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From Wikipedia


Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid. Evaporation is also part of the water cycle.

Evaporation is a type of phase transition; it is the process by which molecules in a liquidstate (e.g., water) spontaneously become gaseous (e.g., water vapor). In general, evaporation can be seen by the gradual disappearance of a liquid from a substance when exposed to a significant volume of gas. Vaporization and evaporation however, are not entirely the same processes.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid. With sufficient heat, the liquid would turn into vapor quickly (see boiling point). When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to escape.

Liquids that do not evaporate visibly at a given temperature in a given gas (e.g., cooking oil at room temperature) 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.

Evaporation is an essential part of the water cycle. 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 plantstomata) are collectively termed evapotranspiration. Evaporation is caused when water is exposed to air and the liquid molecules turn into water vapor, which rises up and forms clouds.


For molecules of a liquid to evaporate, they must be located near the surface, be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase intermolecular forces. 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 temperatures. 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. Three key parts to evaporation are heat, 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 Knudsen layer, where the phase is undetermined. Because this layer is only a few molecules thick, at a macroscopic scale a clear phase transition interface can be seen.

Evaporative equilibrium

If 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, 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:

\ln \left( \frac{ P_2 }{ P_1 } \right) = - \frac{ \Delta H_{ vap } }{ R } \left( \frac{ 1 }{ T_2 } - \frac{ 1 }{ T_1 } \right)

where P1, P2 are the vapor pressures at temperatures T1, T2 respectively, ΔHvap is the enthalpy of vaporization, and R is the universal 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 will boil.

The ability for a molecule of a liquid to evaporate is based largely on the amount of kinetic 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

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 s

From Yahoo Answers

Question:I lose about a inch a day of water this summer but, my salt level goes down too. The chlorine generator installer said that salt would not go down with evaporation.

Answers:No we drink it!

Question:For my 8th rgade science fair project, I'm testing to see if the acidity of water affects the formation of [NaCl] stalactites. I have to add different concentrations of citric acid to each setup, which is salt water in plastic cups with a string between them. The strings asorbs the water, the water evaporates, and the salt crystals are left behind, forming "Stalactites". I was wondering if the different amounts of citric acid will affect its rate of evaporation? If so, how? Thanks!!

Answers:Yes. The addition of any soluble, nonvolatile solute will lower the vapor pressure of the solution. Thus the evaporation rate will also slow down.

Question:1. Alcohol 2. Peroxide 3. oil 4. liquid soap I found that it takes about 600 calories of heat energy to change 1 gram of liquid water into a gas. Does anyone know the amount of heat energy per gram of liquid that it takes to evaporate the others? 1. Alcohol 2. Peroxide 3. vegetable oil 4. liquid dish soap I found that it takes about 600 calories of heat energy to change 1 gram of liquid water into a gas. Does anyone know the amount of heat energy per gram of liquid that it takes to evaporate the others?

Answers:You cannot determine some of this directly because they can only be found through experimental analysis. "Oil" could be relating to vegetable oil, crude oil, baby oil but i am going to assume you are talking about crude oil in which case the purity would affect the rate of evaporation. liquid soap is mostly made of animal fat and would this depends on what else is used in the soap. this does not have enough information to give a direct answer and you also cannot figure this out without experimental analysis of at least the last two. the first two you can find on sigma-aldrich. a chemical manufacturer that we at the university of oregon

Question:What is the evaporation rate of water, ethanol, isopropyl alcohol, and acetone? I need to know the order in which they evaporate.

Answers:gotta look at their boiling points, water = 100c , ethanol 78.4c, isopropyl alcohol = 82.3c, and acetone = 56.53c so from this you can judge that the order is acetone, ethanol, isopropyl alcohol, water

From Youtube

Dead Sea Salt 3: Salts by evaporation :In an experiment ampoules are models for the huge evaporation pans in the southern end of the Dead Sea. Instead of the sun energy a burner - run by a candle - is used. When half the water is evaporated from a 10 mL ampoule the liquid is separated from the salt by decanting it into 5 mL ampoule to go on with the same procedure of evaporating, sedimenting and decanting. URL: www.micrecol.de/sssE5.html