How does evaporation occur? What determines the rate of evaporation of a liquid? Factors affecting this process
Evaporation is the physical process of the transition of a substance from liquid state into gaseous (steam) from the surface of the liquid. The evaporation process is the reverse of the condensation process (transition from a vapor to a liquid state).
The evaporation process depends on the intensity thermal motion molecules: the faster the molecules move, the faster the evaporation occurs. In addition, important factors affecting the evaporation process are the rate of external (with respect to the substance) diffusion, as well as the properties of the substance itself. Simply put, with wind, evaporation occurs much faster. As for the properties of the substance, then, for example, alcohol evaporates much faster than water. An important factor is also the surface area of the liquid from which evaporation occurs: from a narrow decanter it will occur more slowly than from a wide plate.
Consider this process at the molecular level: molecules that have sufficient energy (speed) to overcome the attraction of neighboring molecules break out of the boundaries of the substance (liquid). In this case, the liquid loses some of its energy (cools down). For example, hot tea: we blow on the surface of the liquid to cool it down, in doing so, we speed up the evaporation process.
Absolute humidity
Absolute humidity - the amount of moisture (in kg) contained in one cubic meter air. Due to the small value, it is usually measured in g / m3. But due to the fact that at a certain air temperature, only a certain amount of moisture can be contained in the air (with an increase in temperature, this maximum possible amount of moisture increases, with a decrease in air temperature, the maximum possible amount of moisture decreases), the concept of relative humidity was introduced.
Relative humidity
- the ratio of the partial pressure of water vapor in a gas (primarily in air) to the equilibrium pressure of saturated vapor at a given temperature. An equivalent definition is the ratio of the mass fraction of water vapor in the air to the maximum possible. Measured in percentage.
The saturation vapor pressure of water increases strongly with increasing temperature (see graph). Therefore, with isobaric (that is, at constant pressure) cooling of air with a constant vapor concentration, there comes a moment (dew point) when the vapor is saturated. In this case, the "extra" vapor condenses in the form of fog or ice crystals. The processes of saturation and condensation of water vapor play a huge role in atmospheric physics: the processes of cloud formation and the formation of atmospheric fronts are largely determined by the processes of saturation and condensation, the heat released during the condensation of atmospheric water vapor provides an energy mechanism for the emergence and development of tropical cyclones (hurricanes).
If you leave a container of water uncovered, then after a while the water will evaporate. If we do the same experiment with ethyl alcohol or gasoline, the process is somewhat faster. If a pot of water is heated on a powerful enough burner, the water will boil.
All these phenomena are a special case of vaporization, the transformation of liquid into vapor. There are two types of vaporization evaporation and boiling.
What is evaporation
Evaporation refers to the formation of vapor from the surface of a liquid. Evaporation can be explained as follows.
During collisions, the velocities of the molecules change. Often there are molecules whose speed is so great that they overcome the attraction of neighboring molecules and break away from the surface of the liquid. (Molecular structure of matter). Since even in a small volume of liquid there are a lot of molecules, such cases are obtained quite often, and there is a constant process of evaporation.
Molecules separated from the surface of the liquid form vapor above it. Some of them, due to chaotic motion, return back to the liquid. Therefore, evaporation occurs faster if there is wind, since it carries the vapor away from the liquid (here, the phenomenon of "capture" and detachment of molecules from the surface of the liquid by the wind also takes place).
Therefore, in a closed vessel, evaporation quickly stops: the number of molecules "torn off" per unit time becomes equal to the number of "returned" into the liquid.
Evaporation rate depends on the type of liquid: the smaller the attraction between the molecules of the liquid, the more intense the evaporation.
How more area surface of the liquid, the more molecules are able to leave it. This means that the rate of evaporation depends on the surface area of the liquid.
As the temperature rises, the speed of the molecules increases. Therefore, the higher the temperature, the more intense the evaporation.
What is boiling
Boiling is intense vaporization, which occurs as a result of heating a liquid, the formation of vapor bubbles in it, floating to the surface and bursting there.
During boiling, the temperature of the liquid remains constant.
The boiling point is the temperature at which a liquid boils. Usually, speaking of the boiling point of a given liquid, they mean the temperature at which this liquid boils at normal atmospheric pressure.
During vaporization, the molecules that have separated from the liquid carry away part of it internal energy. Therefore, during evaporation, the liquid is cooled.
Specific heat of vaporization
The physical quantity that characterizes the amount of heat that is required to evaporate a unit mass of a substance is called the specific heat of vaporization. (link more detailed analysis this thread)
In the SI system, the unit of measure for this quantity is J / kg. It is denoted by the letter L.
Details Category: Molecular-kinetic theory Posted on 09.11.2014 21:08 Views: 12413In a liquid state, a substance can exist in a certain temperature range. At a temperature below the lower value of this interval, the liquid turns into a solid. And if the temperature value exceeds the upper limit of the interval, the liquid passes into a gaseous state.
We can observe all this in the example of water. In a liquid state, we see it in rivers, lakes, seas, oceans, a water tap. The solid state of water is ice. It turns into it when, at normal atmospheric pressure, its temperature drops to 0 o C. And when the temperature rises to 100 o C, water boils and turns into steam, which is its gaseous state.
The process of changing a substance into vapor is called vaporization. The reverse process of changing from vapor to liquid is condensation .
Vaporization occurs in two cases: during evaporation and during boiling.
Evaporation
Evaporation is the phase process of the transition of a substance from a liquid state to a gaseous or vaporous state, occurring on the surface of the liquid .
As with melting, heat is absorbed by a substance during evaporation. It is spent on overcoming the cohesive forces of particles (molecules or atoms) of the liquid. The kinetic energy of molecules that have the most high speed, exceeds their potential energy of interaction with other liquid molecules. Due to this, they overcome the attraction of neighboring particles and fly out from the surface of the liquid. The average energy of the remaining particles becomes smaller, and the liquid gradually cools down if it is not heated from the outside.
Since particles are in motion at any temperature, evaporation also occurs. at any temperature. We know that puddles dry up after rain, even in cold weather.
But the rate of evaporation depends on many factors. One of the most important - substance temperature. The higher it is, the greater the speed of particles and their energy, and the greater their number leaves the liquid per unit time.
Fill 2 glasses with the same amount of water. We put one in the sun, and the other we leave in the shade. After a while, we will see that there is less water in the first glass than in the second. She was heated Sun rays and it evaporated faster. Puddles after rain also dry up much faster in summer than in spring or autumn. In extreme heat, rapid evaporation of water from the surfaces of reservoirs occurs. Ponds and lakes are drying up, the beds of shallow rivers are drying up. The higher the temperature environment, the higher the evaporation rate.
With the same volume, the liquid in a wide plate will evaporate much faster than the liquid poured into a glass. It means that evaporation rate depends on the surface area of evaporation . The larger this area, the large quantity molecules flies out of the liquid per unit time.
Under the same external conditions evaporation rate depends on the type of substance . Fill the glass flasks with the same volume of water and alcohol. After a while, we will see that there is less alcohol left than water. It evaporates at a faster rate. This happens because alcohol molecules interact more weakly with each other than water molecules.
affect the rate of evaporation and presence of wind . We know that things after washing dry much faster when they are blown by the wind. The jet of hot air in a hair dryer can quickly dry our hair.
The wind carries away the molecules that have flown out of the liquid, and they do not return back. Their place is taken by new molecules leaving the liquid. Therefore, they become less in the liquid itself. Therefore, it evaporates faster.
Sublimation
Evaporation takes place in solids Oh. We see how the frozen, ice-covered linen gradually dries out in the cold. Ice turns into steam. We smell a pungent odor from the evaporation of the naphthalene solid.
Some substances do not have a liquid phase at all. For example, elemental iodineI 2 - a simple substance, which is black-gray crystals with a purple metallic luster, under normal conditions immediately turns into gaseous iodine - purple vapor with a pungent odor. The liquid iodine that we buy in pharmacies is not its liquid state, but a solution of iodine in alcohol.
The transition process of solids into a gaseous state, bypassing the liquid stage, is called sublimation, or sublimation .
Boiling
Boiling This is also the process of liquid turning into vapor. But vaporization during boiling occurs not only on the surface of the liquid, but throughout its entire volume. Moreover, this process is much more intense than during evaporation.
Put a kettle of water on the fire. Since there is always air dissolved in water, when heated, bubbles appear on the bottom of the kettle and on its walls. These bubbles contain air and saturated water vapor. First they appear on the walls of the teapot. The amount of steam in them increases, and they themselves increase in size. Then, under the influence of the buoyant force of Archimedes, they will break away from the walls, rise up and burst on the surface of the water. When the water temperature reaches 100 ° C, bubbles will form throughout the entire volume of water.
Evaporation occurs at any temperature, and boiling occurs only at a certain temperature, which is called boiling point .
Each substance has its own boiling point. It depends on the amount of pressure.
At normal atmospheric pressure, water boils at a temperature of 100 o C, alcohol - at 78 o C, iron - at 2750 o C. And the boiling point of oxygen is minus 183 o C.
As the pressure decreases, the boiling point decreases. In the mountains where Atmosphere pressure lower, water boils at a temperature of less than 100 o C. And the higher above sea level, the lower the boiling point. And in a pressure cooker, where it is created high blood pressure water boils at temperatures above 100°C.
Saturated and unsaturated steam
If a substance can simultaneously exist in a liquid (or solid) phase and a gaseous one, then its gaseous state is called ferry . Vapor is made up of molecules escaping from a liquid or solid during evaporation.
Pour the liquid into the vessel and close it tightly with a lid. After a while, the amount of liquid will decrease due to its evaporation. Molecules leaving the liquid will concentrate above its surface in the form of vapor. But when the vapor density becomes quite high, some of them will begin to return to the liquid again. And there will be more and more such molecules. Finally, a moment will come when the number of molecules leaving the liquid and the number of molecules returning to it will be equal. In this case, they say that liquid is in dynamic equilibrium with its vapor . This pair is called rich .
If, during vaporization, more molecules fly out of the liquid than return, then such vapor will be unsaturated . Unsaturated vapor is formed when an evaporating liquid is in an open vessel. The molecules leaving it are scattered in space. Not all of them return to the liquid.
Steam condensation
The reverse transition of a substance from a gaseous state to a liquid state is called condensation. During condensation, some of the vapor molecules return to the liquid.
The vapor starts to turn into a liquid (condenses) when certain combination temperature and pressure. This combination is called critical point . Maximum temperature , below which condensation begins is called critical temperature. Above the critical temperature, the gas will never turn into a liquid.
At the critical point, the liquid-vapor interface is blurred. The surface tension of the liquid disappears, the densities of the liquid and its saturated vapor are equalized.
At dynamic equilibrium, when the number of molecules leaving the liquid and returning to it is equal, the processes of evaporation and condensation are balanced.
When water evaporates, its molecules form water vapor , which is mixed with air or other gas. The temperature at which such vapor in the air becomes saturated, begins to condense upon cooling and turns into water droplets, is called dew point .
When there is a large amount of water vapor in the air, it is said that its humidity is increased.
We observe evaporation and condensation very often in nature. Morning fog, clouds, rain - all this is the result of these phenomena. Moisture evaporates from the earth's surface when heated. The molecules of the resulting vapor rise up. Encountering cool leaves or blades of grass on its way, the steam condenses on them in the form of dew drops. A little higher, in the surface layers, it becomes fog. And high in the atmosphere at low temperatures, the cooled vapor turns into clouds consisting of water droplets or ice crystals. Subsequently, rain or hail will fall from these clouds to the earth.
But water droplets form during condensation only when the smallest solid or liquid particles are in the air, which are called condensation nuclei . They can be products of combustion, spraying, dust particles, sea salt over the ocean, particles resulting from chemical reactions in the atmosphere, etc.
desublimation
Sometimes a substance can go from a gaseous state immediately to a solid state, bypassing the liquid stage. Such a process is called desublimation .
Ice patterns that appear on glasses in cold weather are an example of desublimation. During frosts, the soil is covered with hoarfrost - thin ice crystals into which water vapor has turned from the air.
At any temperature, some of the molecules fly off the surface of the liquid, forming vapor above it. The process by which a substance changes from a liquid state to a gaseous state is called vaporization. Vaporization that occurs at any temperature from the open surface of a liquid is called evaporation. Its speed depends on the type of liquid, the size of its free surface, temperature, external pressure, and the presence of an air flow above the liquid that carries away vapor.
The departure of molecules from the surface of a liquid during evaporation is associated with the expenditure of internal energy for the work function A, which the molecule must perform in order to overcome the forces of molecular attraction and the forces of external pressure. This work is done due to the kinetic energy of the molecules. A molecule will leave the liquid only if its kinetic energy is equal to or more work output: (m is the mass of the molecule, v is the component of the velocity of the molecule, directed perpendicular to the surface of the liquid). During vaporization, the liquid is cooled, since the ejected molecules carry away part of its internal energy.
In order for the evaporation of a liquid to occur without changing its temperature, energy must be imparted to the liquid. The scalar value, measured by the amount of energy required to convert a unit mass of liquid into vapor at a constant temperature, is called the specific heat of vaporization.
To convert a unit mass of a liquid into steam at a constant temperature, it is given an amount of heat equal to the specific heat of vaporization. During vaporization, an increase in the volume of a substance occurs. So, water vapor at 100 ° C occupies a volume almost 1700 times greater than the volume of the same mass of water at 100 ° C. Therefore, the substance, evaporating, part specific heat vaporization spends on doing work against the force of external pressure, and part - on increasing its internal potential energy. Therefore, at the same temperature, the internal energy of a unit mass of a substance in the gaseous state is greater than in the liquid state. So, 1 kg water vapor at 100 ° C has on 2*10 6 j more internal energy than 1 kg water at the same temperature.
Experiments have shown that the specific heat of vaporization of a substance depends on its temperature. The higher the temperature of a substance, the lower its specific heat of vaporization. For example, at 0°C, the specific heat of water vaporization 2499 kJ/kg, at 50° С - 2385 kJ / kg, at 100° C - 2257 kJ / kg, at 200°С - 1943 kJ/kg. The decrease in the heat of vaporization is explained by the fact that the higher the temperature of a substance, the greater the kinetic energy of its molecules and the less energy must be additionally imparted to the liquid so that its molecules fly out into the environment.
Name of specific heat of vaporization r kg / j. For the transformation m kg a mass of liquid into vapor requires a certain amount of energy, in particular the amount of heat Q=rm.
Let us assume that the liquid evaporates in a closed vessel. Part of the vapor molecules due to thermal motion, approaching the surface of the liquid, returns to it. In a closed vessel, both the evaporation process and the condensation process take place simultaneously. If the number of molecules escaping from the liquid is more number molecules returning to it, then the vapor above the liquid is called unsaturated. Experiments with unsaturated vapors have shown that they obey gas laws.
In the process of evaporation and condensation, a moment comes, starting from which the number of molecules that have flown out of the liquid per unit time will be equal to the number of molecules returning back to the liquid, that is, a dynamic equilibrium will come between the liquid and vapor. A vapor in dynamic equilibrium with its liquid is called saturated steam. It can be saturated not only in a closed vessel, but also in the atmosphere. So, with fog, water vapor in the air is saturated.
Let's open tap A (Fig. 35) and let a few drops of ether into the flask, which evaporates, forming unsaturated vapor. The more ether we let into the flask, the greater the pressure of its unsaturated vapor becomes. We let in the ether until a little liquid ether appears at the bottom of the flask. The appearance of the latter indicates that the ether vapors have become saturated. From this moment on, the pressure gauge stops showing an increase in pressure - it has become constant, despite the subsequent addition of ether. Hence, The vapor pressure and density at a given temperature is greatest when the vapor is saturated.
If different liquids are placed in turn in a flask and the pressure of their saturated vapors is measured, it turns out that At the same temperature, the saturation vapor pressure of different liquids is different. Ether vapor has the highest pressure, alcohol vapor has the lowest pressure, and water vapor has even less pressure.
At a temperature of 20 ° C, the saturated vapor pressure of these liquids is (in mmHg):
Let us find out whether the pressure of saturated vapor at a constant temperature depends on its volume. Under the piston in a cylinder connected to a pressure gauge, there is a liquid and its saturated vapor (Fig. 36). By changing its volume by moving the piston up and then down, according to the pressure gauge, we see that at a constant temperature, the saturation vapor pressure does not depend on volume, and it is a constant value for a given liquid at a given temperature. This means that saturated vapors do not obey the Boyle-Mariotte law. So, the pressure gauge of a steam boiler at a given temperature always shows the same pressure, regardless of how much volume the saturated steam occupies in it.
This is explained by the fact that when the volume of saturated steam changes, its mass changes. With an increase in volume, the mass of vapor increases (an additional evaporation of the liquid occurs), with a decrease in volume, the mass of vapor decreases (part of it condenses).
Let us find out if it depends constant volume saturation vapor pressure versus its temperature. We heat the saturated steam in the flask (see Fig. 35), placing it in hot water. We see As the temperature rises, the saturation vapor pressure increases. For example, the saturation vapor pressure of water at 50°C is 92.5 mmHg Art., and at 100 ° С - 760 mmHg Art.
Experiments and calculations on the change in saturated vapor pressure from heating show that the pressure increases many times more than it should according to Charles's law, i.e., the dependence of pressure on temperature does not obey this law. This is explained by the fact that the pressure of saturated vapor increases during heating, firstly, due to an increase in the average kinetic energy of the molecules of this vapor and, secondly, due to an increase in the concentration of vapor molecules, i.e., an increase in the total mass of molecules.
As long as the vapor remains saturated, a change in its temperature or volume is always accompanied by a change in the mass of the vapor, i.e. vaporization or condensation.
The property of saturated water vapor to increase its pressure with increasing temperature is used in steam boilers to produce steam having a high pressure, for example 100 atm, at a water boiling point of 310 ° C. To use steam in steam engines it is removed from the boiler, heated, converted into unsaturated. This pair is called overheated he has large stock internal energy. If the steam is not superheated, then it contains liquid droplets.
Having received a pair of ether in a test tube, we will begin to cool them by placing it in a mixture of ice and salt. A coating of liquid ether appears on the walls of the test tube, since when cooled, its vapors turned into a liquid. There are two ways of converting steam into a liquid: increasing the pressure on the steam, compressing it (see Fig. 36) and lowering the temperature of the steam, cooling it. Experiments show that gases can also be turned into a liquid (liquefaction of gases). To do this, they must be simultaneously compressed and cooled until they turn into a liquid.
Quantitatively, evaporation is characterized by the mass of water that evaporates per unit of time from a unit surface. This value is called the evaporation rate. In the SI system, it is expressed in kg / (m 2. s), in the CGS - in g / (cm 2. s).
The rate of evaporation increases with increasing temperature of the evaporating surface. In the process of evaporation, water molecules that turn into vapor spend part of their energy on overcoming cohesive forces and on the work of expansion associated with an increase in the volume of the liquid, which passes into a gaseous state. As a result, the average energy of the molecules that remain in the liquid decreases and the liquid cools. To continue the evaporation process, additional heat is needed, which is called the heat of evaporation. The heat of evaporation decreases with increasing temperature of the evaporating surface.
If evaporation takes place from the surface of the water, then this dependence is expressed by the formula:
Q \u003d Q 0 - 0.65. t, (5.9)
where Q is the heat of vaporization, J/g;
t is the temperature of the surface that evaporates, 0 С;
Q 0 \u003d 2500 J / kg.
If evaporation occurs from the surface of ice or snow, then:
Q \u003d Q 0 - 0.36. t, (5.10)
For practical purposes, the evaporation rate is expressed in terms of the height (in mm) of the layer of water that evaporates per unit of time. A layer of water 1 mm high, which will evaporate from an area of 1 m 2, corresponds to its mass of 1 kg.
According to Dalton's law, the evaporation rate W in kg / (m 2. s) is directly proportional to the moisture deficit calculated from the temperature of the evaporating surface, and inversely proportional to atmospheric pressure:
where E 1 - saturation elasticity, taken from the temperature of the evaporating surface, hPa;
e is the vapor pressure in the ambient air, hPa;
Р – atmospheric pressure, hPa;
A is the coefficient of proportionality, which depends on the wind speed.
It can be seen from Dalton's law that the greater the difference (E 1 - e), the greater the evaporation rate. If the surface that evaporates is warmer than air, then E 1 is greater than the saturation elasticity E at air temperature. In this case, evaporation continues even when the air is saturated with water vapor, that is, if e = E (but E On the contrary, if the evaporating surface is colder than air, then at a fairly high relative humidity it may turn out that E 1 Evaporation rate versus atmospheric pressure due to the fact that in still air, molecular diffusion increases with a decrease in external pressure: the smaller it is, the easier it is for molecules to break away from the evaporating surface. However, atmospheric pressure near the earth's surface fluctuates within relatively small limits. Therefore, it cannot significantly change the rate of evaporation. But it has to be taken into account, for example, when comparing evaporation rates at different heights in mountainous areas. Evaporation rate depends on wind speed. With an increase in wind speed, turbulent diffusion increases, on which the evaporation rate largely depends. The more intense the turbulent mixing, the faster the transfer of water vapor to the environment. If air is transported from land to a water body, then the rate of evaporation from the water body increases, since in the air that flows onto a relatively drier surface, the moisture deficit is greater than it is over the water body. When air is transferred from the water surface to land, the evaporation rate gradually decreases as a result of a decrease in the moisture deficit in the air that is above the water. The rate of evaporation from the surfaces of the seas and oceans is affected by their salinity, since the elasticity of saturation over a solution is less than over fresh water. Evaporation from the soil surface is significantly affected physical properties, state of the active surface, relief and other factors. A smooth surface evaporates less than a rough one, since turbulent mixing is less developed over it than over a rough surface. Light soils, other things being equal, evaporate less than dark soils, since they heat up less. Loose soils with wide capillaries evaporate less than dense soils with narrow capillaries. This is explained by the fact that water rises closer to the soil surface through narrow capillaries than through wide capillaries. The rate of evaporation depends on the degree of soil moisture: the drier the soil, the slower the evaporation. The rate of evaporation is influenced by the terrain. At higher elevations, over which there is intense turbulent mixing, evaporation occurs faster than in lowlands, gullies and valleys, where the air is less mobile. Vegetation cover affects the rate of evaporation. It significantly reduces evaporation directly from the soil surface. However, the plants themselves evaporate a lot of moisture, which they take from the soil. Evaporation of moisture by plants is a physical and biological process and is called transpiration. The complete removal of water vapor from a given surface with the same vegetation cover is called evapotranspiration. It includes evaporation from the earth's surface and from plants. Evaporation is the maximum evaporation possible in a given area from a certain active surface with a sufficient amount of moisture under the meteorological conditions existing here.
