The passing from a liquid phase to a gas phase; in particular, the process by which water is lost from the Earth's surface to the atmosphere as water vapour. It is an important part of the exchange of energy within the Earthatmosphere system which produces atmospheric motions, and therefore climate (the global energy cascade). Rates of evaporation depend on such factors as solar radiation, the temperature difference between the evaporating surface and the overlying air, humidity, and wind.
It is the process whereby atoms or molecules in a liquid state (or solid state if the substance sublimes) gain sufficient energy to enter the gaseous state.The thermal motion of a molecule of liquid must be sufficient to overcome the surface tension and evaporate, that is, its kinetic energy must exceed the work function of cohesion at the surface. Evaporation therefore, proceeds more quickly at higher temperature, at higher flow rates between the gaseous and liquid phase and in liquids with lower surface tension (i.e. Since only a small proportion of the molecules are located near the surface and are moving in the proper direction to escape at any given instant, the rate of evaporation is limited. Also, as the faster-moving molecules escape, the remaining molecules have lower average kinetic energy, and the temperature of the liquid thus decreases.
Gas has less order than liquid or solid matter, and thus the entropy of the system is increased, which always requires energy input. This means that the enthalpy change for evaporation (ΔHevaporation) is always positive. Evaporation is a cooling process.
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.
Evaporative equilibrium
If the evaporation takes place in a closed vessel, the escaping molecules accumulate as a vapour above the liquid.
Evaporation of water
The meaning of evaporation is when a liquid turns into a gas due to heat energy allowing the atoms to escape. An example of both condensation and evaporation is a nuclear reactor which smashes the molecules, produces heat energy to heat the water and turns it into water vapor by evaporation to move turbines and when the water vapor moves out of the turbines it condenses.
The ratio of the heat loss from a pond by evaporation to the heat loss due to convection, independent of wind speed, is given by:
where Qc,pa is the heat loss from the pond by convection, in W/(m·K), Tp and Ta are the Kelvin (or Celsius) temperatures of the water and air, and Pwp and Pa are the vapor pressures of the pond surface and air, and p is the barometric pressure, with all pressures in mm Hg. (Bowen, 1926)
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. If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating. 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. If the liquid contains other substances (such as salts), it will have a lower capacity for evaporation. If the substance is hotter, then evaporation will be faster. 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.Applied evaporation
Forced evaporation
Forced evaporation or distillation is a process used in the separation of mixtures, in which a mixture is heated to drive off the more volatile component with a higher vapor pressure.
Film deposition
Evaporation is a common method of thin film deposition used in industry. The energy distribution of evaporated material tends to be Maxwellian, with a temperature derived from the evaporation source. Evaporation is therefore a gentler process with a better defined beam of source material that can be used to coat just one side of a substrate or even the side of etched surface features, as in MEMS processing.
The three main kinds of evaporation are thermal, electron-beam and resistive. Electron-beam evaporation is used with the highest melting elements. Resistive evaporation is accomplished by passing a large current through a wire or foil of the material that is to be deposited. Molecular beam epitaxy is a particularly sophisticated kind of thermal evaporation.
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