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From Wikipedia

Evaporation

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.

## Theory

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, Î”Hvap 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

Question:I know there are probably a number of factors that can affect the rate of evaporation but is there a general rule of thumb or formula I can use to calculate it? Here's the environment: freshwater concrete fishpond about 2.5 feet deep (assume near vertical walls). The (circular) surface is roughly 10 feet in diameter--which make approx 80 square feet. Assuming an average temp of about 75 degrees, how many inches of evaporation loss should I expect in a 24 hr period? (I need a rough answer so I can determine if the pond is leaking or if it's just evaporation that is causing the bulk of the daily water level drop.)

Answers:Here are a couple of sources to get you going. Edit- If you're trying to find out if your pond is leaking here's something you can try--- Place a five gallon bucket on the second pool step and fill it to match the water level of the pool (probably have to put concrete block in bucket fore you fill it with water to help keep in place). Over the course of 2-3 days the bucket will lose water from evaporation and gain water from rainfall at the same rate as your pool! As long as there is no splash-out or backwashing during that time, if the pool level drops more than the bucket level then you have a leak. Source- http://wiki.answers.com/Q/How_do_you_measure_water_evaporation

Question:Identify the major factor affecting reaction rates that accounts for the following abservations: 1. tadpoles grow more rapidly near the cooling water discharge from a power plant. 2. enzymes accelerate certain biochemical reactions, but are not consummed. 3. Campfires are started with twings rather than logs for wood. 4. Iron and steel corrode more rapidly near the coast of an ocean than in the desert. please help!

Answers:1. increased temperature 2. catalyst 3. increased surface area 4. presence of moisture, or salt (catalyst).

Question:I need to make a lab on how intermolecular forces affect the rate of evaporation. I have to write up the methods and equipment but i don't know how to carry out the experiment. Can anyone give me some idea about how to measure the rate of evaporation of liquids?

Answers:If it were me, I would label an open container (beaker, coffee cup, glass, bowl...) and weigh it. Add the test liquid and reweigh to get the starting weight of the liquid. Record this weight along with the time. Let the open container set and reweigh it from time to time, recording the weights and times. Test a variety of liquids using similar containers. Use an empty container as a control. Set them in various places to see if this changes things. Use a thermometer and record temperatures to see if this is a factor. If any of the liquids are flammable or toxic, take suitable precautions. This would measure the evaporation rates. Do you have some way of measuring the intermolecular forces? I hope this helps you get started. Think about it, write something up and present it to your teacher and ask for advice. Good Luck!! (Try to have some fun.)

Question:I am doing a Weather and Climate lab and I am confused by this question? Can someone please explain to me what it would be? This is the whole question: Hurricanes gain their energy through evaporation of warm ocean waters. How does the subsidence of dry air affect evaporation rate? How does wind speed affect evaporation rate? also there is part 2 to this question: How do sea-surface temperatures affect evaporation rate? What could cause variations in sea-surface temperatures ahead of a hurricane's path?

Answers:I have read twice your question without really understanding. The third time I came to think that "evaporation rate" is the evaporation rate of sea water at the surface. If that is correct then a subsidence of dry air, i.e. the sinking of air at a low relative humidity level, will - of course - increase evaporation. Likewise, the higher the wind speed, the greater the evaporation. This is because, right over the surface of the water, some molecules mix with the air until it is 100 percent saturated. More evaporation can't occur until that air is replaced by new air and the faster it happens, and the drier the air is, the faster evaporation occurs. Of course, since warmer air can contain more moisture than cold one, the warmer the more evaporation too. But I don't think that the adiabatic warming of a subsidence has much to say for the air right above the sea. A subsidence, as it happens in a high pressure, is something that happens very slowly and I can't imagine it has any effect of the rate of sea water evaporation. It is said that hurricanes happen in the southern part of the North Atlantic when the temperature of the surface of the sea reaches 27 C. This usually occur by the end of the summer, in August. I don't see why the sea surface temperature should vary ahead of a hurricane but I am not a specialist, only a sailor and I know that off the Cape Verde islands, where hurricanes are born, the weather is boringly the same in the NE'ly Trade winds. I know, I have been there. The reason hurricanes occur when the sea water is that high is not only because of that but because the weather is not about temperature but the difference of temperature. While it varies a lot at the surface between the poles and the equator, at the tropopause, the top of our stroposphere, the temperature is pretty much the same, around -55 C. The only difference is that the tropopause is nearly twice as high at the equator than the poles. But it is the great difference of temperature between the surface and altitude that is responsible for the hurricanes and all other extreme tropical cyclones. The reason it is born over the sea is that moisture is the fuel of a convection since the wet adiabatic lapse rate is only half that of the dry air. Moist air keeps rising and lowers even further the pressure.