Evaporation

Evaporation over a mug of tea

Evaporation - the process of transition of the substance from liquid state In the gaseous, occurring on the substance surface (pairs). The evaporation process is a condensation inverse process (a transition from a vapor state into liquid). Evaporation (vaporization), transition of a substance from condensed (solid or liquid) phase into gaseous (pairs); Phase transition of the first kind.

There is a more disclosed concept of evaporation in higher physics.

Evaporation - This is a process at which the particles (molecules, atoms) are flying from the surface of the liquid or solid body, and the E K\u003e E p.

general characteristics

The evaporation of the solid is called sublimation (sublimation), and the vaporization in the volume of liquid is boiling. Usually, under evaporation, vaporization on the free surface of the fluid as a result of thermal motion of its molecules at a temperature below the boiling point corresponding to the pressure of the gas medium located above the specified surface. At the same time, molecules with sufficiently large kinetic energy are pulled out of the surface layer of fluid into the gas medium; Some of them are reflected back and captured with liquid, and the rest is irretrievably lost.

Evaporation - an endothermic process, in which the heat of the phase transition is absorbed - the heat of evaporation, spent on overcoming the molecular clutch forces in the liquid phase and to work the expansion when the liquid turns into steam. The specific heat of evaporation refers to 1 praying fluid (molar heat of evaporation, J / mol) or to a unit of its mass (mass heat of evaporation, j / kg). The evaporation rate is determined surface density A pair of jP, penetrating a unit of time into the gas phase from a unit of liquid surface [in mol / (SM 2) or kg / (sm 2)]. The greatest value JP is achieved in vacuo. In the presence of a relatively dense gas medium, evaporation slows down due to the fact that the removal rate of steam molecules from the surface of the liquid into the gas medium becomes small compared with the velocity vehicle. In this case, the surface of the phase section is formed by a layer pAROGAZONAY MOVEY, practically saturated by steam. The partial pressure and the concentration of steam in this layer is higher than in the bulk of the vapor-gas mixture.

The evaporation process depends on the intensity of the heat movement of molecules: the faster the molecule is moving, the faster there is evaporation. In addition, important factors affecting the evaporation process are the rate of outer (with respect to the substance) diffusion, as well as the properties of the substance itself. Simply put, with wind, evaporation is much faster. As for the properties of the substance, then, for example, the alcohol evaporates much faster than water. An important factor It is also an area of \u200b\u200bliquid surface with which evaporation occurs: from a narrow decline it will occur more slowly than from a wide plate.

Molecular level

Consider this process on molecular level: Molecules with sufficient energy (speed) to overcome the attraction of adjacent molecules, break over the boundaries of the substance (liquid). At the same time, the fluid loses part of its energy (cooled). For example, a very hot liquid: we blow on its surface to cool down, while we accelerate the evaporation process.

Thermodynamic equilibrium

The violation of the thermodynamic equilibrium between the liquid and the steam contained in the vapor-gas mixture is explained by the temperature jump on the border of the phase partition. However, usually this jump can be neglected and taking that the partial pressure and the concentration of steam at the phase surface correspond to their values \u200b\u200bfor a saturated pair having a surface temperature of the liquid. If the liquid and the steamed mixture are still and the effect of free convection in them is insignificant, the removal of the vapor formed during the evaporation from the surface of the fluid into the gas medium is mainly due to the molecular diffusion and the appearance of the separated surface of the phase separation (so-called) surface Stephanovsky) The flow of a vapor-gas mixture directed from the surface of the liquid into the gas medium (see diffusion). Distribution of temperatures at various modes of evaporative cooling of the liquid. Heat flows are directed: a - from the liquid phase to the surface of evaporation into the gas phase; b - from the liquid phase only to the surface of evaporation; in - to the surface of evaporation from both phases; G - to the surface of evaporation only by the gas phase.

Baro, thermal diffusion

The effects of bar-and thermal diffusion in engineering calculations are usually not taken into account, but the effect of thermal diffusion can be essential at high heterogeneity of the vapor-gas mixture (with a large difference molar masses its components) and significant temperatures. When moving one or both phases relative to the surface of their section, the role of convective transfer of the substance and the energy of the vapor-gas mixture and liquid increases.

In the absence of supply of energy to the liquid-gas system from external. Heat sources Evaporation can be supplied to the surface layer of fluid from one or both phases. In contrast to the resulting flow of a substance, always directed during evaporation from fluid into the gas medium, heat flows may have different areas Depending on the ratio of temperature of the main mass of the liquid TG, the boundaries of the phase section of the TGR and the gas environment Tg. When contacting a certain number of liquids with a semi-infinite volume or wash it with a flow of a gas medium and at a temperature of a liquid, higher than the gas temperature (TG\u003e TGR\u003e TG), the heat of heat from the fluid side to the surface of the phase partition occurs: (qu) Q: Q: Q: Aggregate, quits - the amount of heat transmitted from the liquid of the gas environment. At the same time, the liquid is cooled (the so-called evaporative cooling). If the equality of TGR \u003d TG is achieved, the heat transfer from the liquid to the gas is stopped ( QuI \u003d 0) and all the heat supplied by the liquid to the surface of the section is spent on evaporation (quantity \u003d Q.).

In the case of a gas medium, not saturated with steam, the partial pressure of the latter in the surface of the phase section and when queries \u003d Q and remains higher than in the bulk of the gas, as a result of which the evaporation and evaporative cooling of the liquid are not stopped and the TGR becomes lower than TG and TG. At the same time, the heat is supplied to the surface of the section from both phases until the equality of TGR \u003d TG is achieved and the heat of heat from the fluid side is stopped, and from the gas environment, it becomes equal to Q. Further evaporation of the fluid occurs at a constant temperature TM \u003d TG \u003d TGR, which is called the freight cooling limit during evaporating cooling or temperature of the wet thermometer (since it shows the wet thermometer of the psychoometer). The TM value depends on the parameters of the vapor-gas medium and the conditions of heat and mass transfer between the liquid and gas phases.

If the liquid and gas medium having different temperatures are in a limited volume that does not receive energy from outside and does not send it outward, evaporation occurs until the thermodynamic equilibrium occurs between the two phases, at which the temperatures of both phases are equalized with the system unchanged enthalpy , and the gas phase is saturated with steam at the temperature of the TAD system. The latter is called the temperature of the adiabatic gas saturation, it is determined only by the initial parameters of both phases and does not depend on the conditions of heat and mass transfer.

Evaporation rate

The rate of isothermal evaporation [kg / (m 2 (c)] with a unidirectional vapor diffusion into a fixed layer of a binary vapor-gas mixture with a thickness D, [M] can be found according to the Stephen formula:, where D is the mutual diffusion coefficient, [m 2 /from]; - gas constant, [J / (kg to)] or [m 2 / (C 2 K)]; T - the temperature of the mixture, [k]; p - pressure of the vapor-gas mixture, [pa]; - Partial vapor pressure on the surface of the section and on the outer border of the layer of the mixture, [Pa].

In general, (moving fluid and gas, non-erotic conditions) in the phase adjacent to the surface of the phase, the pulse transfer is accompanied by the transfer of heat, and in the border layer of gas (vapor-gas mixture), interconnected heat and mass transfer occur. At the same time, experimental coefficients of heat and mass studies are used to calculate the velocity of the evaporation, and in relatively more simple cases - Approximate methods of numerical solutions of the system of differential equations for the conjugate boundary layers of gas and liquid phases.

The intensity of mass transfer during evaporation depends on the difference in the chemical potentials of steam in the surface of the section and in the bulk of the vapor-gas mixture. However, if the baro- and thermal diffusion can be neglected, the difference in chemical potentials is replaced by the difference in partial pressures or vapor concentrations and accept: JP \u003d BP (RP, GR - RP, OSN) \u003d BPR (UE, GR - UP, OSN) or JP \u003d BC ( SP, GR - SP, OSN), where BP, BC is the mass transfer coefficient, P is the pressure of the mixture, RP is a partial pressure of steam, yp \u003d pp / p - molar concentration of vapor, SP \u003d RP / R is a massive concentration of vapor, RP, R - local vapor densities and mixtures; The indices mean: "GR" - at the border of the phase section, "OSN" - in the OSN. Masse mix. The heat flux density given during the evaporation of the liquid is [in j / (m2 c)]: Q \u003d Age (TG - TGR) \u003d RJP + GG (TGR - TG), where AG, AG - the heat transfer coefficient by fluid and gas , [W / (m 2 k)]; R - Heat evaporation, [J / kg].

With very small radii, the curvature of the evaporation surface (for example, when evaporating small drops of liquid), the effect of the surface tension of the fluid leading to the fact that the equilibrium pressure of steam above the surface of the section above the pressure of saturated vapor of the same fluid above the flat surface is taken into account. If TGR ~ TZ, then only heat and mass exchange in the gas phase may be taken into account when calculating evaporation. With a relatively low intensity of mass transfer approximately, an analogy between the processes of heat and mass transfer, from which it follows: NU / NU0 \u003d SH * / SH0, where Nu \u003d g L / LG is the number of nusselt, L is the characteristic size of the evaporation surface, LG - thermal conductivity coefficient PAROUSE MIXES, SH * \u003d BYYG, GRL / DP \u003d BCCG, GRL / D is the number of sherwood for the diffusion component of the flow of steam, Dp \u003d D / RPT diffusion -efficient, referred to the gradient of the partial pressure of steam. The BP and BC values \u200b\u200bare calculated according to the above relations, the NU0 and SH0 numbers correspond to JP: 0 and can be determined according to data for separately occurring heat and mass transfer processes. The number of SH0 for the total (diffusion and convective) pair flow is found by the division sh * on the molar (yg, gr) or mass (SG, GR) the gas concentration at the surface of the section depending on which driving force Mass transferred coefficient b.

Equations

Equations of the similarity for NU and SH * in evaporation include besides conventional criteria (Reynolds Reynolds numbers, Archimede Ar, Prandtl PR or SC SC and GEOM. Parameters) Parameters that take into account the effect of the transverse flux of steam and the degree of heterogeneity of the vapor-gas mixture (the ratio of molar mass or gas constant its components) on profiles, speed, temperature or concentrations in the section of the border layer.

At small JP, which do not disturb the substantially hydrodynamic mode of movement of the vapor-gas mixture (for example, during evaporation of water into atmospheric air) and the similarity of the boundary conditions of temperature fields and concentrations, the influence of additional arguments in the similarity equations is insignificant and they can be neglected, taking that Nu \u003d SH. When evaporation of multicomponent mixtures, these regularities are greatly complicated. In this case, the heat of evaporation of the components of the mixture and the compositions of the liquid and vapor-gas phases, which are among themselves in equilibrium, are different and depend on temperature. When evaporated with a binary liquid mixture, the resulting mixture of vapors in a relatively richer more volatile component, excluding only azeotropic mixtures, evaporate at extremum points (maximum or minimum) of the state curves as clean liquid.

Designs of apparators

The total amount of evaporating fluid increases with an increase in the surface of the contact of the liquid and gas phases, so the design of the devices in which evaporation occurs, an increase in the surface of the evaporation by creating a large mirror of the liquid, crushing it on the jet and droplets or the formation of thin films flowing along the surface of the nozzles. The increase in the intensity of heat and mass transfer during evaporation is also achieved by an increase in the rate of the gas medium relative to the surface of the fluid. However, the increase in this velocity should not lead to excessive liquid under the gas medium and a significant increase in the hydraulic resistance of the device.

Application

Evaporation is widely used in industrial practices for cleaning substances, drying materials, separation of liquid mixtures, air conditioning. Evaporative cooling Water is used in current water supply systems of enterprises.

see also

Literature

• // Encyclopedic Dictionary of Brockhaus and Efron: in 86 volumes (82 tons and 4 extra). - St. Petersburg. , 1890-1907.
• Berman L. D., evaporative cooling of circulation water, 2 ed., M.-L., 1957;
• Fuchs N. A., evaporation and growth of drops in the gaseous medium, M., 1958;
• Berd R., Stuart V., Lightfoot E., Transfer phenomena, trans. from English, M., 1974;
• Berman L. D., "Theoretical Basics of Chem. Technologies, 1974, T.8, No. 6, p. 811-22;
• Sherwood T., Pigford R., Willow, C., Mass transfer, lane. from English, M., 1982. L. D. Berman.

Links

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Synonyms:

Watch what is "evaporation" in other dictionaries:

Transition to va from a liquid or solid aggregate state in gaseous (pairs). Usually under I. understand the transition of fluid into steam, occurring on the free surface of the fluid. I. solid bodies called. sublimation or sublimation. Pressure dependence ... ... Physical encyclopedia

Variousness occurring on a free liquid surface. Evaporation from the surface of the solid is called sublimation ... Large encyclopedic Dictionary

Burning liquids

The combustion of liquids is characterized by two interrelated phenomena - evaporation and combustion of the steam-air mixture above the surface of the fluid. Consequently, the combustion of liquids is accompanied not only chemical reaction (oxidation, turning into flame burning), but also physical phenomena (evaporation and formation over the surface of the liquid of the pair-air mixture), without which the combustion is impossible.

The transition of a substance from a liquid state in vapor-shaped is called vaporization.There are two forms of this process: evaporation and boiling. Evaporation - This is a liquid transition to pairs from a free surface at temperatures below the boiling point of the liquid (see Fig. 4.1). Evaporation occurs as a result of thermal motion of fluid molecules. The speed of movement of molecules varies widely, strongly deviating in both directions from its average value. A part of molecules having a sufficiently large kinetic energy is pulled out of the surface layer of fluid into the gas (air) medium. The excessive energy of the liquid lost molecules is spent on overcoming the interaction forces between molecules and the operation of expansion (increase in volume) during fluid transition to steam. Boiling - This evaporation is not only from the surface, but also from the volume of fluid by forming steam bubbles throughout the volume and the allocation of them. Evaporation is observed at any fluid temperature. Boiling occurs only at a temperature at which the saturated pair pressure will reach the value of an external (atmospheric) pressure.

At the expense of the Brownian movement in the gas zone there is a reverse process - condensation. If the volume above the liquid is closed, then at any temperature of the fluid, a dynamic equilibrium is established between the processes of evaporation and condensation.

Couples, located in equilibrium with liquid, is called a saturated ferry. The equilibrium state corresponds to a pair concentration defined for this temperature. Couple pressure in equilibrium with liquid is called pressure of saturated steam.

Fig. 4.1. Fluid evaporation scheme in: a) open vessel, b) closed vessel

The pressure of a saturated pair (r N.P.) of this fluid at a constant temperature is the magnitude of constant and unchanged for it. The magnitude of the saturated steam pressure is determined by the temperature of the fluid: with increasing temperature, the pressure of the saturated pair increases. This is due to the growth of the kinetic energy of fluid molecules with an increase in temperature. At the same time, an increasing fraction of molecules turns out to have an energy sufficient to go to par.

Thus, over the surface (mirror) of the liquid, there is always a steam-air mixture, which in the equilibrium state is characterized by the pressure of saturated vapor vapor or their concentration. With increasing temperature, the pressure of saturated vapors increases according to the Clayperon-Claziusa equation:

, (4.1)

or in integrated form:

, (4.2)

where r n.p. - the pressure of a saturated pair, Pa;

DN is the heat of evaporation, then the amount of heat that is necessary for transfer to the vapor state of the unit of mass of fluid, KJ / mol;

T - Fluid temperature, K.

The concentration of a saturated pair of the bottom surface of the fluid is associated with its pressure by the ratio:

. (4.3)

From (4.1 and 4.2) it follows that with increasing fluid temperature, the pressure of saturated vapor (or their concentration) increases exponentially. In this regard, at a temperature above the surface of the liquid, a vapor concentration is created equal to the lower concentration limit Flame spread. This temperature is called the lower temperature limit of the flame propagation (NTRP).

Therefore, for any liquid, there is always such a temperature range, in which the concentration of saturated vapor over the mirror will be in the field of ignition, that is, HKPrp £ J n £ BCPRP.

The process of intensive evaporation of the fluid begins at a temperature when the elasticity of the liquid will exceed the external pressure of the gas atmosphere above the liquid. At boiling point, the formation of the steam is in the entire mass of the liquid and flows almost at a constant temperature to the total transition of the liquid (one-component) and steam. Artificially lowering pressure, you can force fluid to boil at lower temperatures than widely use in the technique, since it is easier to find for working at low temperatures suitable material For equipment. Modern vacuum technique has at its disposal powerful rotary pumps capable of creating a vacuum at which the residual pressure does not exceed 0.001 mm RT Art., And inkjet diffusion pumps that create a vacuum to 10V-7-10V-8 MM RT. Art.
Distillation in vacuo is used to obtain high purity metals; Zn, Cd, Mg, Ca, etc. Usually work at pressures, slightly exceeding the elasticity of a pair of distilled metal at the point of its melting. Then distilling liquid metal, it is obtained solid condensate, which allows you to apply very simple design Distillation instrument shown in Fig. 24. The device represents a cylinder, at the bottom of which a vessel with a liquid distilled metal is located. The pairs are condensed in the upper part of the cylinder on a special composite metal cylinder (condenser) in the form of a crystalline peel, which, after the end of the process, is extracted along with the condenser. Before heating, the metal first vacuum pump The air is pumped out of the device, and then from time to time the vacuum is restored due to the flow of air from the outside through the poorness of the instrument. If the device is sealed enough, then in the process of distillation, because at the same time non-condensable gases are not highlighted, the permanent operation of the vacuum pump is not needed.

The device described is extremely simple, it is made of heat-resistant metal alloys from the steel. Which is especially important, its cover and all sealing - sealing parts are cooled with water, i.e. it work room temperatureallowing the use of very perfect seals - rubber, vacuum maps, etc. The use of vacuum allows to clean with distillation at relatively low temperatures (700-900 °) such chemically active and very aggressive metals, such as calcium, magnesium, barium, distillation with atmospheric pressure is impracticable due to the impossibility of selecting material for equipment.
Consider the features of the evaporation process in vacuum.
The state diagram of liquid - pairs with a decrease in pressure have the same nature as the charts for atmospheric pressure, only the liquid lines and the pair are moved to the lower temperature area. It follows that the effectiveness of the separation of components during evaporation of their solution in vacuum is approximately the same as at atmospheric pressure, but is carried out at lower temperatures; The temperature is lower than the deeper the vacuum used. The peculiarity of the work in vacuum is the lack of deposit of small droplets of the liquid together with pairs, always observed when working under atmospheric pressure. With a rapid boiling of liquid, the blades of a pair of fluid rising from the depth of fluid give splashes that are carried out in the capacitor and contaminate distillate. In vacuum (deep enough), the formation of splashes does not occur, since the boiling process is radically different from boiling at atmospheric pressure. In vacuum, the formation of the steam is only on the surface of the liquid, bubbles inside the liquid are not formed, the surface is calm, not boil, therefore, the splashes may occur. Therefore, vacuum distillation gives a cleaner distillate than distillation at atmospheric pressure.
We show on the example the peculiarity of the boiling process in vacuo. Let in one case the water in the vessel with a depth of 250 mm layer boils at an atmospheric pressure (760 mm Hg. Art.). Then the steam, released from the surface of the water, to overcome the external pressure should have an atmospheric pressure (760 mm Hg. Art.), Which develops at the temperature of the water surface of 100 °. The vapor bubble forming at the bottom of the vessel should have greater pressure, since, in addition to the pressure of the atmosphere, it needs to overcome the hydrostatic pressure of the water post 250 mm height, which corresponds to an excess of pressure in 18 mm RT. Art. Thus, the steam standing from the bottom of the vessel should have a pressure of 760 + 18 \u003d 778 mm Hg. st .. What corresponds to the temperature of the water at the bottom of the vessel 100.6 °. Such a slight overheating of water at the bottom (0.6 °) is quite real, and the boiling process goes so that the steam is formed in the entire mass of the layer. Water vigorously boils. And forms splashes when the bubbles are destroyed on the surface.
Now consider the boil of the same layer of water in vacuum 4.58 mm Hg. Art. To boil the surface layer of water should have a temperature of 0 °, at which the elasticity of the saturated steam is equal to 4.58 mm Hg. Art. The bubble forming at the bottom should overcome the hydrostatic pressure of the water column in 250 mm, which corresponds to the pressure of 18 mm RT. Art., and have a total pressure of 4.58 + 18 \u003d 22.58 mm Hg. Art. Such a saturated steam pressure will have at a temperature of ~ 23 °, i.e. so that the pair bubble can be formed at the bottom of the vessel, it is necessary to have a temperature of 23 ° at the bottom. It is impossible to obtain such a difference between the temperatures at the bottom and on the surface, since it will prevent convection currents. Consequently, bubbles in the depth of the layer of liquid will not be formed and the vaporization will be carried out only from the surface of the liquid.
Metal melts have a high thermal conductivity that prevents local fluid overheating, and therefore boiling to form bubbles.
While the pressure in the device does not become very small, molecules are met between the surface of the liquid and the ferry and the movable equilibrium fluid is established - steam. The condensant flows the usual gas stream of steam and the results of the distillation process are determined by the diagram of the state of the liquid - steam.
If the pressure in the device is so small that the length of the free run of molecules becomes more sizes The device, the nature of the distillation process radically changes.
Under these conditions, no exchange of molecules between pairs and liquid, there is no mobile equilibrium fluid - steam is not installed and the state diagram of the liquid - pairs does not describe the evaporation process. Conventional gas strap between evaporator and capacitor. HE is formed, the steam molecule, separated from the liquid surface, follow the straight path, without colliding with other molecules, fall on the cold surface of the capacitor and remain there - condensed; The evaporation process will not fully turn and has a character of molecular evaporation. The result of distillation is determined by the rate of evaporation, depending on the genus evaporated substance and temperature and non-external pressure in the system, if this pressure is quite small. The evaporation rate under these conditions can be calculated using the Langmuir formula:

Accepting a mass of a substance evaporated for a second from a unit of surface, expressing the elasticity of vapor P in millimeters of a mercury pillar and replacing the values \u200b\u200bof R and π them with numerical values, obtain equation (III, 13) in a different form, convenient for practical calculations:

In molecular evaporation, substances can be separated with the same elasticity of steam if their molecular weights are different, which has been proven by experiments on the separation of isotopes.

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\u003e\u003e Physics: The dependence of the pressure of saturated steam on temperature. Boiling

Liquid not only evaporates. At some temperature she boils.
The dependence of the pressure of saturated steam on temperature. The state of a saturated pair, as experience shows (we talked about it in the previous paragraph), is approximately described by the equation of the state of the ideal gas (10.4), and its pressure is determined by the formula

With increasing temperature, pressure grows. As the pressure of the saturated pair does not depend on the volume, then, therefore, it depends only on temperature.
However, addiction r N.P. from T.The experimentally found is not directly proportional to the perfect gas at a constant volume. With increasing temperature, the pressure of the real saturated pair grows faster than the pressure of the perfect gas ( fig.11.1, Plot Krivoy AU). This becomes apparent if you have a perfect gas isoohra through points BUT and IN (bar straight). Why is this happening?

When the fluid is heated in a closed vessel, part of the fluid turns into steam. As a result, according to formula (11.1) the pressure of the saturated steam is growing not only due to the increase in fluid temperature, but also due to an increase in the concentration of molecules (density). Basically, the increase in pressure at an increase in temperature is determined precisely an increase in concentration. The main difference in the behavior of the perfect gas and a saturated pair is that when the temperature change in the temperature is changed in the closed vessel (or when the volume is changed at a constant temperature), the mass of steam changes. The liquid partially turns into pairs, or, on the contrary, the pairs are partially condensed. With perfect gas, nothing like this happens.
When the whole liquid evaporates, steam with further heating will cease to be saturated and its pressure at a constant volume will increase directly in proportion to the absolute temperature (see. fig.11.1, Plot Krivoy Sun).
. As the fluid temperature increases, the evaporation intensity increases. Finally, the liquid begins to boil. When boiling throughout the volume of the liquid, rapidly growing steam bubbles are formed, which float to the surface. The boiling point of the liquid remains constant. This is because all the energy supplied to the fluid is spent on turning it into pairs. Under what conditions does boiling begins?
In the liquid there are always dissolved gases that are released at the bottom and walls of the vessel, as well as on the drains weighted in the liquid, which are vaporization centers. Pair of liquids inside bubbles are saturated. With increasing temperature, the pressure of saturated vapors increases and bubbles increase in size. Under the action of the pushing force, they pop up. If the upper layers of fluid have more low temperatureIn these layers, steam condensation occurs in bubbles. Pressure falls rapidly, and bubbles slam. The slamming occurs so quickly that the walls of the bubble, facing something like an explosion. Many such microcrust creates a characteristic noise. When the fluid warms down enough, the bubbles will stop slapping and surfaced to the surface. Liquid boils. Watch carefully behind the kettle on the stove. You will find that before boiling he almost ceases to noise.
The dependence of the pressure of a saturated steam on temperature explains why the boiling point of the fluid depends on the pressure on its surface. A pair bubble can grow when the saturated pair pressure inside it slightly exceeds the pressure in the liquid, which is from the pressure of the air to the surface of the fluid (external pressure) and the hydrostatic pressure of the liquid column.
We draw attention to the fact that the evaporation of the fluid occurs at temperatures smaller than boiling points, and only from the surface of the liquid, when the formation of the steam is boiling throughout the volume of the fluid.
Boiling begins at a temperature at which the saturated vapor pressure in bubbles is compared with pressure in the liquid.
The more external pressure, the higher the boiling point. So, in a steam boiler at a pressure, reaching 1.6 10 6 Pa, water does not boil and at a temperature of 200 ° C. IN medical institutions In hermetically closed vessels - autoclaves ( fig.11.2) boiling water also occurs when increased pressure. Therefore, the boiling point of the liquid is significantly higher than 100 ° C. Autoclaves are used for sterilization surgical instruments and etc.

And vice versa, reducing the external pressure, we are therefore lowering the boiling point. Pumped out the air and a pair of water from the flask, you can get the water boiled at room temperature ( fig.11.3). When lifting the mountains, the atmospheric pressure decreases, therefore the boiling point decreases. At an altitude of 7134 m (Lenin Peak on the Pamir), the pressure is approximately 4 10 4 Pa \u200b\u200b(300 mm Hg. Art.). Water boils there at about 70 ° C. Cook meat under these conditions is impossible.

Each liquid has its own boiling point, which depends on the pressure of its saturated steam. The higher the pressure of the saturated steam, the lower the boiling point of the fluid, since at lower temperatures the saturated pair pressure becomes equal to atmospheric. For example, at a boiling point of 100 ° C, the pressure of saturated vapor of water is 101 325 Pa (760 mm Hg. Art.), And mercury vapors are just 117 Pa (0.88 mm Hg. Art.). Rotume boils at a temperature of 357 ° C at normal pressure.
The liquid boils when the pressure of its saturated pair becomes equal to the pressure inside the liquid.

???
1. Why increase the boiling temperature with increasing pressure?
2. Why is it to boil a significant increase in the pressure of saturated vapor in bubbles, and not an increase in the pressure of the air available in them?
3. How to make the liquid boil, cooling the vessel? (This question is difficult.)

G. Y. Mikishev, B.B. Bukhovtsev, N.N.Sotsky, Physics 10

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Have you ever left a bottle of water for a few hours under the scorching sun and heard the "hissing" sound, opening it? This sound is caused by steam pressure. In chemistry, the pressure of the steam is the pressure rendered by a liquid vapor, which evaporates in hermetically closed vessel. To find the pressure of steam at this temperature, use the Klapairone Clauses equation :.

Steps

Using Klapairone Clausius equation

Record the Klapairone Clausius equation, which is used to calculate the pair pressure when it changes over time. This formula can be used in most physical and chemical tasks. The equation is as follows: ln (p1 / p2) \u003d (ΔH VAP / R) ((1 / T2) - (1 / T1))Where:

Submold to the values \u200b\u200bof values \u200b\u200bof values \u200b\u200bin the valuation equation. Most tasks are given two temperature values \u200b\u200band pressure value or two pressure values \u200b\u200band temperature value.

• For example, in a vessel is fluid at a temperature of 295 K, and its vapor pressure is 1 atmosphere (1 atm). Find the pressure of vapors at a temperature of 393 K. There are two temperature values \u200b\u200band pressure value, so you can find another pressure value using the Klapairone-Clausius equation. Substituting these values \u200b\u200bin the formula, you will receive: ln (1 / P2) \u003d (ΔH VAP / R) ((1/393) - (1/295)).
• Please note that in the Klapairone-Clausius equation, the temperature is always measured in Kelvin, and the pressure in any units of measurement (but they should be the same for P1 and P2).

1. Substitute constants. The Klapairone Clausius equation contains two constants: R and ΔH Vap. R is always equal to 8.314 J / (K × mol). The value of ΔH VAP (evaporation enthalpy) depends on the substance, the pressure of the steam of which you are trying to find; This constant, as a rule, can be found in the table in textbooks in chemistry or on sites (for example,).

   • In our example, we assume that water is in the vessel. ΔH VAP water is equal to 40.65 kJ / mol or equal to 40650 J / mol.
   • Substitute constants in the formula and get: ln (1 / p2) \u003d (40650/8314) ((1/393) - (1/295)).

2. Decide the equation with the help of algebraic operations.

   • In our example, an unknown variable is under the sign of natural logarithm (LN). To get rid of the natural logarithm, turn both sides of the equation to the degree of mathematical constant "E". In other words, ln (x) \u003d 2 → E ln (x) \u003d e 2 → x \u003d e 2.
   • Now decide the equation:
   • ln (1 / p2) \u003d (40650 / 8,314) ((1/393) - (1/295))
   • ln (1 / P2) \u003d (4889.34) (- 0.00084)
   • (1 / P2) \u003d E (-4.107)
   • 1 / P2 \u003d 0.0165
   • P2 \u003d 0.0165 -1 \u003d 60.76 atm. It makes sense, since the increase in temperature in a hermetically closed vessel by 100 degrees will lead to an increase in vaporization, which will significantly increase the pressure of steam.

   Calculation of the pressure of steam in solutions

   1. Write down the law of Raul. IN real life Clean fluids are rarely found; Often we are dealing with solutions. The solution is obtained by adding a small amount of a certain chemical called "dissolved substance", in large quantity Another chemical called "solvent". In cases of solutions, use Raoul's Law:, where:

      • P Solution - pressure of the solution of solution.
      • P Solvent - Solvent Vapor Pressure.
      • X The solvent is a molar proportion of the solvent.
      • If you do not know what a "mole share" is, read on.

   2. Determine which substance will be a solvent, and any dissolved substance. Recall that the dissolved substance is a substance dissolved in a solvent, and the solvent is a substance dissolving the dissolved substance.

      Find the temperature of the solution, as it will affect the pressure of its pair. The higher the temperature, the higher the pressure of the vapor, since steam formation increases with increasing temperature.

      • In our example, it is assumed that the temperature of the syrup is 298 K (about 25 ° C).

   3. Find the pressure of the solvent vapor. In chemistry reference books, the values \u200b\u200bof many common vapors are given chemical substancesBut, as a rule, such values \u200b\u200bare given at temperatures of substances at 25 ° C / 298 K or at their boiling temperatures. If you have such temperatures in the task, use values \u200b\u200bfrom reference books; in otherwise You need to calculate the pressure of vapors at this temperature of the substance.

      Find the molar proportion of the solvent. To do this, find the ratio of the number of moles of the substance to total number Moles of all substances that are in solution. In other words, the molar proportion of each substance is equal to (the number of moles of the substance) / (the total number of moles of all substances).

   4. Now substitute the data and the found values \u200b\u200bin the Raoul equation, which is in the beginning of this section ( P Solution \u003d P Solvent X Solvent).

      • In our example:
      • P solution \u003d (23.8 mm Hg. Art.) (0,947)
      • P solution \u003d 22.54 mm Hg. Art. It makes sense because large quantities Waters dissolved a small amount of sugar (if measured in a moles; in liters, their number is the same), so the pressure of vapors will slightly decrease.

   Calculation of steam pressure in special cases

   1. Definition of standard conditions. Often, temperature and pressure values \u200b\u200bare used in chemistry as a kind of default values. Such values \u200b\u200bare called standard temperatures and pressure (or standard conditions). In tasks for the pressure of steam, standard conditions are often mentioned, so it is better to remember the standard values:

      • Temperature: 273.15 k / 0˚C / 32 F
      • Pressure: 760 mm Hg / 1 atm. / 101,325 kPa

   2. Rewrite the Klapairone Clausius equation so as to find other variables. The first section of this article showed how to calculate the pressure of the vapor of clean substances. However, not all tasks are required to find P1 or P2 pressure; Many tasks need to calculate the temperature or value of ΔH Vap. In such cases, rewrite the Klapairone-Clausius equation, which makes the unknown value on one side of the equation.

      • For example, an unknown liquid is given, the pair pressure of which is 25 Torr at 273 K and 150 Torr at 325 K. It is necessary to find the enthalpy of evaporation of this fluid (that is, ΔH Vap). Solving this problem:
      • ln (P1 / P2) \u003d (ΔH VAP / R) ((1 / T2) - (1 / T1))
      • (Ln (P1 / P2)) / ((1 / T2) - (1 / T1)) \u003d (ΔH VAP / R)
      • R × (Ln (P1 / P2)) / ((1 / T2) - (1 / T1)) \u003d ΔH Vap Now submold you the values:
      • 8,314 J / (k × mol) × (-1,79) / (- 0.00059) \u003d ΔH Vap
      • 8,314 J / (k × mol) × 3033,90 \u003d ΔH Vap \u003d 25223,83 J / mol

   3. Taking into account the pressure of the pair of the dissolved substance. In our example, from the second section of this article, a dissolved substance - sugar - does not evaporate, but if the dissolved substance produces pairs (evaporated), the pressure of such a pair should be considered. To do this, use the modified species of the Raoul equation: P solution \u003d σ (p substance X substance), where the symbol σ (sigma) means that it is necessary to add the values \u200b\u200bof the pressures of the vapor of all substances from which the solution consists.

      • For example, consider a solution consisting of two chemicals: benzene and toluene. Total solution of 120 milliliters (ml); 60 ml of benzene and 60 ml of toluene. The temperature of the solution is 25 ° C, and the pressure of the vapor at 25 ° C is 95.1 mm Hg. For benzene and 28.4 mm Hg.st. For toluene. It is necessary to calculate the pressure of the solution vapor. We can do it with the help of densities of substances, their molecular masses and vapor pressure values:
      • Mass (benzene): 60 ml \u003d 0.06 l × 876,50 kg / 1000 l \u003d 0.053 kg \u003d 53 g
      • Mass (toluene): 0.06 l × 866.90 kg / 1000 l \u003d 0.052 kg \u003d 52 g
      • Mole (benzene): 53 g × 1 mol / 78.11 g \u003d 0,679 mol
      • Mole (toluene): 52 g × 1 mol / 92.14 g \u003d 0.564 mol
      • Total number of moles: 0,679 + 0.564 \u003d 1,243
      • Moled share (benzene): 0.679 / 1,243 \u003d 0.546
      • Moled share (toluene): 0.564 / 1,243 \u003d 0,454
      • Solution: P solution \u003d P benzene x benzene + p toluene x toluene
      • P solution \u003d (95.1 mm Hg. Art.) (0.546) + (28.4 mm Hg. Art.) (0.454)
      • P Solution \u003d 51.92 mm Hg. Art. + 12.89 mm RT. Art. \u003d. 64.81 mm RT. Art.
   • To use the Clausius Clauseer Equation, the temperature must be specified in the degrees of Kelvin (denoted to). If you have a Celsius temperature, it is necessary to convert it using the following formula: T k \u003d 273 + t c
   • The method described above works, because the energy is directly proportional to the amount of heat. The temperature of the fluid is the only factor ambientfrom which the pressure of vapors depends.