Specific heat of vaporization of ethyl alcohol. Boiling

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 water reaches a temperature of one hundred degrees, all the 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 the physical quantity that describes this phenomenon. 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.

In order to maintain the boiling of water (or other liquid), it is necessary to continuously supply heat to it, for example, to heat it with a burner. In this case, the temperature of the water and the vessel does not rise, but a certain amount of steam is formed for each unit of time. From this follows the conclusion that the transformation of water into vapor requires an influx of heat, just as it takes place during the transformation of a crystal (ice) into a liquid (§ 269). The amount of heat required to convert a unit mass of a 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 easy to see that the same amount of heat must be released when a vapor condenses into a liquid. Indeed, let us lower a tube connected to a boiler into a glass of water (Fig. 488). Some time after the start of heating, air bubbles will begin to come out of the end of the tube dipped into the water. This air slightly raises the temperature of the water. Then the water in the boiler boils, 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. Since the specific heat capacity of steam is approximately the same as that of air, it follows from this observation that such a rapid heating of water occurs precisely due to the condensation of steam.

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 starts to condense in the cup, the thermometer will quickly rise, indicating an increase in temperature.

When a unit mass of vapor condenses into a liquid of the same temperature, an amount of heat is released equal to the specific heat of vaporization. This could be foreseen on the basis of the law of conservation of energy. Indeed, if this were not the case, 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 the increment 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). Pour a certain amount of water into the calorimeter and measure its temperature. Then, for some time, we will introduce vapor of the test liquid from the boiler into the water, taking measures to ensure that only steam flows, without droplets of liquid. To do this, steam is passed through a steamer (Fig. 489). After that, we again measure the water temperature in the calorimeter. By weighing the calorimeter, we can judge by the increase in its mass the amount of vapor condensed into a liquid.

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

Using the law of conservation of energy, it is possible to compose a heat balance equation for this process, which makes it possible 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 temperatures 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 the processes of vaporization occur in nature on a grandiose scale.

Table 20. Specific heat of vaporization of some liquids

Substance		Substance
Ethanol)

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

295.1. Calculate the amount of heat required to heat 20 g of water to the boiling point and turn 20 g of water 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 ? Ignore the heat capacity of the glass.

The process of changing a substance from a liquid state 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. This process can be observed using a simple experiment by heating water in a glass flask. When water is heated, bubbles appear in it after a while, which contain air and saturated water vapor, which is formed during the evaporation of water inside the bubbles. When the temperature rises, the pressure inside the bubbles increases, and under the action of the buoyancy force, they rise up. However, since the temperature of the upper layers of water is lower than the lower ones, the vapor in the bubbles begins to condense and they shrink. When the water warms up throughout the volume, the bubbles with steam rise to the surface, burst, and the steam comes out. Water is boiling. This occurs at a temperature at which the saturation vapor pressure in the bubbles is equal to atmospheric pressure.

The process of vaporization occurring in the entire volume of a 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 rises, the boiling point rises.

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 to 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 fluid is expended to do the work of overcoming the forces of attraction. This happens until all the liquid turns into vapor. Since liquid and vapor have the same temperature during the boiling process, the average kinetic energy of the molecules does not change, only their potential energy increases.

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

Specific heat of vaporization

For the transformation of different substances from a liquid state into 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 with a mass of 1 kg to transform it from a liquid state into a gaseous state at the boiling point.

The unit of specific heat of vaporization is [ L] = J/kg.

To calculate the amount of heat Q, which must be imparted to a substance with a mass mn for its transformation from a liquid state to a gaseous one, it is necessary to have the specific heat of vaporization ( L) times the mass of the substance: Q = Lm.

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

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 a process of 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 molecular movement. 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 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.

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.

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 values ​​up to 2 atmospheric pressures, which leads to an increase in the boiling point of water in it up 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

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. The unit of specific heat of 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. When the bubble is large, as it rises behind it, voids, ruptures, 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 highest sounds (at the "white key" stage, the kettle "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 a stream of cold water, the water in the flask, and with it the water vapor, begin 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 kettle. 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?