In this lesson, we will pay attention to this type of evaporation, such as boiling, discuss its differences from the previously discussed evaporation process, introduce a value such as boiling temperature, and discuss what it depends on. At the end of the lesson, we will introduce a very important quantity that describes the process of vaporization - the specific heat of vaporization and condensation.

Topic: Aggregate states of matter

Lesson: Boiling. Specific heat of vaporization and condensation

In the last lesson, we already looked at one of the types of vapor formation - evaporation - and highlighted the properties of this process. Today we will discuss this type of vaporization, the boiling process, and introduce a value that numerically characterizes the process of vaporization - specific heat vaporization and condensation.

Definition.Boiling(Fig. 1) is a process of intense transition of a liquid into a gaseous state, accompanied by the formation of vapor bubbles and occurring throughout the entire volume of the liquid at a certain temperature, which is called the boiling point.

Let's compare the two types of vaporization with each other. The boiling process is more intense than the evaporation process. In addition, as we remember, the evaporation process occurs at any temperature above the melting point, and the boiling process strictly at a certain temperature, which is different for each substance and is called the boiling point. It should also be noted that evaporation occurs only from the free surface of the liquid, i.e., from the area separating it from the surrounding gases, and boiling occurs from the entire volume at once.

Let's take a closer look at the boiling process. Let's imagine a situation that many of us have repeatedly encountered - heating and boiling water in a certain vessel, for example, a saucepan. During heating, a certain amount of heat will be transferred to the water, which will lead to an increase in its internal energy and an increase in the activity of molecular movement. This process will continue until a certain stage, until the energy of molecular motion becomes sufficient to begin boiling.

Water contains dissolved gases (or other impurities) that are released in its structure, which leads to the so-called occurrence of vaporization centers. That is, it is in these centers that steam begins to be released, and bubbles form throughout the entire volume of water, which are observed during boiling. It is important to understand that these bubbles do not contain air, but steam that is formed during the boiling process. After the formation of bubbles, the amount of steam in them increases, and they begin to increase in size. Often, bubbles initially form near the walls of the vessel and do not immediately rise to the surface; first, increasing in size, they are under the influence of the growing force of Archimedes, and then they break away from the wall and rise to the surface, where they burst and release a portion of steam.

It is worth noting that not all steam bubbles immediately reach the free surface of the water. At the beginning of the boiling process, the water is not yet heated evenly and the lower layers, near which the heat transfer process directly occurs, are even hotter than the upper ones, even taking into account the convection process. This leads to the fact that the steam bubbles rising from below collapse due to the phenomenon of surface tension, before reaching the free surface of the water. In this case, the steam that was inside the bubbles passes into the water, thereby further heating it and accelerating the process of uniform heating of the water throughout the entire volume. As a result, when the water warms up almost evenly, almost all the steam bubbles begin to reach the surface of the water and the process of intense steam formation begins.

It is important to highlight the fact that the temperature at which the boiling process takes place remains unchanged even if the intensity of heat supply to the liquid is increased. In simple words, if during the boiling process you add gas on a burner that heats a pan of water, this will only lead to an increase in the intensity of boiling, and not to an increase in the temperature of the liquid. If we delve more seriously into the boiling process, it is worth noting that areas appear in water in which it can be overheated above the boiling point, but the amount of such overheating, as a rule, does not exceed one or a couple of degrees and is insignificant in the total volume of liquid. Boiling point of water at normal pressure is 100°C.

During the process of boiling water, you can notice that it is accompanied by characteristic sounds of the so-called seething. These sounds arise precisely due to the described process of collapse of steam bubbles.

The boiling processes of other liquids proceed in the same way as the boiling of water. The main difference in these processes is the different boiling temperatures of substances, which at normal atmospheric pressure are already measured tabular values. We indicate the main values ​​of these temperatures in the table.

An interesting fact is that the boiling point of liquids depends on the value atmospheric pressure, which is why we indicated that all values ​​in the table are given at normal atmospheric pressure. When air pressure increases, the boiling point of the liquid also increases; when it decreases, on the contrary, it decreases.

On this dependence of boiling temperature on pressure environment based on the operating principle of such a well-known kitchen appliance as a pressure cooker (Fig. 2). It is a pan with a tight-fitting lid, under which, during the process of steaming water, the air pressure with steam reaches up to 2 atmospheric pressure, which leads to an increase in the boiling point of water in it to . Because of this, the water and food in it have the opportunity to heat up to a temperature higher than usual (), and the cooking process is accelerated. Because of this effect, the device got its name.

Rice. 2. Pressure cooker ()

The situation with a decrease in the boiling point of a liquid with a decrease in atmospheric pressure also has an example from life, but no longer everyday for many people. This example applies to the travel of climbers in high mountain regions. It turns out that in areas located at an altitude of 3000-5000 m, the boiling point of water due to a decrease in atmospheric pressure is reduced to lower values, which leads to difficulties when preparing food on hikes, because for effective heat treatment of products in In this case, it takes significantly longer than under normal conditions. At altitudes of about 7000 m, the boiling point of water reaches , which makes it impossible to cook many products in such conditions.

On the fact that boiling temperatures various substances differ, some technologies for separating substances are based. For example, if we consider heating oil, which is a complex liquid consisting of many components, then during the boiling process it can be divided into several different substances. In this case, due to the fact that the boiling points of kerosene, gasoline, naphtha and fuel oil are different, they can be separated from each other by vaporization and condensation at different temperatures. This process is usually called fractionation (Fig. 3).

Rice. 3 Separation of oil into fractions ()

Like any physical process, boiling must be characterized using some numerical value, this value is called the specific heat of vaporization.

In order to understand the physical meaning of this value, consider the following example: take 1 kg of water and bring it to the boiling point, then measure how much heat is needed to completely evaporate this water (without taking into account heat losses) - this value will be equal to the specific heat of vaporization of water. For another substance, this heat value will be different and will be the specific heat of vaporization of this substance.

The specific heat of vaporization turns out to be very important characteristic V modern technologies metal production. It turns out that, for example, during the melting and evaporation of iron with its subsequent condensation and solidification, a crystal lattice is formed with a structure that provides higher strength than the original sample.

Designation: specific heat of vaporization and condensation (sometimes denoted ).

Unit: .

The specific heat of vaporization of substances is determined using laboratory experiments, and its values ​​for basic substances are listed in the appropriate table.

Substance

In order to maintain the boiling of water (or other liquid), heat must be continuously supplied to it, for example, by heating it with a burner. In this case, the temperature of the water and the vessel does not increase, but for each unit of time a certain amount of steam is formed. From this it follows that the transformation of water into steam requires an influx of heat, just as it occurs when a crystal (ice) is transformed into a liquid (§ 269). The amount of heat required to convert a unit mass of liquid into vapor of the same temperature is called the specific heat of vaporization of a given liquid. It is expressed in joules per kilogram.

It is not difficult to understand that when steam condenses into liquid, the same amount of heat should be released. Indeed, let us lower a tube connected to a boiler into a glass of water (Fig. 488). Some time after heating begins, air bubbles will begin to emerge from the end of the tube immersed in water. This air does not raise the temperature of the water much. Then the water in the boiler will boil, after which we will see that the bubbles coming out of the end of the tube no longer rise up, but quickly decrease and disappear with a sharp sound. These are bubbles of steam condensing into water. As soon as steam comes out of the boiler instead of air, the water will begin to heat up quickly. Because specific heat steam is approximately the same as air, then from this observation it follows that such rapid heating of water occurs precisely as a result of steam condensation.

Rice. 488. While air is coming out of the boiler, the thermometer shows almost the same temperature. When steam comes out instead of air and begins to condense in the glass, the thermometer will quickly rise, indicating an increase in temperature

When a unit mass of steam is condensed into a liquid of the same temperature, an amount of heat is released equal to the specific heat of vaporization. This could have been predicted based on the law of conservation of energy. Indeed, if this were not so, then it would be possible to build a machine in which the liquid first evaporated and then condensed: the difference between the heat of vaporization and the heat of condensation would represent an increase in the total energy of all bodies participating in the process under consideration. And this contradicts the law of conservation of energy.

The specific heat of vaporization can be determined using a calorimeter, similar to how it is done when determining the specific heat of fusion (§ 269). Let's pour a certain amount of water into the calorimeter and measure its temperature. Then we will introduce steam of the test liquid from the boiler into the water for some time, taking measures to ensure that only steam comes out, without droplets of liquid. To do this, steam is passed through a steam chamber (Fig. 489). After this, we again measure the temperature of the water in the calorimeter. By weighing the calorimeter, we can judge by the increase in its mass the amount of vapor that has condensed into liquid.

Rice. 489. Steamer - a device for retaining water droplets moving along with steam

Using the law of conservation of energy, we can create an equation for this process heat balance, which allows us to determine the specific heat of vaporization of water. Let the mass of water in the calorimeter (including the water equivalent of the calorimeter) be equal to the mass of steam - , the heat capacity of water - , the initial and final temperature of water in the calorimeter - and , the boiling point of water - and the specific heat of vaporization - . The heat balance equation has the form

.

The results of determining the specific heat of vaporization of some liquids at normal pressure are given in table. 20. As you can see, this heat is quite large. The high heat of vaporization of water plays an extremely important role in nature, since vaporization processes occur in nature on a grand scale.

Table 20. Specific heat of vaporization of some liquids

Substance		Substance
Ethanol)

Note that the specific heat of vaporization values ​​contained in the table refer to the boiling point at normal pressure. If a liquid boils or simply evaporates at a different temperature, then its specific heat of vaporization is different. As the temperature of a liquid increases, the heat of vaporization always decreases. We will look at the explanation for this later.

295.1. Determine the amount of heat required to heat 20 g of water to boiling point and turn it into steam at .

295.2. What temperature will be obtained if 3 g of steam is introduced into a glass containing 200 g of water at ? Neglect the heat capacity of the glass.

Do you know what the boiling temperature is for soup? 100˚С. No more, no less. At the same temperature, the kettle boils and the pasta is cooked. What does it mean?

Why is it that when a saucepan or kettle is constantly heated with burning gas, the temperature of the water inside does not rise above one hundred degrees? The fact is that when the water reaches a temperature of one hundred degrees, all incoming thermal energy is spent on the transition of water into a gaseous state, that is, evaporation. Up to one hundred degrees, evaporation occurs mainly from the surface, and upon reaching this temperature, the water boils. Boiling is also evaporation, but only throughout the entire volume of the liquid. Bubbles with hot steam form inside the water and, being lighter than water, these bubbles burst to the surface, and the steam from them evaporates into the air.

When heated, the water temperature rises to one hundred degrees. After one hundred degrees, with further heating, the temperature of the water vapor will increase. But until all the water boils away at one hundred degrees, its temperature will not increase, no matter how much energy you apply. We have already figured out where this energy goes - to the transition of water into a gaseous state. But since such a phenomenon exists, it means there must be describing this phenomenon physical quantity. And such a value exists. It is called the specific heat of vaporization.

Specific heat of vaporization of water

The specific heat of vaporization is a physical quantity that shows the amount of heat required to convert a liquid weighing 1 kg into steam at the boiling point. The specific heat of vaporization is designated by the letter L. And the unit of measurement is joule per kilogram (1 J/kg).

The specific heat of vaporization can be found from the formula:

where Q is the amount of heat,
m is body weight.

By the way, the formula is the same as for calculating the specific heat of fusion, the only difference is in the designation. λ and L

The values ​​of the specific heat of vaporization of various substances were experimentally found and tables were compiled from which data for each substance can be found. Thus, the specific heat of vaporization of water is equal to 2.3*106 J/kg. This means that for every kilogram of water it is necessary to spend an amount of energy equal to 2.3 * 106 J to turn it into steam. But at the same time, the water must already have a boiling point. If the water was initially at a lower temperature, then it is necessary to calculate the amount of heat that will be required to heat the water to one hundred degrees.

In real conditions, it is often necessary to determine the amount of heat required for transformation of a certain mass of any liquid into vapor, therefore, more often you have to deal with a formula of the form: Q = Lm, and the values ​​of the specific heat of vaporization for a specific substance are taken from ready-made tables.

The phenomenon of a substance changing from a liquid to a gaseous state is called vaporization. Vaporization can be carried out in the form of two processes: i.

Boiling

The second process of vaporization is boiling. You can observe this process using simple experience, heating water in glass flask. When water is heated, after a while bubbles appear in it, containing air and saturated water vapor, which is formed when the water evaporates inside the bubbles. As the temperature rises, the pressure inside the bubbles increases, and under the influence of buoyant force they rise upward. However, since the temperature upper layers There is less water than the lower ones, the vapor in the bubbles begins to condense, and they shrink. When the water warms up throughout the entire volume, bubbles with steam rise to the surface, burst, and the steam comes out. Water is boiling. This occurs at a temperature at which the saturated vapor pressure in the bubbles is equal to atmospheric pressure.

The process of vaporization that occurs in the entire volume of liquid at a certain temperature is called. The temperature at which a liquid boils is called boiling point.

This temperature depends on atmospheric pressure. As atmospheric pressure increases, the boiling point increases.

Experience shows that during the boiling process, the temperature of the liquid does not change, despite the fact that energy comes from outside. The transition of a liquid into a gaseous state at the boiling point is associated with an increase in the distance between the molecules and, accordingly, with overcoming the attraction between them. The energy supplied to the liquid is consumed to perform work to overcome the forces of attraction. This happens until all the liquid turns into steam. Since liquid and vapor have the same temperature during boiling, the average kinetic energy of the molecules does not change, only their potential energy increases.

The figure shows a graph of the temperature of water versus time during its heating from room temperature to boiling point (AB), boiling point (BC), steam heating (CD), steam cooling (DE), condensation (EF) and subsequent cooling (FG) .

Specific heat of vaporization

To transform different substances from a liquid to a gaseous state, different energy is required, this energy is characterized by a value called the specific heat of vaporization.

Specific heat of vaporization (L) is a value equal to the ratio of the amount of heat that must be imparted to a substance weighing 1 kg to transform it from a liquid state to a gaseous state at the boiling point.

Unit of specific heat of vaporization - [ L] = J/kg.

To calculate the amount of heat Q that must be imparted to a substance with a mass mn for its transformation from a liquid to a gaseous state, the specific heat of vaporization ( L) multiplied by the mass of the substance: Q = Lm.

When steam condenses, a certain amount of heat is released, and its value is equal to the amount of heat that must be expended to convert the liquid into steam at the same temperature.

In this lesson, we will pay attention to this type of evaporation, such as boiling, discuss its differences from the previously discussed evaporation process, introduce a value such as boiling temperature, and discuss what it depends on. At the end of the lesson, we will introduce a very important quantity that describes the process of vaporization - the specific heat of vaporization and condensation.

Topic: Aggregate states of matter

Lesson: Boiling. Specific heat of vaporization and condensation

In the last lesson, we already looked at one of the types of vapor formation - evaporation - and highlighted the properties of this process. Today we will discuss this type of vaporization, the boiling process, and introduce a value that numerically characterizes the process of vaporization - the specific heat of vaporization and condensation.

Definition.Boiling(Fig. 1) is a process of intense transition of a liquid into a gaseous state, accompanied by the formation of vapor bubbles and occurring throughout the entire volume of the liquid at a certain temperature, which is called the boiling point.

Let's compare the two types of vaporization with each other. The boiling process is more intense than the evaporation process. In addition, as we remember, the evaporation process occurs at any temperature above the melting point, and the boiling process strictly at a certain temperature, which is different for each substance and is called the boiling point. It should also be noted that evaporation occurs only from the free surface of the liquid, i.e., from the area separating it from the surrounding gases, and boiling occurs from the entire volume at once.

Let's take a closer look at the boiling process. Let's imagine a situation that many of us have repeatedly encountered - heating and boiling water in a certain vessel, for example, a saucepan. During heating, a certain amount of heat will be transferred to the water, which will lead to an increase in its internal energy and an increase in the activity of molecular movement. This process will continue until a certain stage, until the energy of molecular motion becomes sufficient to begin boiling.

Water contains dissolved gases (or other impurities) that are released in its structure, which leads to the so-called occurrence of vaporization centers. That is, it is in these centers that steam begins to be released, and bubbles form throughout the entire volume of water, which are observed during boiling. It is important to understand that these bubbles do not contain air, but steam that is formed during the boiling process. After the formation of bubbles, the amount of steam in them increases, and they begin to increase in size. Often, bubbles initially form near the walls of the vessel and do not immediately rise to the surface; first, increasing in size, they are under the influence of the growing force of Archimedes, and then they break away from the wall and rise to the surface, where they burst and release a portion of steam.

It is worth noting that not all steam bubbles immediately reach the free surface of the water. At the beginning of the boiling process, the water is not yet heated evenly and the lower layers, near which the heat transfer process directly occurs, are even hotter than the upper ones, even taking into account the convection process. This leads to the fact that the steam bubbles rising from below collapse due to the phenomenon of surface tension, before reaching the free surface of the water. In this case, the steam that was inside the bubbles passes into the water, thereby further heating it and accelerating the process of uniform heating of the water throughout the entire volume. As a result, when the water warms up almost evenly, almost all the steam bubbles begin to reach the surface of the water and the process of intense steam formation begins.

It is important to highlight the fact that the temperature at which the boiling process takes place remains unchanged even if the intensity of heat supply to the liquid is increased. In simple words, if during the boiling process you add gas on a burner that heats a pan of water, this will only lead to an increase in the intensity of boiling, and not to an increase in the temperature of the liquid. If we delve more seriously into the boiling process, it is worth noting that areas appear in water in which it can be overheated above the boiling point, but the amount of such overheating, as a rule, does not exceed one or a couple of degrees and is insignificant in the total volume of liquid. The boiling point of water at normal pressure is 100°C.

During the process of boiling water, you can notice that it is accompanied by characteristic sounds of the so-called seething. These sounds arise precisely due to the described process of collapse of steam bubbles.

The boiling processes of other liquids proceed in the same way as the boiling of water. The main difference in these processes is the different boiling temperatures of substances, which at normal atmospheric pressure are already measured tabular values. We indicate the main values ​​of these temperatures in the table.

An interesting fact is that the boiling point of liquids depends on the value of atmospheric pressure, which is why we indicated that all the values ​​in the table are given at normal atmospheric pressure. When air pressure increases, the boiling point of the liquid also increases; when it decreases, on the contrary, it decreases.

The principle of operation of such a well-known kitchen appliance as a pressure cooker is based on this dependence of the boiling point on ambient pressure (Fig. 2). It is a pan with a tight-fitting lid, under which, during the process of steaming water, the air pressure with steam reaches up to 2 atmospheric pressure, which leads to an increase in the boiling point of water in it to . Because of this, the water and food in it have the opportunity to heat up to a temperature higher than usual (), and the cooking process is accelerated. Because of this effect, the device got its name.

Rice. 2. Pressure cooker ()

The situation with a decrease in the boiling point of a liquid with a decrease in atmospheric pressure also has an example from life, but no longer everyday for many people. This example applies to the travel of climbers in high mountain regions. It turns out that in areas located at an altitude of 3000-5000 m, the boiling point of water due to a decrease in atmospheric pressure is reduced to lower values, which leads to difficulties when preparing food on hikes, because for effective heat treatment of products in In this case, it takes significantly longer than under normal conditions. At altitudes of about 7000 m, the boiling point of water reaches , which makes it impossible to cook many products in such conditions.

Some technologies for separating substances are based on the fact that the boiling points of different substances are different. For example, if we consider heating oil, which is a complex liquid consisting of many components, then during the boiling process it can be divided into several different substances. In this case, due to the fact that the boiling points of kerosene, gasoline, naphtha and fuel oil are different, they can be separated from each other by vaporization and condensation at different temperatures. This process is usually called fractionation (Fig. 3).

Rice. 3 Separation of oil into fractions ()

Like any physical process, boiling must be characterized using some numerical value, this value is called the specific heat of vaporization.

In order to understand the physical meaning of this value, consider the following example: take 1 kg of water and bring it to the boiling point, then measure how much heat is needed to completely evaporate this water (without taking into account heat losses) - this value will be equal to the specific heat of vaporization of water. For another substance, this heat value will be different and will be the specific heat of vaporization of this substance.

The specific heat of vaporization turns out to be a very important characteristic in modern metal production technologies. It turns out that, for example, during the melting and evaporation of iron with its subsequent condensation and solidification, a crystal lattice is formed with a structure that provides higher strength than the original sample.

Designation: specific heat of vaporization and condensation (sometimes denoted ).

Unit: .

The specific heat of vaporization of substances is determined using laboratory experiments, and its values ​​for basic substances are listed in the appropriate table.

Substance