Specific heat of vaporization of water at different temperatures. What is the specific heat of vaporization and how to determine it

In this lesson, we will pay attention to such a type of vaporization as boiling, discuss its differences from the previously considered evaporation process, introduce such a value as the boiling point, 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: Boil. Specific heat of vaporization and condensation

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

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

Let's compare 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 takes place at any temperature above the melting point, and the boiling process - strictly at a certain temperature, which is different for each of the substances 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 that delimits it from the surrounding gases, and boiling occurs immediately from the entire volume.

Let us consider the course of the boiling process in more detail. Let's imagine a situation that many of us have repeatedly encountered - this is heating and boiling water in a certain vessel, for example, in 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 the movement of molecules. This process will proceed up to a certain stage, until the energy of molecular motion becomes sufficient to start boiling.

Dissolved gases (or other impurities) are present in water, which are released in its structure, which leads to the so-called emergence of centers of vaporization. That is, it is in these centers that steam is released, and bubbles form throughout the entire volume of water, which are observed during boiling. It is important to understand that these bubbles are not air, but steam, which is formed during the boiling process. After the formation of bubbles, the amount of vapor 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, they, increasing in size, are under the influence of the growing force of Archimedes, and then break away from the wall and rise to the surface, where they burst and release a portion of steam.

It should be noted that not all steam bubbles reach the free surface of the water at once. At the beginning of the boiling process, the water is still far from evenly heated, and the lower layers, near which the heat transfer process takes place, 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, not yet reaching the free surface of the water. At the same time, the steam that was inside the bubbles passes into the water, thereby additionally heating it and accelerating the process of uniform heating of the water throughout the volume. As a result, when the water is heated almost evenly, almost all steam bubbles begin to reach the surface of the water and the process of intense vaporization 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, gas is added to the burner, which heats the pot of water, this will only increase the intensity of the boil, and not increase the temperature of the liquid. If we delve more seriously into the boiling process, it is worth noting that there are areas in water in which it can be overheated above the boiling point, but the magnitude of such overheating, as a rule, does not exceed one or a couple of degrees and is insignificant in the total volume of the liquid. The boiling point of water at normal pressure is 100°C.

In the process of boiling water, you can notice that it is accompanied by characteristic sounds of the so-called seething. These sounds arise just because of the described process of collapse of steam bubbles.

The processes of boiling other liquids proceed in the same way as the boiling of water. The main difference in these processes is the different boiling points of substances, which at normal atmospheric pressure are already measured tabular values. Let us 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 values in the table are given at normal atmospheric pressure. When the air pressure increases, the boiling point of the liquid also increases, and when it decreases, on the contrary, it decreases.

On this dependence of boiling point on pressure environment based on the principle of operation of such a well-known kitchen appliance like a pressure cooker (fig. 2). It is a pan with a tight-fitting lid, under which, in the process of water vaporization, 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 with the food in it has 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 the highlands. It turns out that in an area located at an altitude of 3000-5000 m, the boiling point of water, due to a decrease in atmospheric pressure, decreases to even lower values, which leads to difficulties in cooking on hikes, because for effective heat treatment products in this case, it takes a much longer time 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 that boiling point various substances differ, some technologies for the separation of substances are based. For example, if we consider the heating of oil, which is a complex liquid consisting of many components, then in the process of boiling 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 referred to as fractionation (Fig. 3).

Rice. 3 Separation of oil into fractions ()

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

In order to understand 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 (excluding heat losses) - this value will be equal to the specific heat of vaporization of water. For another substance, this value of heat 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 technologies metal production. It turns out that, for example, during the melting and evaporation of iron, followed by its condensation and solidification, crystal cell with a structure that provides higher strength than the original sample.

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

unit of measurement: .

The specific heat of vaporization of substances is determined by experiments in laboratory conditions, and its values for the main substances are listed in the appropriate table.

Substance

Boiling is an intense vaporization that occurs when a liquid is heated not only from the surface, but also inside it.

Boiling occurs with the absorption of heat.
Most of the heat supplied is spent on breaking the bonds between the particles of the substance, the rest - on the work done during the expansion of the steam.
As a result, the interaction energy between vapor particles becomes greater than between liquid particles, so the internal energy of the vapor is greater than the internal energy of the liquid at the same temperature.
The amount of heat required to transfer liquid to vapor during the boiling process can be calculated using the formula:

where m is the mass of liquid (kg),
L is the specific heat of vaporization.

The specific heat of vaporization shows how much heat is needed to turn 1 kg of a given substance into steam at the boiling point. Unit specific heat vaporization in the SI system:
[ L ] = 1 J/kg
As the pressure increases, the boiling point of the liquid rises, and the specific heat of vaporization decreases, and vice versa.

During boiling, the temperature of the liquid does not change.
The boiling point depends on the pressure exerted on the liquid.
Each substance at the same pressure has its own boiling point.
With an increase in atmospheric pressure, boiling begins at a higher temperature, with a decrease in pressure - vice versa.
For example, water boils at 100°C only at normal atmospheric pressure.

WHAT HAPPENS INSIDE THE LIQUID WHEN BOILING?

Boiling is the transition of a liquid into vapor with the continuous formation and growth of vapor bubbles in the liquid, inside which the liquid evaporates. At the beginning of heating, the water is saturated with air and has room temperature. When water is heated, the gas dissolved in it is released at the bottom and walls of the vessel, forming air bubbles. They begin to appear long before boiling. Water evaporates into these bubbles. A bubble filled with steam begins to inflate at a sufficiently high temperature.

Having reached a certain size, it breaks away from the bottom, rises to the surface of the water and bursts. In this case, the vapor leaves the liquid. If the water is not heated enough, then the steam bubble, rising into the cold layers, collapses. The resulting water fluctuations lead to the appearance of a huge number of small air bubbles in the entire volume of water: the so-called "white key".

A lift force acts on an air bubble at the bottom of the vessel:
Fpod \u003d Farchimede - Fgravity
The bubble is pressed to the bottom, since pressure forces do not act on the lower surface. When heated, the bubble expands due to the release of gas into it and breaks away from the bottom when the lifting force is slightly greater than the pressing one. The size of a bubble that can break away from the bottom depends on its shape. The shape of the bubbles at the bottom is determined by the wettability of the vessel bottom.

Wetting inhomogeneity and merging of bubbles at the bottom led to an increase in their size. At large sizes When a bubble rises behind it, voids, gaps and eddies are formed.

When the bubble bursts, all the liquid surrounding it rushes inward, and an annular wave occurs. Closing, she throws up a column of water.

When bursting bubbles collapse in a liquid, shock waves of ultrasonic frequencies propagate, accompanied by audible noise. The initial stages of boiling are characterized by the loudest and high sounds(on the stage " white key"teapot" sings ").

(source: virlib.eunnet.net)

TEMPERATURE GRAPH OF CHANGES IN AGGREGATE STATES OF WATER

LOOK AT THE BOOKSHELF!

INTERESTING

Why is there a hole in the lid of the teapot?
To release steam. Without a hole in the lid, steam can slosh water over the kettle's spout.
___

The duration of cooking potatoes, starting from the moment of boiling, does not depend on the power of the heater. The duration is determined by the residence time of the product at the boiling point.
The power of the heater does not affect the boiling point, but only the rate of water evaporation.

Boiling can make water freeze. To do this, it is necessary to pump out air and water vapor from the vessel where the water is located, so that the water boils all the time.

"Pots easily boil over the edge - to bad weather!"
The drop in atmospheric pressure that accompanies worsening weather is the reason why milk "runs away" faster.
___

Very hot boiling water can be obtained at the bottom of deep mines, where the air pressure is much greater than on the surface of the Earth. So at a depth of 300 m, water will boil at 101 ͦ C. With an air pressure of 14 atmospheres, water boils at 200 ͦ C.
Under the bell of the air pump, you can get "boiling water" at 20 ͦ C.
On Mars, we would drink "boiling water" at 45 C.
Salt water boils above 100 ͦ C. ___

In mountainous regions at a considerable height, under reduced atmospheric pressure, water boils at temperatures lower than 100 ͦ Celsius.

Waiting for such a meal to be cooked takes longer.

Pour it cold ... and it will boil!

Normally, water boils at 100 degrees Celsius. Heat the water in the flask on the burner to a boil. Let's turn off the burner. The water stops boiling. We close the flask with a stopper and begin to carefully pour cold water onto the stopper. What is it? The water is boiling again!

..............................

under the jet cold water some water in the flask, and with it the water vapor begins to cool.
The vapor volume decreases and the pressure above the water surface changes...
What do you think, in which direction?
... The boiling point of water at reduced pressure is less than 100 degrees, and the water in the flask boils again!
____

When cooking, the pressure inside the pot - "pressure cooker" - is about 200 kPa, and the soup in such a pot will cook much faster.

You can draw water into the syringe up to about half, close it with the same cork and pull the piston sharply. A lot of bubbles will appear in the water, indicating that the process of boiling water has begun (and this is at room temperature!).
___

When a substance passes into a gaseous state, its density decreases by about 1000 times.
___

The first electric kettles had heaters under the bottom. The water did not come into contact with the heater and boiled for a very long time. In 1923, Arthur Large made a discovery: he placed a heater in a special copper tube and placed it inside the teapot. The water boiled quickly.

Self-cooling cans for soft drinks have been developed in the USA. A compartment with a low-boiling liquid is mounted in the jar. If you crush the capsule on a hot day, the liquid will begin to boil rapidly, taking away heat from the contents of the jar, and in 90 seconds the temperature of the drink drops by 20-25 degrees Celsius.

WHY?

Do you think it is possible to hard boil an egg if the water boils at a temperature lower than 100 degrees Celsius?
____

Will water boil in a pot that is floating in another pot of boiling water?
Why? ___

Can you make water boil without heating it?

This knowledge quickly disappears, and gradually people stop paying attention to the essence of familiar phenomena. Sometimes it is useful to recall theoretical knowledge.

Definition

What is a boil? This is a physical process during which intense vaporization occurs both on the free surface of the liquid and inside its structure. One of the signs of boiling is the formation of bubbles, which consist of saturated steam and air.

It is worth noting the existence of such a thing as boiling point. The rate of steam formation also depends on the pressure. It must be permanent. As a rule, the main characteristic of liquid chemical substances is the boiling point at normal atmospheric pressure. However, this process can also be influenced by factors such as the intensity sound waves, air ionization.

Boiling stages of water

Steam will certainly begin to form during a procedure such as heating. Boiling involves the passage of a liquid through 4 stages:

At the bottom of the vessel, as well as on its walls, small bubbles begin to form. This is the result of the fact that air is contained in the cracks of the material from which the container is made, which expands under the influence of high temperature.
The bubbles begin to increase in volume, as a result of which they break out to the surface of the water. If a upper layer the liquid has not yet reached the boiling point, the cavities sink to the bottom, after which they begin to rise again. This process leads to the formation of sound waves. That is why we can hear noise when water boils.
Floats to the surface the largest number bubbles, which gives the impression After that, the liquid turns pale. Considering visual effect, this stage of boiling is called "white key".
There is intense seething, which is accompanied by the formation of large bubbles that quickly burst. This process is accompanied by the appearance of splashes, as well as the intense formation of steam.

Specific heat of vaporization

Almost every day we are faced with such a phenomenon as boiling. The specific heat of vaporization is a physical quantity that determines the amount of heat. With her help liquid substance can be converted to par. In order to calculate this parameter, you need to divide the heat of evaporation by the mass.

How is the measurement

The specific indicator is measured in the laboratory by conducting appropriate experiments. They include the following:

measured out required amount liquid, which is then poured into the calorimeter;
an initial measurement of the water temperature is carried out;
a flask with the test substance placed in it in advance is installed on the burner;
the vapor emitted by the test substance is launched into the calorimeter;
the water temperature is re-measured;
the calorimeter is weighed, which makes it possible to calculate the mass of the condensed vapor.

bubble boiling mode

Dealing with the question of what boiling is, it is worth noting that it has several modes. So, when heated, steam can form in the form of bubbles. They periodically grow and burst. This mode of boiling is called bubbly. Usually, cavities filled with steam are formed precisely at the walls of the vessel. This is due to the fact that they are usually overheated. This is necessary condition for boiling, because otherwise the bubbles will collapse, not reaching large sizes.

Film boiling mode

What is a boil? The easiest way to explain this process is as vaporization at a certain temperature and constant pressure. In addition to the bubble mode, a film mode is also distinguished. Its essence lies in the fact that when strengthening heat flow individual bubbles combine to form a vapor layer on the walls of the vessel. When a critical indicator is reached, they break through to the surface of the water. This mode of boiling differs in that the degree of heat transfer from the walls of the vessel to the liquid itself is significantly reduced. The reason for this is the same steam film.

Boiling temperature

It should be noted that there is a dependence of the boiling point on the pressure that is exerted on the surface of the heated liquid. So, it is generally accepted that water boils when heated to 100 degrees Celsius. Nevertheless, this indicator can be considered fair only if the atmospheric pressure indicator is considered normal (101 kPa). If it increases, the boiling point will also change upwards. So, for example, in popular pressure cookers, the pressure is about 200 kPa. Thus, the boiling point rises by 20 points (up to 20 degrees).

Mountainous areas can be considered an example of low atmospheric pressure. So, given that it is quite small there, water begins to boil at a temperature of about 90 degrees. Residents of such areas have to spend much more time preparing food. So, for example, to boil an egg, you will have to heat the water by at least 100 degrees, otherwise the protein will not curdle.

The boiling point of a substance depends on the saturation vapor pressure. Its effect on temperature is inversely proportional. For example, mercury boils when heated to 357 degrees Celsius. This can be explained by the fact that the saturated vapor pressure is only 114 Pa (for water, this figure is 101,325 Pa).

Boiling under different conditions

Depending on the conditions and condition of the liquid, the boiling point can vary significantly. For example, it is worth adding salt to the liquid. Chlorine and sodium ions are located between water molecules. Thus, boiling requires an order of magnitude more energy, and, accordingly, time. In addition, such water produces much less steam.

The kettle is used to boil water in living conditions. If a clean liquid is used, then the temperature of this process is the standard 100 degrees. Under similar conditions, distilled water boils. However, it will take a little less time if you take into account the absence of foreign impurities.

What is the difference between boiling and evaporation

Whenever water boils, steam is released into the atmosphere. But these two processes cannot be identified. They are only ways of vaporization, which occurs under certain conditions. So, boiling is the first kind. This process is more intense than due to the formation of steam pockets. It is also worth noting that the evaporation process occurs exclusively on the surface of the water. Boiling applies to the entire volume of the liquid.

What does evaporation depend on?

Evaporation is the process of converting a liquid or solid into a gaseous state. A "flight" of atoms and molecules occurs, the connection of which with the rest of the particles is weakened under the influence of certain conditions. Evaporation rate may vary under the influence of the following factors:

liquid surface area;
the temperature of the substance itself, as well as the environment;
the speed of movement of molecules;
type of substance.

The energy of boiling water is widely used by man in everyday life. This process has become so commonplace and familiar that no one thinks about its nature and features. Nevertheless, a number of interesting facts are associated with boiling:

Probably, everyone noticed that there is a hole in the lid of the teapot, but few people think about its purpose. It is done in order to partially release steam. Otherwise, water may splash out through the spout.
The cooking time for potatoes, eggs and other foodstuffs does not depend on how powerful the heater is. The only thing that matters is how long they were under the influence of boiling water.
The power of the heating device does not affect such an indicator as the boiling point. It can only affect the rate of evaporation of the liquid.
Boiling is not only about heating water. This process can also cause the liquid to freeze. So, in the process of boiling, it is necessary to continuously pump air out of the vessel.
One of the most actual problems for housewives is that milk can "run away". Thus, the risk of this phenomenon increases significantly during weather deterioration, which is accompanied by a drop in atmospheric pressure.
The hottest boiling water is obtained in deep underground mines.
way experimental studies Scientists were able to establish that water on Mars boils at a temperature of 45 degrees Celsius.

Can water boil at room temperature?

By simple calculations, scientists were able to establish that water can boil at the level of the stratosphere. Similar conditions can be recreated using vacuum pump. Nevertheless, a similar experiment can be carried out in simpler, more mundane conditions.

Boil 200 ml of water in a liter flask, and when the container is filled with steam, close it tightly and remove from heat. Having placed it over the crystallizer, you need to wait for the end of the boiling process. Next, the flask is poured cold water. After that, intensive boiling will begin again in the container. This is due to the fact that under the influence of low temperature, the vapor in the upper part of the flask descends.

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

Why does the temperature of the water inside not rise above one hundred degrees when a saucepan or kettle is constantly heated with burning gas? 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 a hundred degrees, evaporation occurs mainly from the surface, and when it reaches this temperature, the water boils. Boiling is also evaporation, but only over the entire volume of the liquid. Hot steam bubbles are formed inside the water and, being lighter than water, these bubbles break out to the surface, and the steam from them escapes into the air.

Up to a hundred degrees, the temperature of the water rises when heated. After a 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 rise, 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 if such a phenomenon exists, then 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 indicates the amount of heat required to turn a 1 kg liquid into vapor at the boiling point. The specific heat of vaporization is denoted by the letter L. And the unit of measurement is the 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 - body weight.

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

Empirically, the values of the specific heat of vaporization of various substances were found and tables were compiled from which data can be found for each substance. Thus, the specific heat of vaporization of water is 2.3*106 J/kg. This means that for every kilogram of water, an amount of energy equal to 2.3 * 106 J must be spent to turn it into steam. But at the same time, the water should 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 the transformation of a certain mass of a liquid into vapor, therefore, more often one has to deal with a formula of the form: Q \u003d Lm, and the values \u200b\u200bof the specific heat of vaporization for a particular substance are taken from ready-made tables.

Topic: Aggregate states of matter

Lesson: Boil. Specific heat of vaporization and condensation

Let us consider the course of the boiling process in more detail. Let's imagine a situation that many of us have repeatedly encountered - this is heating and boiling water in a certain vessel, for example, in a saucepan. During heating, a certain amount of heat will be transferred to water, which will lead to an increase in its internal energy and an increase in the activity of molecular movement. This process will proceed up to a certain stage, until the energy of molecular motion becomes sufficient to start boiling.

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 terms, if you add gas to the burner during the boiling process, which heats the pot of water, this will only increase the intensity of the boil, and not increase the temperature of the liquid. If we delve more seriously into the boiling process, it is worth noting that there are areas in water in which it can be overheated above the boiling point, but the magnitude of such overheating, as a rule, does not exceed one or a couple of degrees and is insignificant in the total volume of the liquid. The boiling point of water at normal pressure is 100°C.

This dependence of the boiling point on the ambient pressure is the basis for the principle of operation of such a well-known kitchen appliance as a pressure cooker (Fig. 2). It is a pan with a tight-fitting lid, under which, in the process of water vaporization, 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 with the food in it has 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 the highlands. It turns out that in an area located at an altitude of 3000-5000 m, the boiling point of water, due to a decrease in atmospheric pressure, decreases to even lower values, which leads to difficulties in cooking on hikes, because for effective thermal processing of food in In this case, much longer time is required 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 the separation of substances are based on the fact that the boiling points of various substances are different. For example, if we consider the heating of oil, which is a complex liquid consisting of many components, then in the process of boiling 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 referred to as fractionation (Fig. 3).

Rice. 3 Separation of oil into fractions ()

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

In order to understand the physical meaning of this quantity, 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 (excluding heat losses) - this value will be equal to the specific heat of vaporization of water. For another substance, this value of heat 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 technologies for the production of metals. It turns out that, for example, during the melting and evaporation of iron, followed by its 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 of measurement: .

The specific heat of vaporization of substances is determined by experiments in laboratory conditions, and its values for the main substances are listed in the appropriate table.

Substance