Evaporation
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[edit] Headline text
Evaporation is the process whereby atoms or molecules in a liquid state gain sufficient energy to enter the gaseous state (the equivalent process in solids is known as sublimation). It is the opposite process of condensation. Evaporation is exclusively a surface phenomena and should not be confused with boiling. Most notably, for a liquid to boil, its vapor pressure must equal the ambient pressure, whereas for evaporation to occur, this is not the case.
Evaporation is a critical component of the water cycle, which is responsible for clouds and rain. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. In hydrology, evaporation and transpiration (which involves evaporation within plant stomata) are collectively termed evapotranspiration.
[edit] Theory
- See also: Kinetic theory
For molecules of a liquid to evaporate,they must be located near the surface, and moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces.[1] Only a small proportion of the molecules meet these criteria, so the rate of evaporation is limited. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperature. As the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases. This phenomenon is also called evaporative cooling. This is why evaporating sweat 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 vapor pressure. For example, laundry on a clothes line will dry (by evaporation) more rapidly on a windy day than on a still day.
[edit] Entropy
Evaporation is a state change from liquid to gas, and as gas has less order than liquid matter, the entropy of the system is increased, which always requires potential energy input. This means that the enthalpy change for evaporation (ΔHevaporation) and the standard enthalpy change of vaporization (ΔH0vap) is always positive, making it an endothermic process and subsequently, a cooling process.
[edit] Evaporative equilibrium
If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapor above the liquid. Many of the molecules return to the liquid, with returning molecules becoming more frequent as the density and pressure of the vapor increases. When the process of escape and return reaches an equilibrium,[2] the vapor is said to be "saturated," and no further change in either vapor 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 the Clausius-Clapeyron relation:
where P1, P2 are the vapor pressures at temperatures T1, T2 respectively. 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 boil.
The ability for a molecule of a liquid to evaporate is largely based on the amount of kinetic energy an individual particle may possess. Even at lower temperatures, individual molecules of a liquid can potentially evaporate if they have more than the minimum amount of kinetic energy required for vaporization.
[edit] Factors influencing rate of evaporation
- 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.
- Concentration of other substances in the air. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating.
- Flow rate of air. This is in part related to the concentration points above. If fresh air 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.
- Concentration of other substances in the liquid. If the liquid contains other substances (such as salts), it will have a lower capacity for evaporation. This is due to Raoult's law.
- Temperature of the substance. If the substance is hotter, then evaporation will be faster.
- Inter-molecular forces. The stronger the forces keeping the molecules together in the liquid state the more energy that must be input in order to evaporate them.
- Surface Area: A substance which has a larger surface area will evaporate faster due to the fact that there are more surface molecules which are able to escape.
Knowing that the highest energy particles are located at the surface of any liquid, evaporation is the process where vaporization occurs only at the surface of a liquid. One might believe that evaporation and boiling have the same chemical characteristics, but this is not true. Evaporation is a surface phenomenon that occurs at all temperatures. The amount of kinetic energy, or heat, applied to a liquid determines how many particles can evaporate. On the other hand, boiling occurs throughout a liquid, and has a definite temperature, or boiling point, for certain liquids (ex, 100o C for H20). Even at cool temperatures, a liquid can still evaporate, but only a few particles would separate from the liquid over a long period of time. So, the temperature is always changing during evaporation, but the temperature stays the same when the potential energy increases during vaporization.
[edit] Applications
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 humidity, heat (from the sun), and wind. In an electric clothes dryer hot air is blown through clothes, allowing water to evaporate very rapidly.
[edit] Combustion vaporisation
The fuel droplets 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.
[edit] Film deposition
Thin films may be deposited by evaporating a substance and condensing it onto a substrate.
[edit] See also
- Heat of vaporization
- Evaporation pan
- Evapotranspiration
- Flash evaporation
- Crystallisation
- Coffee ring
- Latent heat
- Distillation
- Desalination
- Evaporator
- Drying
[edit] References
- ^ "Changes of State", Last modified March 19, 2003. Last accessed Feb. 6, 2007 (UTC).
- ^ Martin A. Silberberg, Chemistry, 4th edition, pp431-434, McGraw-Hill, New York, 2006. ISBN 0-07-296439-1
- Semiconductor Devices: Physics and Technology, by S.M. Sze, ISBN 0-471-33372-7, has an especially detailed discussion of film deposition by evaporation.