Saturated steam properties

Saturated steam and its properties.

Boiling. critical temperature

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

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

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

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

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

Saturated steam properties

The saturated vapor pressure at a given temperature is constant. The saturated vapor pressure is different for different liquids. Consider an experiment that supports this statement.

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

At the initial moment, the height of the column of mercury h= 760 mm, then as the ether evaporates, it decreases, since the pressure on mercury from the ether vapor increases. As soon as the ether poured into the flask ceases to evaporate, the saturation, and the pressure no longer increases, no matter how much ether is poured into the flask.

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

The saturated vapor parameters satisfy the Mendeleev – Cliperon equation

pV = .

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

Saturated steam.

If a vessel with close tightly with liquid, then first the amount of liquid will decrease, and then it will remain constant. With no menn At the same 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 the averagesensitize each other. At the first moment, after the liquid is poured into the vessel and closed, the liquid willevaporate 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 higher the vapor density, the more its molecules are returned 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 for some R th period of time will be equal on average to the number of vapor molecules that have returned in the same time in the liquid b. Steam, nah dressed in dynamic equilibrium with its liquid is called saturated vapor. This is the definition of underscoreit seems that in a given volume at a given temperature there cannot be large quantity pair.

Saturated steam pressure .

What happens to saturated steam if the volume it occupies is reduced? For example, if you compress the vapor in equilibrium with the liquid in the cylinder under the piston, keeping the temperature of the cylinder contents 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 move from gas to liquid than from liquid to gas. After all, the number of molecules leaving the liquid per unit time depends only on temperature, and vapor compression does not change this number. The process continues until dynamic equilibrium and vapor density are re-established, which means that the concentration of its molecules will not assume 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. vapor, in which the liquid is in equilibrium with its vapor, is called the saturated vapor pressure.

Dependence of saturated steam 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 P = nkT With increasing temperature, the pressure increases. Since the saturated vapor pressure does not depend on the volume, then, therefore, it depends only on the temperature. However, the dependence of the p n.p. from T, found experimentally, is not directly proportional, as in an ideal gas at constant volume. With an increase in temperature, the pressure of a real saturated vapor grows faster than the pressure of an ideal gas (Fig.curve 12). Why is this happening? When a liquid is heated in a closed vessel, some of the liquid turns into steam. As a result, according to the formula P = nkT, the saturated vapor pressure increases not only as a result of an increase in the temperature of the liquid, but also as a result of an increase in the concentration of molecules (density) of vapor. Basically, an increase in pressure with increasing temperature is determined precisely by an increase in the concentration center ui. (The main difference in behavior andideal gas and saturated steam is that when the temperature of the steam in a closed vessel changes (or when the volume changes at a constant temperature), the mass of the steam changes. The liquid is partially converted into steam, or, on the contrary, the steam is partially condensedtsya. Nothing of the sort happens with an ideal gas.) When all the liquid has evaporated, the vapor, upon further heating, will cease to be saturated and its pressure at a constant volume will increaseto be directly proportional 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 that occurs throughout the entire volume of a liquid (and not just from its surface). (Condensation is the reverse process.) As the temperature of the liquid rises, the rate of evaporation increases. Finally, the liquid starts 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 is because all the energy supplied to the liquid is spent on converting it into steam. Under what conditions does boil start?

The liquid always contains dissolved gases released at the bottom and walls of the vessel, as well as on dust particles suspended in the liquid, which are centers of vaporization. The vapors of the liquid inside the bubbles are saturated. With an increase in temperature, the pressure of saturated vapor increases and the bubbles increase in size. Under the action of the buoyancy force, they float upward. If the upper layers of the liquid have more low temperature, then vapor condensation in bubbles occurs in these layers. The pressure drops rapidly and the bubbles collapse. The collapse occurs so quickly 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 will stop collapsing and float to the surface. The liquid will boil. Pay close attention to the kettle on the stove. You will find that it almost stops making noise before it boils. The dependence of the 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 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 pressure of the saturated vapor in the bubbles is equal to the pressure in the liquid. The higher the external pressure, the higher the boiling point. Conversely, by decreasing 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 when room temperature... Each liquid has its own boiling point (which remains constant until all the liquid has boiled away), which depends on the pressure of its saturated vapor. The higher the saturated vapor pressure, the lower the boiling point of the liquid.

Air humidity and its measurement.

There is almost always a certain amount of water vapor in the air around us. Air humidity depends on the amount of water vapor contained in it. Raw air contains greater percentage water molecules than dry. Pain Relative humidity is of utmost importance, and it is reported every day in weather forecast reports.

Dew point

The dryness or humidity of the air depends on how close its water vapor is to saturation. If wet air cooled, then the steam 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 steam in the air becomes saturated is called the dew point. The dew point also characterizes the humidity of the air. Examples: dew fall 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. Measuring devices - hygrometers are used to measure air humidity. There are several types of hygrometers, but the main ones are: hair and psychrometric.

Under natural conditions, steam is considered a gas. He might be saturated and unsaturated, which depends on its density, temperature and pressure.

Steam in dynamic equilibrium with its own liquid is saturated.

Dynamic equilibrium between liquid and vapor occurs when the number of molecules escaping from the free surface of the liquid is equal to the number of molecules returning to it.

In an open vessel, dynamic equilibrium is disturbed, and the steam becomes unsaturated, since a certain amount of molecules evaporates into the atmosphere and does not return to the liquid.

Saturated steam is formed in a closed vessel above the free surface of the liquid.

Saturated and unsaturated steam have different properties. Let's explore them.

Concentration of molecules saturated steam does not depend on its volume.

Let be unsaturated steam at a temperature T located in a cylinder with a piston (Fig. 3.2). We begin to slowly compress it to ensure an isothermal process (section AB). First, if the steam is significantly rarefied, the dependence of pressure on volume will correspond to Boy-la-Mariotte's law for an ideal gas: pV= const. Nevertheless, with a decrease in the volume of unsaturated steam (an increase in its density), a deviation from it begins to be observed. Further isothermal compression of vapor leads to the fact that it begins to condense (point B), liquid droplets form in the cylinder, and the vapor becomes saturated. Its density, and hence the concentration of molecules, acquires maximum value for a given temperature. They do not depend on the volume occupied by saturated steam, and are determined by its pressure and temperature.

When compressed saturated steam(section BC) its pressure will not change ( p = const). This is due to the fact that with a decrease in volume, saturated vapor condenses, forming a liquid. Its share in the volume of the cylinder increases all the time, and the volume occupied by saturated steam decreases. This happens until all the saturated steam passes into liquid state(point C).

A further decrease in volume causes a rapid increase in pressure (DC section), since liquids are almost not compressed. Material from the site

So, with isothermal compression unsaturated steam first (at insignificant density) it exhibits the properties of an ideal gas. When does the steam become saturated, its properties obey other laws. In particular, at non-high temperatures, its state is approximately described by the equation p = nkT, when the concentration of molecules does not depend on the volume occupied by the gas. Pressure plot p from the volume V, shown in fig. 3.2 is called isotherm of real gases.

Real gas isotherms characterize its equilibrium state with liquid. Their compatibility allows you to determine the dependence of pressure saturated steam from temperature.

On this page material on topics:

• Isothermal increase in unsaturated steam pressure

• What is characteristic of saturated steam from a molecular point of view

• Saturated steam and its properties in brief

• What is characteristic of saturated steam from a molecular point of view?

• What is characteristic of a saturated gas from a molecular point of view

Questions about this material:

DEFINITION

Evaporation is the process of converting a liquid into steam.

In a liquid (or a 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 find themselves near the surface, then they can overcome the attraction of other molecules and fly out of the liquid, forming vapor above it. The 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 rate of evaporation increases. The evaporation rate 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 is continuously decreasing due to evaporation. But this does not happen in a tightly closed vessel. This is explained by the fact that the opposite process occurs simultaneously with evaporation in a liquid (or solid). 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 converting vapor to liquid is called condensation. The process of converting steam into a solid is commonly referred to as crystallization from steam.

After we pour 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 state is called a 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 cancel each other out.

Fig. 1. Fluid in dynamic equilibrium

Saturated and unsaturated steam

DEFINITION

Saturated steam is a vapor that is in a state of 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 a state of dynamic equilibrium.

In 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 forces of interaction of molecules are large (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 small forces of attraction of molecules, at the same temperatures a significant number of molecules fly out of the liquid and vapor saturation 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 vaporization 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 steam pressure depends only on temperature and does not depend on volume. Moreover, as the temperature rises, the concentration of saturated vapor molecules and, consequently, the density and pressure of the saturated vapor grow rapidly. The specific dependences of the pressure and density of saturated steam 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 Cliperon-Mendeleev equation. However, when compressed or heated, the mass of the saturated vapor changes.

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

Examples of problem solving

EXAMPLE 1

EXAMPLE 2

Liquids tend to evaporate. If we dripped a drop of water, ether and mercury onto the table (just don't do it at home!), We could observe how the drops gradually disappear - evaporate. Some liquids evaporate faster, others more slowly. The process of evaporation of a liquid is also called vaporization. And the reverse process of transformation of vapor 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, bypassing the liquid phase);
• sublimation, it is also sublimation (transition from solid to gaseous state, bypassing the liquid).

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

How saturated and unsaturated steam is formed

But back to vaporization. We will continue to experiment and pour liquid - water, for example, in 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 the liquid molecules together.

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

If we close the vessel, the vapor molecules will gradually become more and more. And more and more of them will return to the liquid. This will increase the vapor pressure. This will fix the pressure gauge connected to the vessel.

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

Saturated steam properties

To illustrate them clearly, we will use one more experiment. Invoke the full power of your imagination to present it. So, let's take a mercury manometer, consisting of two elbows - communicating tubes. Mercury is poured into both, one end is open, the other is sealed, and over the mercury there is still a certain amount of ether and its saturated vapor in it. If you lower and raise the unsealed knee, the level of mercury in the sealed one will also rise and fall.

In this case, the amount (volume) of the saturated vapor of the ether will also change. The difference in the levels of the 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 does matter. The higher the temperature, the higher the pressure. The pressure of saturated steam rises faster with increasing temperature than it does with unsaturated steam. The temperature and pressure of unsaturated steam are linearly related.

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

More about temperature and saturated steam

The steam temperature also affects the condensation rate. Just as the temperature of a liquid determines the rate of evaporation - the number of molecules that fly out from the surface of a liquid per unit of 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 temperature critical for a substance is reached, the density of the liquid and vapor is the same. If the temperature of the vapor is higher than the critical temperature for the substance, the physical differences between liquid and saturated vapor disappear.

Determination of the pressure of saturated steam mixed with other gases

We said that the saturated vapor pressure remains unchanged at a constant temperature. We determined the pressure under “ideal” conditions: when there is liquid and vapor of only one substance in a vessel or flask. 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 them in both closed vessels with ether. As usual, we will 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 the cylinder with a closed ether vessel allows you to find out the pressure of the saturated vapor of the 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 rises, vapor bubbles form. When the pressure of the saturated vapor is greater than or equal to the pressure of the gas in the bubbles, the liquid evaporates inside the bubbles. And those expand and rise to the surface.

Liquids boil at different temperatures. V 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, the boiling point of water also decreases as you climb the mountains.

By the way, boiling is impossible in a hermetically sealed vessel 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 the relative humidity of the air. It is the ratio of the partial pressure of water vapor to the pressure of saturated vapor and is determined by the formula: φ = p / p about * 100%.

With a decrease in air temperature, the concentration of water vapor in it increases, i.e. they become richer. This temperature is called the dew point.

Let's summarize

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

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

site, with full or partial copying of the material, a link to the source is required.