evaporation
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Evaporation is a type of that occurs on the of a as it changes into the gas phase. The surrounding gas must not be saturated with the evaporating substance. When the molecules of the liquid collide, they transfer energy to each other based on how they collide with each other. When a molecule near the surface absorbs enough energy to overcome the , it will escape and enter the surrounding air as a gas. When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling. 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 the . The sun (solar energy) drives evaporation of water from oceans, lakes, in the soil, and other sources of water. In , evaporation and (which involves evaporation within plant ) are collectively termed . 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.


Theory

For s of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, and have sufficient to overcome liquid-phase s. 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 called . This is why evaporating 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 higher . 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, (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 a , where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface cannot be seen. Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at room ) 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 liquids ''are'' evaporating. It is just that the process is much slower and thus significantly less visible.


Evaporative equilibrium

If evaporation takes place in an enclosed area, the escaping molecules accumulate as a above the liquid. Many of the return to the liquid, with returning molecules becoming more frequent as the and of the vapor increases. When the process of escape and return reaches an , the vapor is said to be "saturated", and no further change in either 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 the : : \ln \left( \frac \right) = - \frac \left( \frac - \frac \right) where ''P''1, ''P''2 are the vapor pressures at temperatures ''T''1, ''T''2 respectively, Δ''H''vap is the , and ''R'' is the . 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 will . The ability for a molecule of a liquid to evaporate is based largely on the amount of 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 used here is a common example; however, the vapor phase can be other gases. ; 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 the 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 the . ;: Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves. ;: 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. ; of the substance: 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. In the US, the National Weather Service measures the actual rate of evaporation from a standardized "pan" open water surface outdoors, at various locations nationwide. 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 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 isn't 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.'


Thermodynamics

Evaporation is an , in that heat is absorbed during evaporation.


Applications

* Industrial applications include many and processes; recovering salts from solutions; and drying a variety of materials such as lumber, paper, cloth and chemicals. * The use of evaporation to dry or concentrate samples is a common preparatory step for many laboratory analyses such as and . Systems used for this purpose include s and s. * When clothes are hung on a laundry line, even though the ambient temperature is below the boiling point of water, water evaporates. This is accelerated by factors such as low , heat (from the sun), and wind. In a , hot air is blown through the clothes, allowing water to evaporate very rapidly. * The , a traditional Indian porous clay container used for storing and cooling water and other liquids. * The , a traditional Spanish porous clay container designed to cool the contained water by evaporation. * s, which can significantly cool a building by simply blowing dry air over a filter saturated with water.


Combustion vaporization

Fuel s 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 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.


Film deposition

s may be 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. The is often used to estimate the rate of evaporation in these instances.


See also

* (evaporation) * * * * * * * flux (a.k.a. eddy correlation, eddy flux) * * * * * * * * * * (phase transfer from solid directly to gas) *


References


Further reading

* Has an especially detailed discussion of film deposition by evaporation.


External links

{{Authority control Gases