Evaporation and condensation graph. Phase transitions. Melting and crystallization
Themes USE codifier : the change aggregate states substances, melting and crystallization, evaporation and condensation, liquid boiling, energy change in phase transitions.
Ice, water and water vapor are examples of three aggregate states substances: solid, liquid and gaseous. What kind of aggregate state a given substance is in depends on its temperature and other external conditions in which it is located.
When external conditions change (for example, if the internal energy of the body increases or decreases as a result of heating or cooling), phase transitions can occur - changes in the aggregate states of the body's substance. We will be interested in the following phase transitions.
Melting(solid liquid) and crystallization(liquid is a solid).
Steam generation(liquid vapor) and condensation(vapor liquid).
Melting and crystallization
Most solids are crystalline, i.e. have crystal lattice- a strictly defined, periodically repeating arrangement of its particles in space.
Particles (atoms or molecules) of a crystalline solid perform thermal vibrations near fixed equilibrium positions - knots crystal lattice.
For example, the nodes of the crystal lattice table salt- these are the tops of cubic cells of "three-dimensional checkered paper" (see Fig. 1, in which larger balls denote chlorine atoms (image from the site en.wikipedia.org.)); if the water is allowed to evaporate from the salt solution, the remaining salt will be a heap of small cubes.
Rice. 1. Crystal lattice
By melting is called the transformation of a crystalline solid into a liquid. You can melt any body - for this you need to heat it to melting point, which depends only on the substance of the body, but not on its shape or size. The melting point of a given substance can be determined from tables.
On the contrary, if you cool a liquid, then sooner or later it will go into a solid state. The transformation of a liquid into a crystalline solid is called crystallization or hardening... Thus, melting and crystallization are mutually inverse processes.
The temperature at which the liquid crystallizes is called crystallization temperature... It turns out that the crystallization temperature is equal to the melting temperature: at this temperature, both processes can occur. So, when ice melts, and water crystallizes; what exactly occurs in each specific case - it depends on external conditions (for example, whether heat is supplied to the substance or removed from it).
How does melting and crystallization take place? What is their mechanism? To understand the essence of these processes, let us consider the graphs of the dependence of body temperature on time during heating and cooling - the so-called graphs of melting and crystallization.
Melting schedule
Let's start with the melting graph (Fig. 2). Let at the initial moment of time (point on the graph) the body is crystalline and has a certain temperature.
Rice. 2. Melting schedule
Then heat begins to be supplied to the body (say, the body was placed in a melting furnace), and the body temperature rises to a value - the melting point of the given substance. This is a section of the graph.
At the site, the body receives the amount of heat
where is the specific heat capacity of a solid substance, is the body mass.
When the melting point (at a point) is reached, the situation changes qualitatively. Despite the fact that heat continues to be supplied, the body temperature remains unchanged. On the site there is melting body - its gradual transition from a solid state to a liquid. Inside the site we have a mixture of solid and liquid, and the closer to the point, the less solid remains and the more liquid appears. Finally, at the point from the original solid there was nothing left: it completely turned into a liquid.
The area corresponds to the further heating of the liquid (or, as they say, melt). In this area, the liquid absorbs the amount of heat
where is the specific heat of the liquid.
But now we are most interested in the phase transition section. Why does the mixture temperature not change in this area? The heat is supplied!
Let's go back to the beginning of the heating process. An increase in the temperature of a solid in a section is the result of an increase in the intensity of oscillations of its particles in the nodes of the crystal lattice: the supplied heat is used to increase kinetic energy of body particles (in fact, some of the supplied heat is spent on doing work to increase the average distance between particles - as we know, bodies expand when heated. However, this part is so small that it can be disregarded.).
The crystal lattice looses more and more, and at the melting temperature the range of oscillations reaches the limiting value at which the forces of attraction between the particles are still able to ensure their ordered arrangement relative to each other. The solid begins to "crack at the seams", and further heating destroys the crystal lattice - this is how melting begins at the site.
From this moment on, all the supplied heat goes to work to break the bonds that hold the particles in the nodes of the crystal lattice, i.e. to increase potential particle energies. In this case, the kinetic energy of the particles remains the same, so that the body temperature does not change. At the point, the crystal structure disappears completely, there is nothing more to destroy, and the supplied heat is again used to increase the kinetic energy of the particles - to heat the melt.
Specific heat of fusion
So, for the transformation of a solid into a liquid, it is not enough to bring it to the melting point. It is necessary additionally (already at the melting point) to impart a certain amount of heat to the body for the complete destruction of the crystal lattice (i.e., for the passage of the section).
This amount of heat is used to increase the potential energy of particle interaction. Consequently, the internal energy of the melt at a point is greater than the internal energy of a solid at a point by an amount.
Experience shows that the value is directly proportional to body weight:
The coefficient of proportionality does not depend on the shape and size of the body and is a characteristic of the substance. It is called specific heat of fusion of a substance... The specific heat of fusion of this substance can be found in the tables.
The specific heat of fusion is numerically equal to the amount of heat required to transform into a liquid one kilogram of a given crystalline substance, brought to the melting point.
So, the specific heat of melting of ice is equal to kJ / kg, lead - kJ / kg. We see that almost times more energy is required to destroy the crystal lattice of ice! Ice belongs to substances with a high specific heat of melting and therefore does not melt immediately in spring (nature has taken its own measures: if ice had the same specific heat of melting as lead, the entire mass of ice and snow would melt with the first thaws, flooding everything around).
Crystallization schedule
Now let's move on to considering crystallization- the reverse melting process. We start from the point of the previous drawing. Suppose that at the point the heating of the melt stopped (the stove was turned off and the melt was exposed to air). Further changes in the melt temperature are shown in Fig. (3).
Rice. 3. Crystallization schedule
The liquid cools down (section) until its temperature reaches the crystallization temperature, which coincides with the melting point.
From this moment, the temperature of the melt ceases to change, although heat still leaves it in environment... On the site there is crystallization melt - its gradual transition to a solid state. Inside the site, we again have a mixture of solid and liquid phases, and the closer to the point, the more solid becomes and the less liquid. Finally, at the point the liquid does not remain at all - it has completely crystallized.
The next section corresponds to further cooling of the solid, which has arisen as a result of crystallization.
Again, we are interested in the phase transition section: why does the temperature remain unchanged, despite the loss of heat?
Back to the point again. After stopping the supply of heat, the temperature of the melt decreases, since its particles gradually lose kinetic energy as a result of collisions with molecules of the environment and radiation of electromagnetic waves.
When the temperature of the melt drops to the crystallization temperature (point), its particles will slow down so much that the forces of attraction will be able to "unfold" them properly and give them a strictly defined mutual orientation in space. This will create conditions for the nucleation of a crystal lattice, and it will actually begin to form due to the further escape of energy from the melt into the surrounding space.
At the same time, a counter process of energy release will begin: when the particles take their places in the nodes of the crystal lattice, their potential energy decreases sharply, due to which their kinetic energy increases - the crystallizing liquid is a source of heat (you can often see birds sitting at the ice hole. They are warming up there!) ... The heat released during crystallization exactly compensates for the heat loss to the environment, and therefore the temperature in the area does not change.
At the point, the melt disappears, and along with the completion of crystallization, this internal "generator" of heat also disappears. Due to the ongoing dissipation of energy into the external environment, a decrease in temperature will resume, but the formed solid body (area) will only cool down.
As experience shows, during crystallization in the area, exactly the same the amount of heat that was absorbed during melting at the site.
Vaporization and condensation
Steam generation is the transition of a liquid to a gaseous state (in steam). There are two methods of vaporization: vaporization and boiling.
Evaporation called vaporization that occurs at any temperature from free surface liquids. As you remember from the sheet "Saturated vapor", the cause of evaporation is the escape from the liquid of the fastest molecules that are able to overcome the forces of intermolecular attraction. These molecules form vapor above the surface of the liquid.
Different liquids evaporate at different rates: the greater the forces of attraction of molecules to each other, the smaller the number of molecules per unit time will be able to overcome them and fly out, and the lower the rate of evaporation. Ether, acetone, alcohol (they are sometimes called volatile liquids) evaporate quickly, water evaporates more slowly, oil and mercury evaporate much more slowly than water.
The rate of evaporation increases with increasing temperature (in the heat, the laundry will dry out more quickly), since the average kinetic energy of the liquid molecules increases, and thus the number of fast molecules capable of leaving its limits increases.
The rate of evaporation depends on the surface area of the liquid: the larger the area, the more molecules gain access to the surface, and the evaporation is faster (which is why it is carefully straightened when hanging the laundry).
Simultaneously with evaporation, the opposite process is also observed: vapor molecules, making a random movement above the surface of the liquid, partially return back into the liquid. The transformation of vapor into liquid is called condensation.
Condensation slows down the evaporation of the liquid. So, in dry air, laundry will dry faster than in damp air. It will dry faster in the wind: the steam is carried away by the wind, and evaporation is more intense
In some situations, the condensation rate may be equal to the evaporation rate. Then both processes compensate each other and a dynamic equilibrium sets in: from a tightly sealed bottle, the liquid does not evaporate for years, and in this case there is saturated steam.
We constantly observe the condensation of water vapor in the atmosphere in the form of clouds, rains and dew falling in the morning; it is evaporation and condensation that provide the water cycle in nature, supporting life on Earth.
Since evaporation is the departure of the fastest molecules from the liquid, the average kinetic energy of the liquid molecules decreases during the evaporation process, i.e. the liquid cools down. You are well aware of the feeling of coolness and sometimes even chilliness (especially with the wind) when you leave the water: water evaporating over the entire surface of the body carries away heat, while the wind accelerates the evaporation process (now it is clear why we are blowing hot tea. It is even better to draw the air in, because then dry ambient air comes to the surface of the tea, and not humid air from our lungs ;-)).
You can feel the same coolness if you rub a piece of cotton wool dipped in a volatile solvent (say, acetone or nail polish remover) over your hand. In forty-degree heat, thanks to the increased evaporation of moisture through the pores of our body, we keep our temperature at a normal level; had it not been for this thermoregulatory mechanism, we would have simply died in such heat.
On the contrary, in the process of condensation, the liquid heats up: the vapor molecules, when returning to the liquid, are accelerated by the forces of attraction from the side of the nearby liquid molecules, as a result of which the average kinetic energy of the liquid molecules increases (compare this phenomenon with the release of energy during the crystallization of a melt!).
Boiling
Boiling is the vaporization that occurs throughout the volume liquids.
Boiling is possible because a certain amount of air is always dissolved in the liquid, which has got there as a result of diffusion. When the liquid is heated, this air expands, air bubbles gradually increase in size and become visible to the naked eye (in a pot of water, they deposit the bottom and walls). Inside the air bubbles there is saturated steam, the pressure of which, as you remember, quickly increases with increasing temperature.
The larger the bubbles become, the greater the Archimedean force acts on them, and at a certain moment the bubbles begin to detach and rise. Rising upward, the bubbles fall into the less heated layers of the liquid; steam condenses in them, and the bubbles are compressed again. The collapse of the bubbles causes the familiar noise that precedes the kettle boiling. Finally, over time, all the liquid warms up evenly, the bubbles reach the surface and burst, throwing out air and steam - the noise is replaced by gurgling, the liquid boils.
Bubbles, thus, serve as "conductors" of vapor from the inside of the liquid to its surface. During boiling, along with ordinary evaporation, the liquid is converted into steam throughout the entire volume - the evaporation of air bubbles into the interior, followed by the withdrawal of steam to the outside. That is why the boiling liquid evaporates very quickly: a kettle, from which water would evaporate for many days, will boil away in half an hour.
Unlike evaporation, which occurs at any temperature, the liquid begins to boil only when it reaches boiling point- exactly the temperature at which air bubbles are able to float and reach the surface. At the boiling point, the saturated vapor pressure becomes equal to the external pressure on the liquid(in particular, atmospheric pressure). Accordingly, the higher the external pressure, the higher the temperature the boiling will begin.
At normal atmospheric pressure (atm or Pa), the boiling point of water is equal to. So the pressure of saturated water vapor at a temperature is Pa. This fact is necessary to know for solving problems - it is often considered known by default.
At the top of Elbrus, atmospheric pressure is equal to atm, and the water there will boil at a temperature. And under a pressure of atm, water will begin to boil only at.
The boiling point (at normal atmospheric pressure) is a strictly defined value for a given liquid (boiling points given in tables of textbooks and reference books are the boiling points of chemically pure liquids. The presence of impurities in a liquid can change the boiling point. For example, tap water contains dissolved chlorine and some salts, therefore, its boiling point at normal atmospheric pressure may differ slightly from). So, alcohol boils at, ether - at, mercury - at. Please note: the more volatile a liquid is, the lower its boiling point. In the boiling point table, we also see that oxygen boils at. This means that at ordinary temperatures oxygen is a gas!
We know that if the kettle is removed from the heat, the boiling stops immediately - the boiling process requires a continuous supply of heat. At the same time, the temperature of the water in the kettle after boiling stops changing, remaining the same all the time. Where does the supplied heat go?
The situation is similar to the melting process: heat is used to increase the potential energy of molecules. In this case, to do the work of removing molecules at such distances that the forces of attraction will be unable to keep the molecules close to each other, and the liquid will go into a gaseous state.
Boiling schedule
Consider a graphical representation of the process of heating a liquid - the so-called boil graph(fig. 4).
Rice. 4. Boiling schedule
The site precedes the beginning of the boil. On the site, the liquid boils, its mass decreases. At the point, the liquid boils away completely.
To pass the section, i.e. in order for the liquid, brought to the boiling point, to completely turn into steam, a certain amount of heat must be supplied to it. Experience shows that this amount of heat is directly proportional to the mass of the liquid:
The aspect ratio is called specific heat of vaporization liquids (at boiling point). The specific heat of vaporization is numerically equal to the amount of heat that must be supplied to 1 kg of liquid taken at the boiling point in order to completely convert it into steam.
So, when the specific heat of vaporization of water is equal to kJ / kg. It is interesting to compare it with the specific heat of melting of ice (kJ / kg) - the specific heat of vaporization is almost seven times higher! This is not surprising: after all, to melt ice, you only need to destroy the ordered arrangement of water molecules in the nodes of the crystal lattice; in this case, the distances between the molecules remain approximately the same. But to convert water into steam, you need to do a lot more work to break all bonds between molecules and remove molecules at significant distances from each other.
Condensation schedule
The process of vapor condensation and subsequent cooling of the liquid looks on the graph symmetrically to the process of heating and boiling. Here is the corresponding condensation schedule for the case of centigrade water vapor, which is most often encountered in problems (Fig. 5).
Rice. 5. Condensation schedule
At the point, we have water vapor at. Condensation is occurring on the site; inside this area - a mixture of steam and water at. There is no more steam at the point, there is only water at. The plot is the cooling of this water.
Experience shows that during the condensation of the vapor of the mass (i.e., when passing through the section) exactly the same amount of heat is released that was spent on the transformation into vapor of the liquid of the mass at a given temperature.
Let's compare the following amounts of heat for the sake of interest:
Which is released during the condensation of g of water vapor;
, which is released when the resulting centigrade water cools to a temperature, say,.
J;
J.
These numbers clearly show that a steam burn is much worse than a boiling water burn. When boiling water gets on the skin, “only” is released (the boiling water cools down). But with a steam burn, an order of magnitude more heat will first be released (steam condenses), centigrade water is formed, after which the same value will be added when this water cools.
The phenomenon of the transformation of a substance from a liquid state into a gaseous state is called vaporization... Steam generation can be carried out in the form of two processes: evaporation and
Evaporation
Evaporation occurs from the surface of the liquid at any temperature. So, puddles dry up at 10 ° C, and at 20 ° C, and at 30 ° C. Thus, evaporation is the process of converting a substance from a liquid to a gaseous state, which occurs from the surface of a liquid at any temperature.
From the point of view of the structure of matter, the evaporation of a liquid is explained as follows. Liquid molecules, participating in continuous movement, have different speeds. The fastest molecules, located at the interface between the surface of water and air and having a relatively high energy, overcome the attraction of neighboring molecules and leave the liquid. Thus, above the liquid is formed steam.
Since molecules with a higher internal energy are emitted from the liquid during evaporation, compared to the energy of the molecules remaining in the liquid, the average velocity and average kinetic energy of the liquid molecules decrease and, therefore, the temperature of the liquid decreases.
Evaporation rate liquid depends on the type of liquid. Thus, the rate of evaporation of ether is greater than the rate of evaporation of water and vegetable oil. In addition, the rate of evaporation depends on the movement of air above the surface of the liquid. The proof is that the laundry dries faster in the wind than in a windless place under the same external conditions.
Evaporation rate depends on the temperature of the liquid. For example, water at 30 ° C evaporates faster than water at 10 ° C.
It is well known that water poured into a saucer evaporates faster than water of the same mass poured into a glass. Therefore, it depends on the surface area of the liquid.
Condensation
The process of converting a substance from a gaseous state to a liquid is called condensation.
The condensation process occurs simultaneously with the evaporation process. Molecules escaping from a liquid and being above its surface participate in chaotic motion. They collide with other molecules, and at some point in time, their velocities can be directed towards the surface of the liquid, and the molecules will return to it.
If the vessel is open, then the evaporation process occurs faster than condensation, and the mass of the liquid in the vessel decreases. The vapor generated above the liquid is called unsaturated .
If the liquid is in a closed vessel, then at first the number of molecules escaping from the liquid will be greater than the number of molecules returning to it, but over time the vapor density over the liquid will increase so much that the number of molecules leaving the liquid will equal the number of molecules, returning to it. In this case, a dynamic equilibrium of the liquid with its vapor occurs.
Steam in a state of dynamic equilibrium with its liquid is called saturated steam .
If a vessel with a liquid containing saturated steam is heated, then at first the number of molecules escaping from the liquid will increase and will be greater than the number of molecules returning to it. In the course of time, equilibrium will be restored, but the vapor density above the liquid and, accordingly, its pressure will increase.
The liquid turns into vapor (gas) by evaporation and boiling. These processes are united by one name "vaporization", but there is a difference between these processes.
Evaporation occurs from the free surface of any liquid constantly. The physical nature of evaporation is the emission from the surface of molecules with high velocity and kinetic energy of thermal motion. In this case, the liquid is cooled. In industry, this effect is used in cooling towers to cool water.
Boiling (like evaporation) is a transition of a substance into a vaporous state, but it occurs throughout the entire volume of a liquid and only when heat is supplied to the liquid. Upon further heating, the temperature of the liquid remains constant, while the liquid continues to boil.
The boiling point depends on the vapor pressure above the liquid; with decreasing pressure, the boiling point decreases and vice versa. By reducing the vapor pressure above the liquid, it is possible to lower the boiling point of the liquid to its freezing point, and by choosing substances with the desired properties, you can get almost any low temperature.
The amount of heat required for the transition of 1 kg of liquid to a vapor state is called the specific heat of vaporization r, kJ / kg.
The temperature at which evaporation occurs is called the saturation temperature. Steam can be wet or dry (no liquid droplets). The steam can be superheated and have a superheat temperature above the saturation temperature.
These processes are used in vapor compression refrigeration machines. A boiling liquid is a refrigerant, and the apparatus in which it boils, taking heat from the substance to be cooled, is an evaporator. The amount of heat supplied to the boiling liquid is determined by the formula:
where M- the mass of liquid turning into steam; r- heat of vaporization.
The boiling point of the liquid depends on the pressure. This relationship is depicted by the vapor pressure pressure curve.
For the most common refrigerant in the refrigeration industry, ammonia, such a curve is shown in Fig. 3, from which it can be seen that at a pressure equal to atmospheric (0.1 MPa) the boiling point of ammonia corresponds to -30 ° C, and at 1.2 MPa - + 30 ° C.
The transformation of a saturated vapor into a liquid is called condensation, which occurs at the condensation temperature, which is also dependent on pressure. Condensing and Evaporating Temperature at a Specific Pressure homogeneous substance are the same. This effect is used in evaporative condensers to transfer the heat of condensation to air.
Sublimation
The substance can go from solid state directly to vapor. This process is called sublimation. The heat absorbed from the ambient air is spent on overcoming the adhesion forces of molecules and the influence of external pressure, which prevents this process.
Under normal conditions, not many substances sublimate - solid carbon dioxide (dry ice), iodine, camphor, etc.
For cooling and obtaining low temperatures, dry ice is used, which provides a temperature of -78.3 ° C at atmospheric pressure, and by lowering the pressure it is possible to reach -100 ° C.
>> Physics: Evaporation and Condensation
When vaporized, a substance passes from a liquid state to a gaseous state (vapor). There are two types of vaporization: vaporization and boiling.
Evaporation- This is vaporization that occurs from the free surface of the liquid.
How does evaporation take place? We know that the molecules of any liquid are in continuous and disorderly motion, some of them moving faster, others slower. The forces of attraction to each other prevent them from flying out. If, however, a molecule with a sufficiently high kinetic energy appears at the surface of the liquid, then it will be able to overcome the forces of intermolecular attraction and fly out of the liquid. The same will be repeated with another fast molecule, with the second, third, etc. Escaping outside, these molecules form vapor above the liquid. The formation of this vapor is evaporation.
Since the fastest molecules fly out of the liquid during evaporation, the average kinetic energy of the molecules remaining in the liquid becomes less and less. As a result the temperature of the evaporating liquid decreases: the liquid cools... That is why, in particular, a person in wet clothes feels colder than in dry ones (especially in the wind).
At the same time, everyone knows that if you pour water into a glass and leave it on the table, then, despite evaporation, it will not cool continuously, becoming colder and colder until it freezes. What prevents this? The answer is very simple: heat exchange of water with the warm air surrounding the glass.
Cooling of a liquid during evaporation is more noticeable in the case when evaporation occurs quickly enough (so that the liquid does not have time to recover its temperature due to heat exchange with the environment). Volatile liquids with low intermolecular attraction forces evaporate quickly, for example, ether, alcohol, gasoline. If you drop such a liquid on your hand, we will feel cold. Evaporation from the surface of the hand, such a liquid will cool and take some heat from it.
Rapidly evaporating substances are widely used in technology. For example, in space technology, descent vehicles are coated with such substances. When passing through the planet's atmosphere, the body of the apparatus heats up as a result of friction, and the substance covering it begins to evaporate. Evaporating, it cools the spacecraft, thereby saving it from overheating.
Cooling of water during its evaporation is also used in devices for measuring air humidity, - psychrometers(from the Greek "psychros" - cold). The psychrometer (fig. 81) consists of two thermometers. One of them (dry) shows the air temperature, and the other (the reservoir of which is tied with cambric dipped in water) - a lower temperature due to the intensity of evaporation of moist cambric. The drier the measured humidity, the stronger the evaporation and therefore the lower the wet bulb reading. And vice versa, the higher the air humidity, the less intense the evaporation and therefore the higher the temperature this thermometer shows. Based on the readings of dry and humidified thermometers, using a special (psychrometric) table, the air humidity is determined, expressed as a percentage. The highest humidity is 100% (with such air humidity, dew appears on objects). For humans, the most favorable humidity is considered to be in the range from 40 to 60%.
With the help of simple experiments, it is easy to establish that the evaporation rate increases with an increase in the temperature of the liquid, as well as with an increase in its free surface area and in the presence of wind.
Why does the liquid evaporate faster in the presence of wind? The fact is that simultaneously with evaporation on the surface of the liquid, the reverse process also occurs - condensation
... Condensation occurs due to the fact that some of the vapor molecules, randomly moving above the liquid, return to it again. The wind carries away the molecules ejected from the liquid and does not allow them to return back.
Condensation can also occur when the vapor is not in contact with the liquid. It is condensation, for example, that explains the formation of clouds: the molecules of water vapor rising above the ground, in the colder layers of the atmosphere, are grouped into tiny droplets of water, the accumulations of which are clouds. Condensation of water vapor in the atmosphere also results in rain and dew.
During evaporation, the liquid cools down and, becoming colder than the environment, begins to absorb its energy. Conversely, during condensation, a certain amount of heat is released into the environment, and its temperature rises somewhat.
??? 1. What two types of vaporization exist in nature? 2. What is evaporation? 3. What determines the rate of evaporation of a liquid? 4. Why does the liquid temperature drop during evaporation? 5. How is it possible to prevent the descent spacecraft from overheating during their passage through the planet's atmosphere? 6. What is condensation? 7. What phenomena are explained by steam condensation? 8.What instrument is used to measure air humidity? How does it work?
Experimental tasks ... 1. Pour the same amount of water into two identical saucers (for example, three tablespoons). Place one saucer in a warm place and the other in a cold place. Measure the time it takes for the water to evaporate in both saucers. Explain the difference in evaporation rate. 2. Pipette a drop of water and alcohol onto a sheet of paper. Measure the time it takes for them to evaporate. Which of these liquids has less attractive forces between molecules? 3. Pour the same amount of water into the glass and saucer. Measure the time it takes for it to evaporate in them. Explain the difference in its evaporation rate.
S.V. Gromov, N.A. Homeland, Physics grade 8
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The process of transition of a substance from a liquid state to a gaseous state is called vaporization, the reverse process of converting a substance from a gaseous state to a liquid state is called condensation. There are two types of vaporization - vaporization and boiling. Consider first the evaporation of a liquid. Evaporation is the process of vaporization that occurs from an open surface of a liquid at any temperature. From the point of view of molecular kinetic theory, these processes are explained as follows. Liquid molecules, participating in thermal motion, continuously collide with each other. This leads to the fact that some of them acquire kinetic energy sufficient to overcome molecular attraction. Such molecules, being at the surface of the liquid, fly out of it, forming vapor (gas) above the liquid. Vapor molecules ~ moving chaotically, they hit the surface of the liquid. In this case, some of them can go into liquid. These two processes of ejection of liquid molecules and ah return to liquid occur simultaneously. If the number of escaping molecules is greater than the number of returning molecules, then the mass of the liquid decreases, i.e. the liquid evaporates, if on the contrary, the amount of liquid increases, i.e. steam condensation is observed. A case is possible when the masses of the liquid and the vapor located above it do not change. This is possible when the number of molecules leaving the liquid is equal to the number of molecules returning to it. This state is called dynamic equilibrium.
A steam
In dynamic equilibrium with its fluid, called saturated
. If there is no dynamic equilibrium between vapor and liquid, then it is called unsaturated. Obviously, saturated steam at a given temperature has a certain density, called equilibrium.
This determines the invariability of the equilibrium density, and, consequently, the pressure of the saturated vapor from its volume at a constant temperature, since a decrease or increase in the volume of this vapor leads to condensation of vapor or to the evaporation of liquid, respectively. Saturated steam isotherm at a certain temperature in coordinate plane P, V is a straight line parallel to the V axis. With an increase in the temperature of the thermodynamic system liquid - saturated vapor, the number of molecules leaving the liquid over time exceeds the number of molecules returning from the vapor to the liquid. This continues until the increase in vapor density leads to the establishment of dynamic equilibrium at a higher temperature. At the same time, the pressure also increases. saturated vapors... Thus, the saturated vapor pressure depends only on temperature. Such a rapid increase in the saturated vapor pressure is due to the fact that with an increase in temperature, there is an increase not only in the kinetic energy of the translational motion of molecules, but also in their concentration, i.e. number of molecules per unit volume
During evaporation, the fastest molecules leave the liquid, as a result of which the average kinetic energy of the translational motion of the remaining molecules decreases, and, consequently, the temperature of the liquid also decreases (see §24). Therefore, in order for the temperature of the evaporating liquid to remain constant, a certain amount of heat must be continuously supplied to it.
The amount of heat that must be communicated to a unit mass of liquid in order to convert it into steam at a constant temperature is called the specific heat of vaporization.
The specific heat of vaporization depends on the temperature of the liquid, decreasing with its increase. During condensation, the amount of heat expended on the evaporation of the liquid is released. Condensation is the process of converting from a gaseous state to a liquid state.
2. Air humidity.
The atmosphere always contains a certain amount of water vapor. The degree of humidity is one of the essential characteristics of weather and climate and in many cases is of practical importance. So, storage of various materials (including cement, gypsum and other building materials), raw materials, products, equipment, etc. should take place at a certain humidity. The premises, depending on their purpose, are also subject to corresponding humidity requirements.
A number of quantities are used to characterize moisture content. Absolute humidity p is the mass of water vapor contained in a unit volume of air. It is usually measured in grams per cubic meter (g / m3). Absolute humidity is related to the partial pressure P of water vapor by the Mendeleev - Claypeyron equation, where V is the volume occupied by vapor, m, T and m are the mass, absolute temperature and molar mass of water vapor, R is the universal gas constant (see (25.5)) ... Partial pressure is the pressure exerted by water vapor without taking into account the action of other types of air molecules. Hence, since p = m / V is the density of water vapor.
