1. Evaporation and condensation
The process of transition of a substance from a liquid state into a gaseous condition is called a vaporization, the inverse process of the conversion of the substance from the gaseous state into liquid is called condensation. There are two types of vaporization - evaporation and boiling. Consider first evaporation of the fluid. Evaporation is called the charging process, occurring with an open surface of the fluid at any temperature. From the point of view of the molecular-kinetic theory, these processes are explained as follows. Liquid molecules, participating in thermal motion, are continuously faced 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, are flying out of it, forming a pair (gas) above the liquid. Molecules of steam ~ moving chaotically, hit the surface of the liquid. In this case, some of them can go into the liquid. These two process of departure of fluid molecules and ah reverse return to the liquid occur simultaneously. If the number of departing molecules is greater than the number of returning, then there is a decrease in the mass of the liquid, i.e. The liquid evaporates, if on the contrary, the amount of fluid increases, i.e. There is a condensation of steam. There is a case when the masses of the liquid and the pair, which is 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 condition is called dynamic equilibrium.
BUT par
In dynamic equilibrium with its liquid, called saturated
. If there is no dynamic equilibrium between the steam and liquid, then it is called unsaturated.Obviously, saturated steam at a given temperature has a certain density called equilibrium.
This causes the immutability of equilibrium density, and consequently, the pressure of a saturated steam from its volume at a constant temperature, since the decrease or an increase in the volume of this vapor leads to steam condensation or to evaporate the liquid, respectively. The rich steam isotherm at a certain temperature in the coordinate plane P, V is a straight, parallel axis V. with an increase in the temperature of the thermodynamic system. Liquid - saturated couples. The number of molecules leaving fluid over a while exceeds the number of molecules returning from steam into a liquid. This continues until the increase in the density of the steam does not lead to the establishment of a dynamic equilibrium at a higher temperature. This increases the pressure of saturated vapor. Thus, the pressure of saturated vapors depends only on temperature. Such a rapid increase in the pressure of the saturated steam is due to the fact that with increasing temperature, there is an increase of not only the kinetic energy of the translational movement of molecules, but also of their concentration, i.e. Numbers of molecules per unit volume
During evaporation, the liquid leave the fastest molecules, as a result of which the average kinetic energy of the translational movement of the remaining molecules decreases, and therefore, the fluid temperature decreases (see §24). Therefore, that the temperature of the evaporating fluid remains constant, it is necessary to continuously sum up a certain amount of heat.
The amount of heat that must be reported by the unit of mass of the fluid, to convert it to steam at a constant temperature is called the specific heat of the vaporization.
The specific heat of the vapor formation depends on the temperature of the fluid, decreasing with its increase. When condensation, the amount of heat spent on evaporation of the fluid is highlighted. Condensation is the process of transformation from a gaseous state into liquid.
2. Air humidity.
The atmosphere always contains some water vapor. The degree of moisture is one of the essential characteristics of weather and climate and has practical importance in many cases. So, the storage of various materials (including cement, plaster and other building materials), raw materials, products, equipment, etc. Must occur with a certain humidity. To the premises, depending on their purpose, the relevant moisture requirements are also imposed.
A number of values \u200b\u200bare used to characterize humidity. The absolute humidity of p is the mass of the water vapor contained in the air volume unit. It is usually measured in grams into a cubic meter (g / m3). Absolute humidity is associated with partial pressure P of a water vapor by the Mendeleev equation - Cloipairone, where V is the volume occupied by steam, m, t and m - mass, absolute temperature and molar weight of water PAPA, R is a universal gas constant (see (25.5)) . Partial pressure is called pressure, which has water vapor without taking into account the action of air molecules of another variety. Hence, since p \u003d m / v- density of water vapor.
The conversion phenomenon of the substance from the liquid state into the gaseous is called vaporization. Vaporization can be carried out in the form of two processes: evaporation and
Evaporation
Evaporation occurs from the surface of the fluid at any temperature. So, the pudges dry and at 10 ° C, and at 20 ° C, and at 30 ° C. Thus, evaporation is called the process of converting a substance from a liquid state into a gaseous, occurring from the surface of the fluid at any temperature.
From the point of view of the structure of the substance, the evaporation of the fluid is explained as follows. Liquid molecules, participating in continuous motion, have different speeds. The fastest molecules on the boundary of the surface of water and air and having relatively greater energy overcome the attraction of neighboring molecules and leave the liquid. Thus, the liquid is formed par.
Since the fluid during evaporation, molecules with greater internal energy fly out from the energy of the molecules remaining in the liquid, then the average speed and the average kinetic energy of the fluid molecules are reduced and, therefore, the fluid temperature decreases.
Evaporation rate Liquid depends on the kind of fluid. So, the speed of evaporation of the ether is more than the rate of evaporation of water and vegetable oil. In addition, the rate of evaporation depends on the movement of the air above the surface of the fluid. Proof can be the fact that lingerie will dry faster in the wind than in a windless place under the same external conditions.
Evaporation rate Depends on fluid temperature. For example, water at a temperature of 30 ° C evaporates faster than water at 10 ° C.
It is well known that water, poured in a saucer, evaporate faster than the water of the same mass, poured into a glass. Therefore, depends on the surface area of \u200b\u200bthe liquid.
Condensation
The process of transformation of the substance from the gaseous state into liquid is called condensation.
The condensation process occurs simultaneously with the process of evaporation. Molecules, flying out of the liquid and above its surface, are involved in chaotic movement. They face other molecules, and at some point of their speeds can be directed to the surface of the liquid, and the molecules will return to it.
If the vessel is open, the process of evaporation occurs faster than condensation, and the mass of fluid in the vessel decreases. Couples formed above the liquid is called unsaturated .
If the fluid is in a closed vessel, then first the number of molecules departing from the liquid will be greater than the number of molecules returned to it, but over time, the density of steam above the liquid will increase so that the number of molecules leaving fluid will become equal to the number of molecules, returning to it. In this case, the dynamic equilibrium of the liquid with its ferry occurs.
Couples in a state of dynamic equilibrium with its liquid is called saturated ferry .
If a liquid vessel in which saturated steam is, heathed, then the number of molecules departing from the liquid will increase and will increase and will increase than the number of molecules returning to it. Over time, the equilibrium will be restored, but the density of steam above the liquid and, accordingly, its pressure will increase.
All gases are yawl. There are no fundamental differences between the concepts of gas and steam in pairs of any substance. Water steam yawl. Real gas and widely used in various industries. This is due to the widespread spread of water, its cheap and harmlessness to human health. Water steam is obtained in the process of evaporation of water when heat to it heat.
Vaporization Naz. The process of transition of fluid into steam.
Evaporation Naz. Variousness occurring only from the surface of the liquid and at any temperature. The intensity of evaporation depends on the nature of the liquid and temperature.
Boiling Naz. Variousness in the entire mass of fluid.
The process of transforming steam into a liquid, carried out when heat from it and being a process, reverse vaporization, called. condensation. This process, as well as vaporization, occurs at a constant temperature.
Retreat or sublimation Naz. The process of transition of the substance from a solid state directly into steam.
Process, inverse sublimation process, i.e. The process of switching the pair directly into solid state, called. desublimation.
Saturated steam. When evaporation of fluid in a limited volume simultaneously, the reverse process occurs, i.e. The phenomenon of the liquefaction. As you evaporate and filling the space of space above the liquid, the intensity of evaporation decreases and the intensity of the opposite process increases. At some point, when the condensation speed becomes equal to evaporation rate, a dynamic equilibrium occurs in the system. In this state, the number of molecules departing from the liquid will be equal to the number of molecules returning back to it. Consequently, in the vapor space, with the equilibrium state, the maximum number of molecules will be. Couples at this condition has the maximum density and called. saturated. Under saturated, pair, which is equilibrium with the liquid, from which it is formed. A saturated pair has a temperature that is a function of its pressure equal to the pressure of the medium in which the boiling process occurs. With an increase in the volume of saturated steam at a constant temperature, there is a transition of a certain amount of fluid into steam, with a decrease in the same volume at a constant temperature - the transition of steam into the liquid, but both in the first and in the second cases the steam pressure remains constant.
Dry saturated par It turns out when evaporating the entire fluid. The volume and temperature of dry steam are pressure functions. As a consequence, the condition of the dry pair is determined by one parameter, for example, pressure or temperature.
Wet saturated parobtained with incomplete evaporation of liquid, yawl. A mixture of steam with the smallest droplets of the liquid, common evenly throughout its mass and in it in suspended state.
Mass fraction of dry pair in a wet pair of Naz. degree of dryness or massive steaming and denoted by x. Mass fraction of fluid in a wet pair of Naz. degree of humidity And denotes y. Obviously, y \u003d 1-x. The degree of dryness and the degree of moisture express or in the fractions of a unit or in percent.
For dry pair x \u003d 1, and for water x \u003d 0. In the process of vaporization, the degree of dryness steam gradually increases from zero to one.
When communicating a dry pair of heat at constant pressure, it will increase it. Couples obtained in this process, called. overheated.
Since the specific volume of the superheated steam is greater than the specific volume of the saturated pair (because P \u003d const, TPER\u003e TN), then the density of the superheated steam is less than the density of the saturated steam. Therefore, overheated pairs of Jawl. unsaturated. In its physical properties, superheated pairs approach ideal gases.
10.3. R, v.- Diagrama water vapor
Consider the peculiarities of the charging process. Suppose in the cylinder there is 1 kg of water at a temperature of 0 s, to the surface of which the piston p is the pressure of the piston. The volume of water under the piston is equal to the specific volume at 0 s, denoted by (\u003d 0.001m / kg), we will consider to simplify that the water is yawl. practically incompressible liquid and has the highest density at 0 s, and not at 4 s (more precisely 3.98 s). When the cylinder is heated and the heat transfer of water will increase its temperature, the volume increases, and when T \u003d T H, corresponding to P \u003d P 1, the water will boil and the vaporization will begin.
All changes in the state of the liquid and the pair will be celebrated in P, v. coordinates (Fig. 10.1).
The process of formation of superheated steam when p \u003d const consists of three consistently implemented physical processes:
1. Heated fluid to T N;
2. Variousness at t H \u003d const;
3. Steating steam, accompanied by an increase in temperature.
At p \u003d p 1 of these processes in P, v. - The diagram corresponds to the segments A-A, A -A, and -D. In the interval between points A and and the temperature will be constant and equal to TN1 and the pair will be wet, and it will be closer to that the degree of dryness will be less (x \u003d 0), and in TA, corresponding to the condition of dry steam, x \u003d 1. If the steam formation process will go at a higher pressure (p 2\u003e p 1), the volume of water will remain almost the same. Volume V corresponding to boiling water, slightly increase (), because T H2\u003e T H1, and the volume, since the process of vaporization at a higher pressure and high temperature flows more intensively. Consequently, with an increase in pressure, the difference in volumes (segment) increases, and the difference in volumes (segment) decreases. A similar picture will be and then when the warehouse process comes with greater pressure (p 3\u003e p 2;;, because t H3\u003e t H2).
If in Fig.10.1, connect points with one and two strokes lying on the isobars
different pressures, we get a line; .
each of which has a completely definite value. For example, the A-B-C line expresses the dependence of the specific volume of water at 0 s, from pressure. It is almost parallel to the axis of the ordinate, because Water is a practically incompressible liquid. The line gives the dependence of the specific volume of boiling water from pressure. This line is called. lower border curve. In p, v. - Diagram, this curve separates the water area from the field of saturated vapor. The line shows the dependence of the specific volume of dry steam from pressure and called. upper border curve. It separates the area of \u200b\u200bsaturated steam from the overheated (unsaturated) pair.
The point of the meeting of the border curves is called. critical point K.. This point corresponds to a certain limit critical condition of the substance when there is no distinction between the liquid and the ferry. At this point there is no area of \u200b\u200bthe charging process. The parameters of the substance at the same time is called. Critical. For example, for water: RK \u003d 22,1145 MPa; TK \u003d 647,266 K; VK \u003d 0.003147 m / kg.
Critical temperature of Yavl. Maximum saturated pair temperature. At temperatures above the critical may be only overheated pairs and gases. For the first time, the concept of critical temperature was given in 1860. D.I. Mendeleev. He defined it as such a temperature above which the gas cannot be translated into a liquid, whatever high pressure to it is applied.
Not always, however, the process of vaporization is performed as shown in Fig.10.1. If the water is purified from mechanical impurities and gases dissolved in it, the vaporization can begin at temperatures above T n (sometimes at 15-20 K) due to the lack of vaporization centers. Such water is called overheated. On the other hand, during the rapid isobar cooled overheated pair, the condensation can begin not to begin. And with a slightly lower temperature. Such pairs is called. controlled or proped. When solving the issue, in which aggregate state there may be substances (pairs or water) with the specified P and T R and v. Or T and V must always keep in mind the following. At p \u003d const for superheated steam and T d\u003e t n (see Fig. 10.1); for water, on the contrary, and t<Т н; при Т=const для перегретого пара и р е <р н; для воды и р n >r n. Knowing these relations and using tables for a saturated pair, you can always determine in which of the three regions 1, 2 or 3 (see Fig. 10.2) is a working fluid with specified parameters, i.e. Is liquid (area 1) saturated (region 2) or overheated (area 3) steam.
For the supercritical region, during the likely border "Water - Couples", it is conventionally taken by a critical isotherm (barchpunctive curve). At the same time, the substance is located in one-phase homogeneous state on the left and right of this isotherm, possessing, for example, in Ty the properties of the liquid, and in T.z - the properties of steam.
While taking place from a free surface of the liquid.
Sublimation, or sublimation, i.e. The transition of a substance from a solid state into a gaseous, is also called evaporation.
From everyday observations it is known that the number of any liquid (gasoline, ether, water), which is in an open vessel, gradually decreases. The liquid does not disappear without a trace - it turns into steam. Evaporation is one of the species variations. Another look is boiling.
Evaporation mechanism.
How is evaporation? The molecules of any liquid are in a non-interrupful and disorderly movement, and the higher the fluid temperature, the greater the kinetic energy of molecules. The average value of kinetic energy has a certain amount. But each molecule, kinetic energy can be both more and less than the average. If a molecule with kinetic energy is near the surface, sufficient to overcome the intermolecular attraction forces, it will depart from the liquid. The same is pursuing with another fast molecule, with the second, third, etc., the outward outward, these molecules form a pair liquid. The formation of this couple is evaporation.
Energy absorption during evaporation.
Since faster molecules, the average kinetic energy of the molecules remaining in the fluid becomes becoming less and less flying from the liquid. This means that the internal energy of evaporating fluid decreases-Xia. Therefore, if there is no inflow of energy to fluid from the outside, the temperature of the evaporating fluid decreases, the liquid is cooled (that is why, in particular, a man in wet clothes is colder than in dry, especially in the wind).
However, when evaporation of water, poured into a glass, we do not notice lowering its temperature. How to explain it? The fact is that evaporation in this case occurs slowly, and the water temperature is maintained by constant due to the heat exchange with the surrounding air, from which the required amount of heat flows into the liquid. It means that the evaporation of fluid pro coming without changing its temperature, the fluid must be informed.
The amount of heat that must be reported to inform the liquid to form a pair mass at a constant temperature is called heat steam formation.
Fluid evaporation rate.
Unlike boilingThe evaporation occurs at any temperature, however, with an increase in fluid temperature, the evaporation rate increases. The higher the temperature of the fluid, the more fast moving molecules, has sufficient kinetic-kuyu energy to overcome the forces of attraction of neighboring particles and fly out of the limits of the liquid bone, and the faster there is evaporation.
Evaporation rate depends on the kind of liquid. Volatile fluids will quickly evaporate, in which the forces of intermolecular interaction are small (for example, ether, alcohol, gasoline). If the cap is such a liquid on the hand, we will notice the cold. After evaporating from the surface of the hand, such a liquid will cool and select a certain amount of warmth.
The rate of evaporation of the fluid depends on the area of \u200b\u200bits free surface. This is explained by the fact that the fluid evaporates from the surface, and the greater the area of \u200b\u200bthe free surface of the liquid bone, the more molecules at the same time flies into the air.
In the open vessel, the mass of fluid due to evaporation gradually decreases. This ensures that the majority of the steam molecules are dissipated in the air, without returning to the liquid (in contrast to what is happening in the closed vessel). But the small part of them is returned to the liquid, thereby slowing the evaporation. Therefore, with wind, which takes steam molecules, the evaporation of the fluid is faster.
The use of evaporation in the technique.
Evaporation plays an important role in energy, refrigeration, in drying processes, evaporative cooling. For example, in cosmic technology, fast-packed substances cover the descent devices. When passing through the atmosphere of the planet, the body of the apparatus is heated as a result of friction, and the coating of its substance is started to evaporate. After evaporated, it cools the spacecraft, thereby saving it from the re-grade.
Condensation.
Condensation(from lat. condensatio.- Seal, condensation) - transition of a substance from a gas-shaped state (steam) into a liquid or solid state.
It is known that if there is a wind, the liquid evaporates faster. Why? The fact is that one-time with evaporation from the surface of the liquid there is condensation. Condensation occurs due to the fact that a part of a steam molecules, randomly moving over the liquid, returns to it again. The wind makes the molecule flying out of the fluid and does not give them to return.
Condensation can occur when steam does not come into contact with the liquid. It is condensation that explains, for example, the formation of clouds: water vapor molecules, lifting over the ground, in the colder layers of the atmosphere are grouped into the smallest droplets of water whose clusters are clouds. The consequence of the condensation of water vapor in the atmosphere is rain and dew.
When evaporated, the liquid is cooled and, becoming colder than the environment, starts to absorb its energy. When condensation, on the contrary, there is a selection of a certain amount of heat into the environment, and its temperature is somewhat rising. The amount of heat released during the condensation of the mass unit is equal to the heat of evaporation.
Themes of the EGE codifier: Changes in aggregate states of matter, melting and crystallization, evaporation and condensation, boiling liquid, change of energy in phase transitions.
Lod, water and water vapor - examples of three aggregate states Substances: solid, liquid and gaseous. In which aggregate state is this substance - depends on its temperature and other external conditions in which it is located.
With the change in external conditions (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 aggregative state of the body of the body. We will be interested in the following phase transitions.
Melting (solid body liquid) and crystallization (liquid solid body).
Vaporization (Par Liquid) and condensation (steam liquid).
Melting and crystallization
Most solid bodies are crystal. have crystal lattice - Strictly defined, periodically repeated in the space location of its particles.
Particles (atoms or molecules) of crystalline solid body make heat fluctuations near fixed equilibrium positions - nodes Crystal lattice.
For example, the nodes of the crystal lattice of the table salt are the vertices of cubic cells "three-dimensional cellular paper" (see Fig. 1, on which the larger balls indicate chlorine atoms (image from EN.Wikipedia.org.)); If you have to evaporate water from salt solution, then the remaining salt will be the jet of small cubes.
Fig. 1. Crystal lattice
Melting It is called the conversion of the crystalline solid body into the liquid. You can melt any body - for this you need to heat it to melting temperature, which depends only on the body of the body, but not from its shape or sizes. The melting point of this substance can be determined from the tables.
On the contrary, if you cool the liquid, then sooner or later it will turn into a solid state. The transformation of the liquid into the crystalline solid body is called crystallization or holding. Thus, melting and crystallization are mutually reverse processes.
The temperature at which the feeders crystallizes is called crystallization temperature. It turns out that the crystallization temperature is equal to the melting point: at this temperature, both processes can flow. So, when ice is melted, and the water is crystallized; what exactly It occurs in each particular case - depends on external conditions (for example, whether heat is supplied to the substance or removed from it).
How do melting and crystallization occur? What are their mechanism? To clarify the essence of these processes, we consider the graphs of the body temperature dependence on time when it is heated and cooling is the so-called melting and crystallization graphs.
Melting graph
Let's start with the melting graph (Fig. 2). Let in the initial moment of time (the point on the graph) the body is crystalline and has some temperature.
Fig. 2. Melting schedule
Then heat begins to the body (say, the body was placed in a melting furnace), and the body temperature rises to the magnitude of the melting point of this substance. This is a plot of graphics.
On the site the body gets the amount of heat
where - the specific heat capacity of the solid body, is the mass of the body.
When the melting point is reached (at point), the situation changes qualitatively. Despite the fact that the heat continues to be supplied, the body temperature remains unchanged. On the plot occurs melting Body is its gradual transition from solid state into liquid. Inside the section, we have a mixture of solid substance and liquid, and the closer to the point, the less solid substance remains and the more fluid appears. Finally, at the point from the initial solid body there was nothing left: it completely turned into a liquid.
The plot corresponds to the further heating of the fluid (or, as they say, melt). In this section, the liquid absorbs the amount of heat
where is the specific heat capacity of the liquid.
But now we are most interested in - a phase transition site. Why does not change the temperature of the mixture on this site? It is warmly summed up!
Let's return back to the beginning of the heating process. Increasing the temperature of the solid body on the site is the result of increasing the intensity of the oscillations of its particles in the nodes of the crystal lattice: the resulting heat goes to an increase kinetic The energy of body particles (in fact, some of the input heat is spent on performing work to increase the average distances between the particles - as we know, the bodies are expanding when heated. However, this part is so small that it can not be taken into account.).
The crystalline lattice looms all the stronger and stronger, and at the melting point of the swing, the oscillations reaches the limit value at which the force of attraction between the particles is still capable of providing their ordered location relative to each other. The solid body begins to "crack along the seams", and further heating destroys the crystal lattice - it begins melting on the site.
From that moment on, all the summed heat goes to the work on the rupture of the connections, holding the particles in the nodes of the crystal lattice, i.e. on an increase potential Energy particles. The kinetic energy of the particles remains the same, so the body temperature does not change. At the point, the crystal structure disappears completely, there is nothing more to destroy, and the resulting heat again goes to an increase in the kinetic particle energy - on the heating of the melt.
Specific heat melting
So, to convert the solid body into a liquid to bring it to the melting point. It is necessary additionally (already at melting point) to inform the body a certain amount of heat for complete destruction of the crystal lattice (i.e., for the passage of the site).
This amount of heat goes to an increase in the potential energy of the interaction of particles. Consequently, the inner energy of the melt at a point is larger than the internal energy of the solid body at the point by magnitude.
Experience shows that the magnitude is directly proportional to the mass of the body:
The proportionality coefficient does not depend on the shape and size of the body and is the characteristic of the substance. It is called specific heat melting substance. The specific heat of melting of this substance can be found in the tables.
The specific heat of melting is numerically equal to the amount of heat required to convert one kilogram of a given crystalline substance, brought to the melting point.
Thus, the specific heat of melting ice is equal to KJ / kg, lead - KJ / kg. We see that to destroy the crystal lattice of ice requires almost many times more energy! The ice refers to substances with a large-specific melting heat and therefore the spring does not immediately (nature accepted its measures: possessing the ice of the same specific heat of melting, like lead, all the mass of ice and snow would melt with the first thaws, floating everything around).
Crystallization schedule
Now let's get to consideration crystallization - Reverse melting process. We start from the point of the previous drawing. Suppose that at the point heating the melt stopped (the stove turned off and the melt was put on the air). Further change in the temperature of the melt is represented in Fig. (3).
Fig. 3. Crystallization schedule
The liquid cools down (section) until its temperature reaches the crystallization temperature, which coincides with the melting point.
From this point on, the melt temperature ceases to change, although heat is still due to it into the environment. On the plot occurs crystallization The melt is its gradual transition to solid state. Inside the site, we again have a mixture of solid and liquid phases, and the closer to the point, the more the solid substance becomes and the smaller - the liquid. Incover, codes of fluid will be-atpolnicly.
The next section corresponds to the further cooling of the solid body resulting from crystallization.
We are again interested in a phase transition site: why is the temperature remain unchanged, despite the care of heat?
Repeat back to the point. After the heat supply is stopped, the melt temperature is reduced, since its particles gradually lose the kinetic energy as a result of collisions with environmental molecules and radiation of electromagnetic waves.
When the melt temperature decreases to the crystallization temperature (point), its particles slow down so much that the forces of attraction will be able to "deploy" them properly and give them a strictly defined mutual orientation in space. This will arise the conditions for the origin of the crystal lattice, and it will actually begin to form due to the further care of the energy from the melt into the surrounding space.
At the same time, the oncoming process will begin: when the particles occupy their places in the nodes of the crystal lattice, their potential energy decreases sharply, due to which their kinetic energy increases - the crystallizing fluid is the source of heat (often the corruses can be seen the sitting birds. They are warm!) . The heat released during crystallization is precisely compensated for the heat loss into the environment, and therefore the temperature on the site does not change.
At the point, the melt disappears, and together with the completion of the crystallization disappears and this internal "generator" of heat disappears. Due to the continuing energy scattering into the outer medium, the temperature decrease will be resumed, but only cooling will already be the resulting solid body (plot).
As experience shows, the crystallization on the site stands out exactly the same The amount of heat that was absorbed when melting on the site.
Various and condensation
Vaporization - this is a liquid transition to a gaseous state (in par). There are two ways to vaporization: evaporation and boiling.
Evaporation called vaporization that occurs at any temperature with free surface liquids. How do you remember from the "saturated steam" sheet, the cause of evaporation is the departure of the liquid of the fastest molecules that can overcome the forces of the intermolecular attraction. These molecules form steam above the surface of the liquid.
Different fluids evaporate with different speeds: the greater the force of attraction of molecules to each other - the smaller number of molecules per unit of time will be able to overcome and fly outward, and the less the speed of evaporation. Ether, acetone, alcohol evaporates quickly (they are sometimes called volatile liquids), slower - water, much slower water is evaporated and mercury.
The evaporation rate is growing with an increase in temperature (in the heat of linen he dries rather), since the average kinetic energy of the fluid molecules is increasing, and thus the number of fast molecules that can leave its limits increases.
The evaporation rate depends on the surface area of \u200b\u200bthe liquid: the greater the area, the greater the number of molecules get access to the surface, and evaporation is faster (which is why it is carefully straightened by linen).
Simultaneously with evaporation, the reverse process is observed: steam molecules, making a messy movement above the surface of the liquid, partially returned back into the liquid. Transformation of steam into liquid is called condensation.
Condensation slows down the evaporation of the fluid. So, in dry air linen dries faster than in the wet. It will dry faster and in the wind: steam is demolished by the wind, and evaporation goes more intensively
In some situations, the condensation speed may be equal to evaporation rate. Then both processes compensate each other and there is a dynamic balance: from a tightly closed bottle, the liquid does not disappear for years, and above the surface of the liquid in this case is located saturated steam.
Condensation of water vapor in the atmosphere we constantly observe the clouds, rain and falling in the mornings of dew; It is evaporation and condensation that provide a cycle of water in nature, supporting life on Earth.
Since evaporation is leaving the fluid of the fastest molecules, in the process of evaporation, the average kinetic energy of the fluid molecules is reduced, i.e. The liquid cools. You are familiar with the feeling of coolness and sometimes even zrayability (especially in the wind), when you leave the water: water, evaporating all over the surface of the body, takes heat, the wind accelerates the process of evaporation (the game is clear, why are we blowing on hot tea. By the way, we blow on hot tea. By the way, Better at the same time to pull the air into my senses, because the surface of the tea then the dry surrounding air comes, and not wet air out of our lungs ;-)).
The same coolness can be felt if you spend on hand a piece of cotton wool moistened in a fly solvent (let's say, in acetone or liquid for removing varnish). To the forty-portus heat, thanks to the enhanced evaporation of moisture through the pores of our body, we preserve our temperature at the level of normal; Do not be this thermostatic mechanism, in such a heat we would simply died.
On the contrary, in the condensation process, the liquid heats up: steam molecules when returning to the liquid, they are accepted by attracting the fluid molecules near the nearby, as a result of which the average kinetic energy of the fluid molecules increases (compare this phenomenon with the release of energy during melt crystallization!).
Boiling
Boiling - this is a vaporization that happens throughout the volume liquids.
Boiling turns out to be possible because the liquid always dissolved some amount of air that has happened there as a result of diffusion. When the fluid is heated, this air expands, air bubbles gradually increase in size and become visible to the naked eye (in a water saucepan, they are precipitated the bottom and walls). Inside air bubbles there is a saturated pair, the pressure of which, as you remember, is growing rapidly with increasing temperature.
The larger the bubbles become, the bigger the strength acts on them. Lifting up, bubbles fall into less heated layers of liquid; Couples are condensed in them, and bubbles are compressed again. The collapse of bubbles causes the noise to us preceding the boiling of the kettle. Finally, over time, all fluid is evenly warming up, bubbles reach the surface and burst, throwing out the outside of air and steam - the noise is replaced by bouffaging, fluid boils.
The bubbles, thus, serve as "conductors" of steam from inside a liquid onto its surface. When boiling, along with conventional evaporation, there is a conversion of a liquid into pairs throughout the volume - evaporation inside the air bubbles followed by the output of the steam outward. That is why the boiling liquid will fly very quickly: a kettle, from which water would evaporate a lot of days, will pop up for half an hour.
Unlike evaporation, occurring at any temperature, the liquid begins to boil only when reached boiling temperatures - It is the temperature at which air bubbles turn out to be able to float and get to the surface. At boiling temperature, the saturated pair pressure becomes equal to external pressure on the liquid (in particular, atmospheric pressure). Accordingly, the more external pressure, the boiling will begin at a higher temperature.
With normal atmospheric pressure (ATM or PA), the boiling point of water is equal. therefore the pressure of saturated water vapor at a temperature equals PA. This fact needs to be known to solve problems - often it is considered to be known by default.
At the top of Elbrus, the atmospheric pressure is equal to ATM, and water will boil there at temperatures. And under pressure ATM, the water will start boiled only at.
The boiling point (at normal atmospheric pressure) is strictly defined for this fluid value (the boiling point, cited in tables of textbooks and reference books, is the boiling points of chemically clean liquids. The presence of impurities in the fluid can change the boiling point. Let's say tap water contains dissolved chlorine and Some salts, so its boiling point at normal atmospheric pressure may differ slightly from). So, the alcohol is boiling at, ether - with mercury - at. Please note: the more volatile is fluid, the lower its boiling point. In the boiling point table, we also see that oxygen boils at. So, at ordinary temperatures, oxygen is gas!
We know that if you remove the kettle from the fire, then the boil will immediately stop - the boiling process requires continuous heat supply. At the same time, the temperature of the water in the kettle after boiling ceases to change, remaining equal all the time. Where does the resulting heat go?
The situation is similar to the melting process: heat goes to an increase in the potential energy of molecules. In this case, to perform work to remove molecules at such distances that the attraction forces will be unable to retain the molecules near each other, and the liquid will switch to the gaseous state.
Chart of boiling
Consider a graphical representation of the fluid heating process - the so-called chart of boiling (Fig. 4).
Fig. 4. Chart of boiling
The site precedes the start of boiling. In the area, fluid boils, its mass decreases. At the point, the liquid rolls completely.
To pass the site, i.e. So that the fluid brought to the boiling point is completely turned into pairs, it must be reduced to a certain amount of heat. Experience shows that this amount of heat is directly proportional to the mass of the liquid:
The ratio of proportionality is called specific warmer vaporization Fluids (at boiling point). The specific heat of the vaporization is numerically equal to the amount of heat that needs to be brought to 1 kg of liquid taken at the boiling point to completely turn it into pairs.
So, with the specific heat of the water vaporization is equal to KJ / kg. It is interesting to compare it with the specific heat of melting ice (KJ / kg) - the specific heat of the vaporization almost seven times more! This is not surprising: because for melting ice you only need to destroy the ordered location of water molecules in the nodes of the crystal lattice; At the same time, the distances between molecules remain about the same. But for the conversion of water to steam, you need to make much more work on the rupture of all links between molecules and the removal of molecules at considerable distances from each other.
Condensation schedule
The process of condensation of steam and the subsequent cooling of the fluid looks on the graph symmetrically the process of heating and boiling. Here is the corresponding condensation schedule For the case of the graduate water vapor, which is most common in tasks (Fig. 5).
Fig. 5. Condensation schedule
At the point we have water steam at. There is condensation on the site; Inside this section is a mixture of steam and water at. At the point there is no more couple, there is only water at. The plot is the cooling of this water.
Experience shows that when condensing a pair of mass (i.e., when the site passing), exactly the same amount of heat, which was spent on the transformation of mass fluid at a temperature of the fluid.
Let's compare the following amounts of warmth:
Which is distinguished by condensation of a water vapor;
which stands out when cooled of the resulting graduate water to the temperature, say ,.
J;
J.
These numbers clearly show that the burn of the ferry is much worse than burn boiling water. If the skin gets, boiling water is allocated "only" (boiling water cools). But with a ferry burn, you will first stand out for an order of magnitude more heat (steam condensed), it is formed a graduate water, after which the same value is added when the water is cooled.
