Vaporization of water. Boiling

We all know that water in a kettle boils at 100°C. But have you noticed that the temperature of water does not change during the boiling process? The question is - where does the generated energy go if we constantly keep the container on fire? It goes into converting liquid into steam. Thus, in order to transfer water to gaseous state a constant supply of heat is required. How much it is needed to convert a kilogram of liquid into steam of the same temperature is determined by a physical quantity called the specific heat of vaporization of water.

The physical meaning of the quantity

Boiling requires energy. Most of it is used to break the chemical bonds between atoms and molecules, resulting in the formation of vapor bubbles, and the smaller part is used to expand the vapor, that is, so that the formed bubbles can burst and release it. Since the liquid puts all its energy into the transition to the gaseous state, its "forces" run out. For constant renewal of energy and prolongation of boiling, more and more heat must be brought to the container with liquid. A boiler, gas burner or any other heating device can provide its inflow. During boiling, the temperature of the liquid does not increase, the process of formation of steam of the same temperature takes place.

Different liquids require different amounts of heat to turn into vapor. Which one - shows the specific heat of vaporization.

You can understand how this value is determined from an example. Take 1 liter of water and bring it to a boil. Then we measure the amount of heat needed to evaporate all the liquid, and we get the value of the specific heat of vaporization for water. For other chemical compounds, this indicator will be different.

In physics, the specific heat of vaporization is denoted by the Latin letter L. It is measured in joules per kilogram (J / kg). It can be derived by dividing the heat expended on evaporation by the mass of the liquid:

This value is very important for production processes based on modern technologies. For example, they are guided by it in the production of metals. It turned out that if iron is melted and then condensed, with further hardening, a stronger crystal lattice is formed.

What is equal to

The value of specific heat for various substances (r) was determined in the course of laboratory studies. Water at normal atmospheric pressure boils at 100 °C, and the heat of vaporization of water is 2258.2 kJ/kg. This indicator for some other substances is given in the table:

However, this indicator can change under the influence of certain factors:

1. Temperature. As it increases, the heat of vaporization decreases and can be zero.
   

   t, °C	r, kJ/kg
   	2500
   10	2477
   20	2453
   50	2380
   80	2308
   100	2258
   200	1940
   300	1405
   374	115
   374,15

2. Pressure. As the pressure decreases, the heat of vaporization increases, and vice versa. The boiling point is directly proportional to pressure and can reach critical 374°C.

   p, Pa	bp, °C	r, kJ/kg
   0,0123	10	2477
   0,1234	50	2380
   1	100	2258
   2	120	2202
   5	152	2014
   10	180	1889
   20	112	1638
   50	264	1638
   100	311	1316
   200	366	585
   220	373,7	184,8
   Critical 221.29	374,15	-

3. The mass of the substance. The amount of heat involved in the process is directly proportional to the mass of the resulting steam.

The ratio of evaporation and condensation

Physicists have found that the reverse evaporation process - condensation - steam spends exactly the same amount of energy as was spent on its formation. This observation confirms the law of conservation of energy.

Otherwise, it would be possible to create an installation in which the liquid would evaporate and then condense. The difference between the heat required for evaporation and the heat sufficient for condensation would lead to the accumulation of energy that could be used for other purposes. In fact, a perpetual motion machine would be created. But this is contrary to physical laws, and therefore impossible.

How is it measured

1. Specific heat The evaporation of water is measured experimentally in physical laboratories. For this, calorimeters are used. The procedure is as follows:
2. A certain amount of liquid is poured into the calorimeter.

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 introduce very important value describing 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, that is, 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 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, 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 the principle of operation of such a well-known kitchen appliance as a pressure cooker is based (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 various 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, 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.

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. 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 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?

Boiling, as we have seen, is also evaporation, only it is accompanied by the rapid formation and growth of vapor bubbles. It is obvious that during boiling it is necessary to bring a certain amount of heat to the liquid. This amount of heat goes to the formation of steam. Moreover, different liquids of the same mass require different amounts of heat to turn them into steam at the boiling point.

Experiments have shown that the evaporation of water weighing 1 kg at a temperature of 100 °C requires 2.3 x 10 6 J of energy. For the evaporation of 1 kg of ether taken at a temperature of 35 °C, 0.4 10 6 J of energy is needed.

Therefore, in order for the temperature of the evaporating liquid not to change, a certain amount of heat must be supplied to the liquid.

The physical quantity showing how much heat is needed to turn a liquid of mass 1 kg into vapor without changing the temperature is called the specific heat of vaporization.

The specific heat of vaporization is denoted by the letter L. Its unit is 1 J / kg.

Experiments have established that the specific heat of vaporization of water at 100 °C is 2.3 10 6 J/kg. In other words, it takes 2.3 x 10 6 J of energy to convert 1 kg of water into steam at a temperature of 100 °C. Therefore, at the boiling point, the internal energy of a substance in the vapor state is greater than the internal energy of the same mass of substance in the liquid state.

Table 6
Specific heat of vaporization of certain substances (at the boiling point and normal atmospheric pressure)

In contact with a cold object, water vapor condenses (Fig. 25). In this case, the energy absorbed during the formation of steam is released. Precise experiments show that, when condensed, steam gives off the amount of energy that went into its formation.

Rice. 25. Steam condensation

Consequently, when 1 kg of water vapor is converted at a temperature of 100 °C into water of the same temperature, 2.3 x 10 6 J of energy is released. As can be seen from a comparison with other substances (Table 6), this energy is quite high.

The energy released during the condensation of steam can be used. At large thermal power plants, the steam used in the turbines heats water.

The water heated in this way is used for heating buildings, in baths, laundries and for other domestic needs.

To calculate the amount of heat Q required to convert a liquid of any mass, taken at the boiling point, into vapor, you need to multiply the specific heat of vaporization L by the mass m:

From this formula, it can be determined that

m=Q/L, L=Q/m

The amount of heat released by steam of mass m, condensing at the boiling point, is determined by the same formula.

Example. How much energy is required to turn 2 kg of water at 20°C into steam? Let's write down the condition of the problem and solve it.

Questions

1. What is the energy supplied to the liquid during boiling?
2. What is the specific heat of vaporization?
3. How can one show experimentally that energy is released when steam condenses?
4. What is the energy released by 1 kg water vapor during condensation?
5. Where in technology is the energy released during the condensation of water vapor used?

Exercise 16

1. How should one understand that the specific heat of vaporization of water is 2.3 10 6 J/kg?
2. How should one understand that the specific heat of condensation of ammonia is 1.4 10 6 J/kg?
3. Which of the substances listed in Table 6, when converted from a liquid state to a vapor, has an increase in internal energy more? Justify the answer.
4. How much energy is required to turn 150 g of water into steam at 100°C?
5. How much energy must be expended in order to bring water of mass 5 kg, taken at a temperature of 0 ° C, to a boil and evaporate it?
6. What amount of energy will be released by water of mass 2 kg when cooled from 100 to 0 °C? What amount of energy will be released if instead of water we take the same amount of steam at 100 °C?

Exercise

1. According to table 6, determine which of the substances, when converted from a liquid state to a vapor, the internal energy increases more strongly. Justify the answer.
2. Prepare a report on one of the topics (optional).
3. How dew, frost, rain and snow are formed.
4. The water cycle in nature.
5. Metal casting.

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.