therefore, the liquid level in the vessel will begin to decrease. If the evaporation process is faster than the condensation process, it is said that there is unsaturated vapor above the liquid (Fig. 44.5).

There is always water vapor in the air, but it is usually unsaturated, so evaporation predominates over condensation. That's why puddles dry up.

Above the surface of the seas and oceans, the vapor is also unsaturated, so they gradually evaporate. Why doesn't the water level go down?

The fact is that the rising steam cools and condenses, forming clouds and clouds. They turn into rain clouds and rain down. And rivers carry water back to the seas and oceans.

3. Dependence of saturated vapor pressure on temperature

The main property of saturated steam is that
Saturated vapor pressure does not depend on volume, but depends only on temperature.

This property of saturated steam is not so easy to understand because it appears to contradict the ideal gas equation of state

pV = (m/M)RT, (1)

from which it follows that for the bottom mass of gas at a constant temperature, the pressure is inversely proportional to the volume. Maybe this equation is not applicable for saturated steam?

The answer is: the ideal gas equation of state describes steam well, both saturated and unsaturated. But the mass of saturated steam m on the right side of equation (1) changes during isothermal expansion or compression - and in such a way that the pressure of the saturated steam remains unchanged. Why is this happening?

The fact is that when the volume of a vessel changes, steam can remain saturated only if “its” liquid is in the same vessel. By isothermally increasing the volume of the vessel, we seem to “pull” molecules from the liquid, which become vapor molecules (Fig. 44.6, a).

This is why this happens. As the volume of steam increases, its concentration initially decreases - but for a very short period of time. As soon as the steam becomes unsaturated, the evaporation of the liquid in the same vessel begins to “outpace” condensation. As a result, the mass of the vapor increases rapidly until it becomes saturated again. The steam pressure will then return to the same level.

1. Using Figure 44.6, b, explain why as the volume of saturated steam decreases, its mass decreases.

So, when saturated steam expands or contracts, its mass changes due to a change in the mass of the liquid contained in the same vessel.

The dependence of saturated water vapor pressure on temperature was measured experimentally. A graph of this relationship is shown in Figure 44.7. We see that the saturated vapor pressure increases very quickly with increasing temperature.

The main reason for the increase in saturated vapor pressure with increasing temperature is the increase in vapor mass. As you will see for yourself by completing the following task, when the temperature increases from 0 ºС to 100 ºС, the mass of saturated steam in the same volume increases by more than 100 times!

The table shows the values of saturated water vapor pressure at certain temperatures.

This table will help you with the next task. Also use formula (1).

2. A hermetically sealed vessel with a volume of 10 liters contains water and saturated steam. The temperature of the contents of the vessel is increased from 0 ºС to 100 ºС. Consider that the volume of water compared to the volume of steam can be neglected.
a) How many times did the absolute temperature increase?
b) How many times would the vapor pressure increase if it remained saturated?
c) How many times would the mass of steam increase if it remained saturated?
d) What would be the final mass of the vapor if it remained saturated?
e) At what minimum mass of water in the initial state will the steam remain saturated?
f) What will be the vapor pressure in the final state if the initial mass of water is 2 times less than that found in the previous paragraph?

3. What increases faster with increasing temperature - saturated vapor pressure or its density?
Clue. Formula (1) can be written as

4. An empty hermetically sealed vessel with a volume of 20 liters was filled with saturated water vapor at a temperature of 100 ºC.
a) What is the vapor pressure?
b) What is the mass of the steam?
c) What is the vapor concentration?
d) What will the vapor pressure be when it cools to 20 ºC?
e) What are the masses of steam and water at 20 ºС?
Clue. Use the table above and formula (1).

4. Boiling

Based on the above graph (Fig. 44 7) and table, you probably noticed that at the boiling point of water (100 ºС), the pressure of saturated water vapor is exactly equal to atmospheric pressure (dotted line in graph 44.7). Is this a coincidence?

No, not by chance. Let's consider the boiling process.

Let's put experience
We will heat water in an open transparent vessel. Soon bubbles will appear on the walls of the vessel. This releases air dissolved in water.

Water begins to evaporate inside these bubbles, and the bubbles are filled with saturated steam. But these bubbles cannot grow as long as the saturated vapor pressure is less than the pressure in the liquid. In an open, shallow vessel, the pressure in the liquid is almost equal to atmospheric pressure.

Let's continue heating the water. The saturated vapor pressure in the bubbles increases rapidly with increasing temperature. And as soon as it becomes equal to atmospheric pressure, intense evaporation of the liquid into the bubbles will begin.

They will grow quickly, rise up and burst on the surface of the liquid (Fig. 44.8). This is boiling.

In a shallow vessel, the pressure in the liquid is almost equal to the external pressure. Therefore we can say that
boiling of a liquid occurs at a temperature at which the pressure p n of saturated vapor is equal to the external pressure p external:

p n = p ext. (2)

It follows that the boiling point depends on pressure. Therefore, it can be changed by changing the fluid pressure. As the pressure increases, the boiling point of the liquid increases. This is used, for example, to sterilize medical instruments: water is boiled in special devices - autoclaves, where the pressure is 1.5-2 times higher than normal atmospheric pressure.

High in the mountains where atmospheric pressure significantly less than normal atmospheric temperature, it is not easy to cook meat: for example, at an altitude of 5 km, water already boils at a temperature of 83 ºС.

5. Using formula (2) and the table above, determine the boiling point of water:
a) at a pressure equal to one fifth of normal atmospheric pressure;
b) at a pressure 2 times greater than atmospheric pressure.

The boiling of water at reduced pressure can be observed in the following experiment.

Let's put experience
Bring the water in the flask to a boil and close the flask tightly. When the water has cooled a little, turn the flask over and water the bottom cold water. The water will boil, although its temperature is significantly lower than 100 ºС (Fig. 44.9).

6. Explain this experience.

7. To what height could boiling water be raised with a piston if it did not cool down?

Additional questions and tasks

8. In a cylindrical vessel under the piston long time contains water and water vapor. The mass of water is 2 times the mass of steam. Slowly moving the piston, the volume under the piston is increased from 1 liter to 6 liters. The temperature of the contents of the vessel remains equal to 20 ºС all the time. Consider that the volume of water is negligible compared to the volume of steam.
a) What kind of steam is under the piston at the beginning?
b) Explain why the pressure in the vessel will not change until the volume under the piston becomes equal to 3 liters.
c) What is the pressure in the vessel when the volume under the piston is 3 liters?
d) What is the mass of steam in the vessel when the volume under the piston is 3 liters?
Clue. In this case, the entire volume of the vessel is filled with saturated steam.
e) How many times did the mass of steam increase when the volume under the piston increased from 1 liter to 3 liter?
f) What is the mass of water in the initial state?
Clue. Take advantage of the fact that in the initial state the mass of water is 2 times the mass of steam.
g) How will the pressure in the vessel change when the volume under the piston changes from 3 l to 6 l?
Clue. For unsaturated steam, the equation of state for an ideal gas with constant mass is valid.
h) What is the pressure in the vessel when the volume under the piston is 6 liters?
i) Draw an approximate graph of the vapor pressure under the piston as a function of volume.

9. The two sealed U-tubes were tilted as shown in Figure 44.10. Which tube contains only saturated steam above the water, and which contains air with steam? Justify your answer.

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 in a given volume at a given temperature there cannot be more pair.

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 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 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 at which the 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?

Liquids always contain 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, producing 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.

Of great importance relative humidity air, 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 readings of the dry and moistened thermometers. saturation = 100 ° C and specific characteristics 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 >> Biology

Flammability limits (by volume). Pressure saturated vapors at T = -38 oC... exposure solar radiation, concentration saturation will be determined by either temperature... exposure to solar radiation, concentration saturation will not be determined by temperature...

Before answering the question posed in the title of the article, let’s figure out what steam is. The images that come to mind for most people 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, and 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, more precisely by the ratio 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 significantly greater – solid state, if the energy of thermal motion is significantly greater - 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? Average speed thermal motion of molecules is equal to a certain value depending on temperature. However individual speeds molecules are different: most of them have velocities close to the average value, but some have velocities 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 a greater number of reverse transitions of molecules into the 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. Ideal gas- it 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

Saturated means that the object has the largest of possible values 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 reference temperature must be constant.
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 a constant temperature, it follows that they are independent of the volume of saturated steam; it turns out that for saturated steam, pressure and volume are independent of each other.
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.
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.

Liquids tend to evaporate. If we dropped a drop of water, ether and mercury onto the table (just don’t do this at home!), we could observe how the drops gradually disappear - evaporate. Some liquids evaporate faster, others slower. The process of evaporation of liquid is also called vaporization. And the reverse process of turning steam into liquid is condensation.

These two processes illustrate phase transition- the process of transition of substances from one state of aggregation to another:

evaporation (transition from liquid to gaseous state);
condensation (transition from a gaseous state to a liquid);
desublimation (transition from a gaseous state to a solid state, bypassing the liquid phase);
sublimation, also known as sublimation (transition from solid to gaseous state, bypassing liquid).

Now, by the way, suitable season to observe the process of desublimation in nature: frost and hoarfrost on trees and objects, frosty patterns on windows - its result.

How saturated and unsaturated steam is formed

But let's return to vaporization. We will continue to experiment and pour liquid - water, for example, into an open vessel, and connect a pressure gauge to it. Invisible to the eye, evaporation occurs in the vessel. All liquid molecules are in continuous motion. Some move so fast that they kinetic energy turns out to be stronger than the one that binds liquid molecules together.

Having left the liquid, these molecules continue to move chaotically in space, the vast majority of them disperse in it - this is how unsaturated steam. Only a small part of them returns back to the liquid.

If we close the vessel, the number of vapor molecules will gradually increase. And more and more of them will return to the liquid. This will increase the steam pressure. This will be recorded by a pressure gauge connected to the vessel.

After some time, the number of molecules flying out of the liquid and returning to it will be equal. The steam pressure will stop changing. As a result steam saturation thermodynamic equilibrium of the liquid-vapor system will be established. That is, evaporation and condensation will be equal.

Properties of saturated steam

To illustrate them clearly, we use another experiment. Use all the power of your imagination to imagine it. So, let's take a mercury manometer, consisting of two elbows - communicating tubes. Both are filled with mercury, one end is open, the other is sealed, and above the mercury there is still a certain amount of ether and its saturated vapor. If you lower and raise the unsealed knee, the mercury level in the sealed one will also fall and rise.

In this case, the amount (volume) of saturated ether vapor will also change. The difference in the levels of mercury columns in both legs of the manometer shows the saturated vapor pressure of the ether. It will remain unchanged all the time.

This implies the property of saturated steam - its pressure does not depend on the volume it occupies. The saturated vapor pressure of different liquids (water and ether, for example) is different at the same temperature.

However, the temperature of the saturated steam matters. The higher the temperature, the higher the pressure. The pressure of saturated steam increases with increasing temperature faster than it does with unsaturated steam. The temperature and pressure of unsaturated steam are related linearly.

Another interesting experiment can be done. Take an empty flask without liquid vapor, close it and connect the pressure gauge. Gradually, drop by drop, add liquid into the flask. As the liquid enters and evaporates, the saturated vapor pressure is established, the highest for a given liquid at a given temperature.

More about temperature and saturated steam

The temperature of the steam also affects the rate of condensation. Just as the temperature of a liquid determines the rate of evaporation - the number of molecules that fly out from the surface of the liquid per unit time, in other words.

For saturated steam, its temperature is equal to the temperature of the liquid. The higher the temperature of the saturated vapor, the higher its pressure and density, the lower the density of the liquid. When the critical temperature for a substance is reached, the density of the liquid and vapor is the same. If the vapor temperature is higher than the critical temperature for the substance, the physical differences between the liquid and saturated vapor are erased.

Determination of saturated vapor pressure in a mixture with other gases

We talked about the saturated vapor pressure being constant at a constant temperature. We determined the pressure under “ideal” conditions: when a vessel or flask contains liquid and vapor of only one substance. Let us also consider an experiment in which molecules of a substance are scattered in space in a mixture with other gases.

To do this, take two open glass cylinders and place closed vessels with ether in both. As usual, let's connect the pressure gauges. We open one vessel with ether, after which the pressure gauge records the increase in pressure. The difference between this pressure and the pressure in a cylinder with a closed vessel of ether allows us to find out the pressure of the saturated vapor of ether.

About pressure and boiling

Evaporation is possible not only from the surface of the liquid, but also in its volume - then it is called boiling. As the temperature of the liquid increases, vapor bubbles form. When the saturated vapor pressure is greater than or equal to the gas pressure in the bubbles, the liquid evaporates into the bubbles. And they expand and rise to the surface.

Liquids boil at different temperatures. Under normal conditions, water boils at 100 0 C. But with a change in atmospheric pressure, the boiling point also changes. So, in the mountains, where the air is very rarefied and the atmospheric pressure is lower, as you rise into the mountains the boiling point of water decreases.

By the way, boiling in a hermetically sealed vessel is impossible at all.

Another example of the relationship between vapor pressure and evaporation is demonstrated by such a characteristic of the content of water vapor in the air as relative air humidity. It is the ratio of the partial pressure of water vapor to the pressure of saturated vapor and is determined by the formula: φ = r/r o * 100%.

As the air temperature decreases, the concentration of water vapor in it increases, i.e. they become more saturated. This temperature is called the dew point.

Let's sum it up

Using simple examples, we analyzed the essence of the evaporation process and the unsaturated and saturated steam formed as a result. You can observe all these phenomena around you every day: for example, see puddles drying up on the streets after rain or a mirror fogged up from steam in the bathroom. In the bathroom, you can even observe how steam formation first occurs, and then the moisture accumulated on the mirror condenses back into water.

You can also use this knowledge to make your life more comfortable. For example, in winter the air in many apartments is very dry, and this has a bad effect on well-being. You can use a modern humidifier device to make it more humid. Or, in the old fashioned way, place a container of water in the room: gradually evaporating, the water will saturate the air with its vapors.

website, when copying material in full or in part, a link to the original source is required.