Aerosol of microscopic water droplets suspended in the air above a cup of hot tea after the water vapor has sufficiently cooled and condensed. Water vapor is an invisible gas, but the clouds of condensed droplets refract and scatter the sunlight and are thus visible.Droplets of water vapor in a pan.Demonstration of evaporative cooling. When the sensor is dipped inethanol and then taken out to evaporate, the instrument shows progressively lower temperature as the ethanol evaporates.Rain evaporating after falling on hot pavement
Evaporation is a type ofvaporization that occurs on thesurface of aliquid as it changes into the gas phase.[1] A highconcentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as whenhumidity affects rate of evaporation of water.[2] When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome thevapor pressure, it will escape and enter the surrounding air as a gas.[3] When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling.[4]
On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid. The evaporation will continue until an equilibrium is reached when the evaporation of the liquid is equal to its condensation. In an enclosed environment, a liquid will evaporate until the surrounding air is saturated.
Evaporation is an essential part of thewater cycle. The sun (solar energy) drives evaporation of water fromoceans, lakes,moisture in the soil, and other sources of water. Inhydrology, evaporation andtranspiration (which involves evaporation within plantstomata) are collectively termedevapotranspiration. Evaporation of water occurs when the surface of the liquid is exposed, allowing molecules to escape and form water vapor; this vapor can then rise up and form clouds. With sufficient energy, the liquid will turn into vapor.
Formolecules of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, and have sufficientkinetic energy to overcome liquid-phaseintermolecular forces.[5] When only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid decreases. This phenomenon is also calledevaporative cooling. This is why evaporatingsweat cools the human body.Evaporation also tends to proceed more quickly with higher flow rates between the gaseous and liquid phase and in liquids with highervapor pressure. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day. Three key parts to evaporation are heat,atmospheric pressure (determines the percent humidity), and air movement.
On a molecular level, there is no strict boundary between the liquid state and the vapor state. Instead, there is aKnudsen layer, where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clearphase transition interface cannot be seen.[6]
Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at roomtemperature) have molecules that do not tend to transfer energy to each other in a pattern sufficient to frequently give a molecule the heat energy necessary to turn into vapor. However, these liquidsare evaporating. It is just that the process is much slower and thus significantly less visible.
Evaporative equilibrium
Vapor pressure of water vs. temperature. 760 Torr = 1 atm.
If evaporation takes place in an enclosed area, the escaping molecules accumulate as avapor above the liquid. Many of themolecules return to the liquid, with returning molecules becoming more frequent as thedensity andpressure of the vapor increases. When the process of escape and return reaches anequilibrium,[5] the vapor is said to be "saturated", and no further change in eithervapor pressure and density or liquid temperature will occur. For a system consisting of vapor and liquid of a pure substance, this equilibrium state is directly related to the vapor pressure of the substance, as given by theClausius–Clapeyron relation:
whereP1,P2 are the vapor pressures at temperaturesT1,T2 respectively, ΔHvap is theenthalpy of vaporization, andR is theuniversal gas constant. The rate of evaporation in an open system is related to the vapor pressure found in a closed system. If a liquid is heated, when the vapor pressure reaches the ambient pressure the liquid willboil.
The ability for a molecule of a liquid to evaporate is based largely on the amount ofkinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can evaporate if they have more than the minimum amount of kinetic energy required for vaporization.
Factors influencing the rate of evaporation
Note: Air is used here as a common example of the surrounding gas; however, other gases may hold that role.
Concentration of the substance evaporating in the air
If the air already has a high concentration of the substance evaporating, then the given substance will evaporate more slowly.
Flow rate of air
This is in part related to the concentration points above. If "fresh" air (i.e., air which is neither already saturated with the substance nor with other substances) is moving over the substance all the time, then the concentration of the substance in the air is less likely to go up with time, thus encouraging faster evaporation. This is the result of theboundary layer at the evaporation surface decreasing with flow velocity, decreasing the diffusion distance in the stagnant layer.
The amount of minerals dissolved in the liquid
Inter-molecular forces
The stronger the forces keeping the molecules together in the liquid state, the more energy one must get to escape. This is characterized by theenthalpy of vaporization.
A substance that has a larger surface area will evaporate faster, as there are more surface molecules per unit of volume that are potentially able to escape.
the higher the temperature of the substance the greater the kinetic energy of the molecules at its surface and therefore the faster the rate of their evaporation.
Photomolecular effect
The amount of light will affect the evaporation. When photons hits the surface area of the liquid they can make individual molecules break free and disappear into the air without any need for additional heat.[7]
In the US, the National Weather Service measures, at various outdoor locations nationwide, the actual rate of evaporation from astandardized "pan" open water surface. Others do likewise around the world. The US data is collected and compiled into an annual evaporation map. The measurements range from under 30 to over 120 inches (3,000 mm) per year.
Because it typically takes place in a complex environment, where 'evaporation is an extremely rare event', the mechanism for the evaporation of water is not completely understood. Theoretical calculations require prohibitively long and large computer simulations. 'The rate of evaporation of liquid water is one of the principal uncertainties in modern climate modeling.'[8][9]
Industrial applications include manyprinting andcoating processes; recovering salts from solutions; anddrying a variety of materials such as lumber, paper, cloth and chemicals.
When clothes are hung on a laundry line, even though the ambient temperature is below theboiling point of water, water evaporates. This is accelerated by factors such as lowhumidity, heat (from the sun), and wind. In aclothes dryer, hot air is blown through the clothes, allowing water to evaporate very rapidly.
Thematki/matka, a traditional Indian porous clay container used for storing and cooling water and other liquids.
Thebotijo, a traditional Spanish porous clay container designed to cool the contained water by evaporation.
Evaporative coolers, which can significantly cool a building by simply blowing dry air over a filter saturated with water.
Combustion vaporization
Fueldroplets vaporize as they receive heat by mixing with the hot gases in the combustion chamber. Heat (energy) can also be received by radiation from any hot refractory wall of the combustion chamber.
Pre-combustion vaporization
Internal combustion engines rely upon the vaporization of the fuel in the cylinders to form a fuel/air mixture in order to burn well.The chemically correct air/fuel mixture for total burning of gasoline has been determined to be about 15 parts air to one part gasoline or 15/1 by weight. Changing this to a volume ratio yields 8000 parts air to one part gasoline or 8,000/1 by volume.
Thin films may bedeposited by evaporating a substance and condensing it onto a substrate, or by dissolving the substance in a solvent, spreading the resulting solution thinly over a substrate, and evaporating the solvent. TheHertz–Knudsen equation is often used to estimate the rate of evaporation in these instances.
^Gusarov, A. V.; Smurov, I. (2002). "Gas-dynamic boundary conditions of evaporation and condensation: Numerical analysis of the Knudsen layer".Physics of Fluids.14 (12): 4242.Bibcode:2002PhFl...14.4242G.doi:10.1063/1.1516211.
Sze, Simon Min (25 September 2001).Semiconductor Devices: Physics and Technology. Wiley.ISBN0-471-33372-7. Has an especially detailed discussion of film deposition by evaporation.
External links
Look upevaporation in Wiktionary, the free dictionary.
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