Boiling, as we have seen, is also evaporation, only it is accompanied by the rapid formation and growth of steam bubbles. Obviously, during boiling it is necessary to supply a certain amount of heat to the liquid. This amount of heat is used to form steam. Moreover, different liquids of the same mass require different amounts of heat to turn them into vapor at the boiling point.
Experiments have established that evaporation of water weighing 1 kg at a temperature of 100 °C requires 2.3 10 6 J of energy. To evaporate 1 kg of ether taken at a temperature of 35 °C, 0.4 10 6 J of energy is required.
Therefore, in order for the temperature of the evaporating liquid not to change, a certain amount of heat must be supplied to the liquid.
A physical quantity showing how much heat is needed to convert a liquid weighing 1 kg into vapor without changing 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 equal to 2.3 10 6 J/kg. In other words, to convert 1 kg of water into steam at a temperature of 100 °C, 2.3 10 6 J of energy is required. Consequently, 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 boiling point and normal atmospheric pressure)
In contact with a cold object, water vapor condenses (Fig. 25). This releases the energy absorbed during the formation of steam. Precise experiments show that, when condensing, steam releases the amount of energy that went into its formation.
Rice. 25. Steam condensation
Consequently, when 1 kg of water vapor at a temperature of 100 °C is converted into water of the same temperature, 2.3 10 6 J of energy are released. As can be seen from comparison with other substances (Table 6), this energy is quite high.
The energy released during steam condensation can be used. At large thermal power plants, steam exhausted from turbines is used to heat 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 transform a liquid of any mass taken at the boiling point into vapor, the specific heat of vaporization L must be multiplied 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. What amount of energy is required to convert 2 kg of water, taken at a temperature of 20 °C, into steam? Let's write down the conditions of the problem and solve it.
Questions
- What is the energy supplied to the liquid during boiling spent on?
- What does the specific heat of vaporization show?
- How can you show experimentally that energy is released when steam condenses?
- What is the energy released by 1 kg of water vapor during condensation?
- Where in technology is the energy released during the condensation of water vapor used?
Exercise 16
- How should we understand that the specific heat of vaporization of water is 2.3 10 6 J/kg?
- How should we understand that the specific heat of condensation of ammonia is 1.4 10 6 J/kg?
- Which of the 6 substances given in table, when converted from liquid to vapor, has internal energy that increases the most? Justify your answer.
- What amount of energy is required to convert 150 g of water into steam at a temperature of 100 °C?
- How much energy must be expended to bring 5 kg of water, taken at a temperature of 0 °C, to a boil and evaporate it?
- What amount of energy will be released by water weighing 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
- Using Table 6, determine which of the substances has a greater increase in internal energy when turning from liquid to vapor. Justify your answer.
- Prepare a report on one of the topics (optional).
- How dew, frost, rain and snow are formed.
- The water cycle in nature.
- Metal casting.
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 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 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 physical quantity describing 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 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.
Everyone knows that water in a kettle boils at a temperature of 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 vapor. Thus, for water to transform into a gaseous state, a constant supply of heat is required. How much of 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.
Boiling requires energy. Most of it is used to break chemical bonds between atoms and molecules, resulting in the formation of steam bubbles, and a smaller part is used to expand the steam, that is, so that the resulting bubbles can burst and release it. Since the liquid puts all its energy into transitioning into a gaseous state, its “forces” run out. To constantly renew energy and prolong boiling, more and more heat must be supplied to the container with liquid. A boiler, gas burner or any other heating device can provide its supply. During boiling, the temperature of the liquid does not increase; steam is formed at the same temperature.
Different liquids require different amounts of heat to change into vapor. Which one is shown by 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 required to evaporate all the liquid, and obtain the value of the specific heat of vaporization for water. For other chemical compounds this figure 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 spent on evaporation by the mass of the liquid:
This value is very important for production processes based on modern technologies. For example, they focus on it in the production of metals. It turned out that if iron is melted and then condensed, upon further hardening a stronger crystal lattice is formed.
What is it equal to
The specific heat value for various substances (r) was determined during laboratory studies. Water at normal atmospheric pressure boils at 100 °C, and the heat of evaporation of water is 2258.2 kJ/kg. This indicator for some other substances is given in the table:
| Substance | boiling point, °C | r, kJ/kg |
|---|---|---|
| Nitrogen | -196 | 198 |
| Helium | -268,94 | 20,6 |
| Hydrogen | -253 | 454 |
| Oxygen | -183 | 213 |
| Carbon | 4350 | 50000 |
| Phosphorus | 280 | 400 |
| Methane | -162 | 510 |
| Pentane | 36 | 360 |
| Iron | 2735 | 6340 |
| Copper | 2590 | 4790 |
| Tin | 2430 | 2450 |
| Lead | 1750 | 8600 |
| Zinc | 907 | 1755 |
| Mercury | 357 | 285 |
| Gold | 2 700 | 1 650 |
| Ethanol | 78 | 840 |
| Methyl alcohol | 65 | 1100 |
| Chloroform | 61 | 279 |
However, this indicator may change under the influence of certain factors:
- Temperature. As it increases, the heat of evaporation decreases and can be equal to 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 - Pressure. As pressure decreases, the heat of vaporization increases, and vice versa. The boiling point is directly proportional to pressure and can reach a critical value of 374 °C.
p, Pa t boil., °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 - - Mass of the substance. The amount of heat involved in the process is directly proportional to the mass of steam formed.
Relationship between evaporation and condensation
Physicists have found that the opposite process to evaporation - condensation - steam spends exactly the same amount of energy as was used to form it. 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 result in a storage of energy that could be used for other purposes. In essence, a perpetual motion machine would be created. But this contradicts physical laws, which means it is impossible.
How is it measured?
- The specific heat of evaporation of water is measured experimentally in physical laboratories. For this purpose calorimeters are used. The procedure looks like this:
- A certain amount of liquid is poured into the calorimeter.
Boiling is 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 supplied heat is spent on breaking the bonds between particles of the substance, the rest - on the work done during the expansion of steam.
As a result, the interaction energy between vapor particles becomes greater than between liquid particles, so the internal energy of vapor is greater than the internal energy of liquid at the same temperature.
The amount of heat required to convert liquid into steam during the boiling process can be calculated using the formula:
where m is the mass of the liquid (kg),
L is the specific heat of vaporization.
The specific heat of vaporization shows how much heat is needed to convert 1 kg of a given substance into steam at the boiling point. Unit of specific heat of vaporization in the SI system:
[L] = 1 J/kg
With increasing pressure, the boiling point of the liquid increases, 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, and with a decrease in pressure, vice versa.
For example, water boils at 100 °C only at normal atmospheric pressure.
WHAT HAPPENS INSIDE A LIQUID WHEN BOILING?
Boiling is the transition of a liquid into vapor with the continuous formation and growth of vapor bubbles in the liquid, into which the liquid evaporates. At the beginning of heating, the water is saturated with air and is at room temperature. When water is heated, 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 swell 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, steam leaves the liquid. If the water is not warmed up enough, the steam bubble, rising into the cold layers, collapses. The resulting fluctuations in water lead to the appearance of a huge number of small air bubbles throughout the entire volume of water: the so-called “white key”.
An air bubble with a volume at the bottom of the vessel is acted upon by a lifting force:
Funder = Farchimedes - Fgravity
The bubble is pressed to the bottom because no pressure forces 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 force. The size of the 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 bottom of the vessel.
Inhomogeneity of wetting and merging of bubbles at the bottom led to an increase in their size. With large bubble sizes, when rising behind it, voids, breaks and turbulences are formed.
When a bubble bursts, all the liquid surrounding it rushes in, creating a ring wave. Closing, it throws up a column of water.
When bursting bubbles collapse, shock waves of ultrasonic frequencies propagate in the liquid, accompanied by an 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 SCHEDULE OF CHANGES IN THE STATES OF WATER
LOOK AT THE BOOKSHELF!
INTERESTING
Why do they make a hole in the lid of the teapot?
To release steam. Without a hole in the lid, steam can splash water out of the kettle 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 time the product remains at boiling point.
The power of the heater does not affect the boiling point, but only affects the rate of evaporation of water.
Boiling can cause water to 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 - bad weather!”
The drop in atmospheric pressure that accompanies worsening weather is the reason that 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. At 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 at temperatures above 100 ͦ C. ___
In mountainous regions at significant altitudes and at low atmospheric pressure, water boils at temperatures lower than 100 ͦ Celsius.
It takes longer to wait for such a meal to be cooked.
Pour in some cold water... and it will boil!
Typically water boils at 100 degrees Celsius. Heat the water in the flask on a burner until it boils. Let's turn off the burner. The water stops boiling. Close the flask with a stopper and begin to carefully pour cold water onto the stopper in a stream. What's it like? The water is boiling again!
..............................Under a stream of cold water, the water in the flask, and with it the water vapor, begins to cool.
The volume of vapor decreases and the pressure above the water surface changes...
Which direction do you think?
... 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 pan - "pressure cooker" - is about 200 kPa, and the soup in such a pan will cook much faster.
You can fill the syringe with water up to about half, close it with the same stopper and sharply pull the plunger. A mass 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 took a very long time to boil. In 1923, Arthur Large made a discovery: he placed a heater in a special copper tube and placed it inside a kettle. The water was boiling quickly.
Self-cooling cans for soft drinks have been developed in the USA. The jar has a compartment with a low-boiling liquid built into it. If you crush the capsule on a hot day, the liquid will begin to boil rapidly, taking heat away from the contents of the jar, and in 90 seconds the temperature of the drink drops by 20-25 degrees Celsius.
WELL, WHY SO?
What do you think, is it 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? ___
Is it possible to make water boil without heating it?
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 influenced by 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.
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