What does saturated steam depend on? What is the difference between saturated steam and unsaturated steam?

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, is the right 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 their kinetic energy is stronger than that, which 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 constant constant temperature saturated steam pressure. 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. IN 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 thin and Atmosphere pressure Below, 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 saturated vapor pressure 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.

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STEAM FORMATION.

SATURATED AND UNSATURATED STEAM.

1. Vaporization.

Between the molecules of a substance in a liquid or solid state, attractive forces act. They are quite large for a solid substance. This leads to the fact that the molecules of a solid substance are inactive; they can only oscillate around their equilibrium position. In a liquid, molecules are not so strongly attracted to each other; they can move short distances and jump to adjacent equilibrium positions. However, as a result of the exchange of energies during collisions of molecules or as a result of the supply of energy from the outside, any individual molecule can receive such an amount of kinetic energy that will allow it to overcome the attractive forces of neighboring molecules and leave the surface of a liquid or solid. Some of these molecules, having lost their energy, return back to the liquid or solid, but the most energetic ones, which were able to move to a distance of about 10 -9 m, where the forces of attraction practically no longer act, become free.

The transition of a substance from a solid or liquid state to a gaseous state is called vaporization, and the collection of molecules of a substance that have left the surface of a liquid or solid is called ferry of this substance.

Most often, vaporization refers to the transition of a substance into a gaseous state from a liquid. Vaporization occurring from a solid state is called sublimation or sublimation.

Vaporization from a liquid state is divided into evaporation And boiling.

2. Evaporation and its intensity.

Evaporation is vaporization that occurs at any temperature only from the free surface of a liquid into air or vacuum, accompanied by a decrease in the temperature of the liquid.

The mechanism of evaporation and the resulting cooling of the liquid can be explained from the point of view of MCT.

As mentioned above, only those molecules leave the surface of a liquid whose kinetic energy exceeds the value of the work required to overcome the forces of molecular attraction from neighboring molecules and the release of the molecule from the surface of the liquid into the air. This work is called work function. As a result, the average kinetic energy of the remaining molecules decreases and, consequently, the temperature of the liquid decreases.

The intensity of evaporation depends on several factors:

on the temperature of the liquid;

on the free surface area;

on the rate of vapor removal from the surface of the liquid;

from external pressure;

depending on the type of liquid.

The higher the temperature, the larger the free surface area, the greater the rate of vapor removal from the surface of the liquid, the lower the external pressure, the more intense the evaporation.

The process of transition of a substance from a gaseous state to a liquid or solid is called condensation.

3.Saturated and unsaturated pairs.

Consider two vessels with liquid - one is open, the other is closed with a lid. In both vessels, both evaporation of liquid and condensation of steam occurs.

However, in the first case, evaporation prevails over condensation, since the molecules of the liquid have the opportunity to leave the confines of the vessel and they will not return to the liquid, and in their place other molecules emerge from the surface of the liquid into the air. The number of N 1 molecules leaving the surface in 1 s exceeds the number of N 2 molecules returning back. If the evaporation process prevails over the condensation process, then the resulting steam is called unsaturated.

In a hermetically sealed vessel, initially the number of N 1 molecules leaving the surface in 1 s exceeds the number of N 2 molecules returning back. Therefore, the vapor density above the liquid surface, as well as its pressure, increase. But as density and pressure increase, the number of molecules returning to the liquid within 1 second increases. After some time, the rates of evaporation and condensation become equal, i.e. the number of N 1 molecules leaving the liquid is equal to the number of N 2 returning. It is said that a dynamic equilibrium has been established between the vapor and its liquid.

Steam in a state of dynamic equilibrium with its liquid is called rich.

4. Boiling.

Boiling is the formation of vapor that occurs both from the surface and throughout the entire volume of a liquid at a constant temperature.

The boiling mechanism can be explained as follows.

There are always bubbles of adsorbed gas on the walls of the vessel. In addition, a liquid always contains a certain amount of dissolved gas (air), the degree of dissolution of which decreases with increasing temperature, and which, when heated, also begins to be released in the form of bubbles. Liquid evaporates inside the bubbles. Therefore, in addition to air, there is saturated steam inside the bubbles, its pressure increases with increasing temperature. Consequently, the bubbles swell. The Archimedes force acting on the bubbles becomes greater than their gravity, and they begin to float. The further behavior of the bubbles depends on how hot the liquid is.

If the liquid is not yet uniformly heated and its upper layers are colder than the lower ones, then as the bubbles float up, the vapor inside them condenses, and the pressure inside the bubbles decreases. Consequently, the volume of bubbles decreases. The Archimedes force, which depends on the volume of the bubbles, also becomes smaller, the upward movement of the bubbles slows down and, before reaching the surface of the liquid, the bubbles disappear.

If the liquid is heated evenly, then as the bubbles float up, their volume will increase, since the force of the hydrostatic pressure of the liquid acting on the bubbles decreases. An increase in volume leads to an increase in Archimedes' force. Therefore, the upward movement of bubbles accelerates. The bubbles reach the free surface, burst, and saturated steam escapes. This moment is called boiling of the liquid. In this case, the saturated vapor pressure in the bubbles is almost equal to the external pressure.

The temperature at which the saturated vapor pressure is equal to the external pressure is called boiling point.

The boiling point depends on:

1) from external pressure (the greater it is, the higher the boiling point);

2) from the presence of an impurity (usually the boiling point increases with increasing impurity concentration);

3) from air or other gases dissolved in the liquid (with a decrease in the amount of dissolved air, the temperature rises);

4) on the condition of the walls of the vessel (in vessels with smoother walls, the liquid boils at a higher temperature);

5) depending on the type of liquid.

5. Comparison of the properties of saturated steam and ideal gas.

1.The pressure and density of saturated vapor are constant and do not depend on the volume of space above the evaporating liquid. For an ideal gas, pressure and density decrease with increasing volume.

Saturated steam Ideal gas

2. With increasing temperature at a constant volume, the increase in saturated vapor pressure does not occur according to a linear law, as for an ideal gas, but much faster. This is explained by the fact that the increase in pressure occurs not only due to an increase in kinetic energy, but also due to an increase in the number of evaporated molecules.

For the same reason, the density of saturated vapor does not remain constant, it increases.

3.The pressure and density of saturated vapor depend on the type of liquid and are determined by the heat of vaporization. The lower the heat of vaporization, the greater the pressure and density of saturated steam.

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

2.7.1 Evaporation and condensation

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

In Fig. Figure 2.16 shows an approximate graph of the velocity distribution of liquid molecules. The blue background shows the majority of molecules whose velocities are grouped around the average value. The red “tail” of the graph is a small number of “fast” molecules, the speeds of which significantly exceed the average speed of the bulk of liquid molecules.

Number of molecules

Fast molecules

Speed ​​of molecules

Rice. 2.16. Distribution of molecules by speed

When such a very fast molecule finds itself 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 leaving 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 returns back to the liquid.

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

2.7.2 Dynamic balance

What happens if a vessel with liquid is hermetically sealed? The vapor density above the liquid surface will begin to increase; vapor particles will increasingly interfere with other liquid molecules flying out, and the evaporation rate will begin to decrease. At the same time it will start

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

p n = n RT:

the condensation rate will increase, since with increasing vapor concentration the number of molecules returning to the liquid will increase.

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

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

The pressure and density of saturated steam are designated pн 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 steam always exceeds the pressure and density of unsaturated steam.

2.7.3 Properties of saturated steam

It turns out that the state of saturated steam (and even more so of unsaturated steam) can be approximately described by the equation of state of an ideal gas (Mendeleev-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. Let us 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 steam is compressed isothermally, then its density will increase at the first moment, the condensation rate will exceed the evaporation rate, and part of the vapor will condense into liquid until dynamic equilibrium occurs again, in which the vapor density will return to its previous value.

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

2. The pressure of saturated steam does not depend on its volume.

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

As we see, Boyle-Mariotte's law, which is valid for ideal gases, does not hold true for saturated steam. This is not surprising, since it was obtained from the Mendeleev-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 liquid evaporation increases. The dynamic equilibrium is disrupted at the first moment, and additional

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

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

Thus, when saturated steam is heated or cooled isochorically, its mass changes, so Charles’s law does not work in this case. The dependence of saturated vapor pressure on temperature will no longer be a linear function.

4. Saturated vapor pressure increases with temperature faster than according to a linear law.

In fact, 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 ideal gas laws.

isochore pair

Rice. 2.17. Dependence of steam pressure on temperature

At point 2 all liquid evaporates; with a further increase in temperature, the steam becomes unsaturated, and its pressure increases linearly according to Charles’s law (section 2–3).

Let us recall that the linear increase in pressure of an ideal gas is caused by an increase in the intensity of impacts of molecules on the walls of the vessel. When saturated steam is heated, the molecules begin to beat not only harder, but also more often because the steam becomes larger. Simultaneous action These two factors cause an exponential increase in saturated 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 air 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

the ratio is expressed as a percentage:

" = p 100%: pн

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

" = 100%:n

One of the devices that measures air humidity is a psychrometer. 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 cools, and the greater the difference between its readings and the readings of the dry thermometer. From this difference, air humidity is determined using a special psychrometric table.

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, it is a solid state; if the energy of thermal motion is much greater, it is a gaseous state; if the energies are comparable, it is a liquid state.

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 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.
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 related proportional dependence. 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.
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.

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 whose kinetic energy is greater than the potential energy of their interaction with other particles of the substance. If such molecules find themselves near the surface, they can overcome the attraction of other molecules and fly out of the liquid, forming vapor above it. Evaporation of solids is also often called sublimation or sublimation.

Evaporation occurs at any temperature at which the substance may be in a liquid or solid state. However, the rate of evaporation depends on 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 container continuously decreases due to evaporation. But this does not happen in a tightly closed vessel. This is explained by the fact that simultaneously with evaporation in a liquid (or solid), the reverse process occurs. Vapor molecules move chaotically over the liquid, so some of them, under the influence of the attraction of free surface molecules, fall back into the liquid. The process of turning steam into liquid is called condensation. The process of turning steam into solid usually called 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 vapor molecules returning to the liquid. This condition is called state of dynamic equilibrium(Fig. 1). In a state of dynamic equilibrium between liquid and vapor, evaporation and condensation occur simultaneously, and both processes compensate each other.

Fig.1. Fluid in a state of dynamic equilibrium

Saturated and unsaturated steam

DEFINITION

Saturated steam is steam in a state of dynamic equilibrium with its liquid.

The name “saturated” emphasizes that no more vapor can be present in a given volume at a given temperature. 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 a state of dynamic equilibrium.

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

Since the intensity of the steam condensation process is proportional to the concentration of steam molecules, and the intensity of the evaporation process depends only on temperature and increases sharply with its growth, the concentration of molecules in saturated steam 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 quickly increase. The 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 Clayperon-Mendeleev equation. However, when compressed or heated, the mass of saturated steam changes.

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

Examples of problem solving

EXAMPLE 1

EXAMPLE 2