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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:"get a much greater air flow, allowing evaporative cooling to occur" Does this mean that without air flow evaporative cooling is not possible? if not/ if so please explain :D thnx "As water evaporate from a surface" is that sweat?

Answers:Not necessarily. Pre-existing airflow encourages this mode of cooling, although it can exist even if the air is initially quiescent. What does prevent it, prevents air flow, which is a layer of solid material which traps air in "pockets". Suppose you have a hot bowl of soup on a table that you'd like to cool to safe eating temperature. You see steam wafting off the top surface. This is evaporative cooling, since the steam used to be water and heat from the rest of the soup is used to cause it to vaporize, thus cooling the soup. Your soup is cooled by natural convection and natural evaporative cooling, both of which are driven by the gravitational field and the buoyant effects of hot and humid air. Waiting for your soup to cool, you become impatient and blow on it. By doing this, you impose a forced air flow over the soup surface. This forced air flow encourages convection and evaporation stronger than a natural condition of air. Now, you soup is cooled at a faster rate via forced convection and forced evaporative cooling than if you had been more patient and let buoyancy effects cool your soup. So in quiescent (think "quiet") air, evaporative cooling and natural convective cooling exist without a pre-existing airflow. If a layer of downfibers covers the soup, evaporative cooling and natural convective cooling aren't possible. This is because, although they exist without pre-existing airflow, buoyancy drives a newly existing airflow in order for these modes of cooling to work. With a downfiber layer covering the soup, it can only be cooled by conduction and radiation (which work without flow of any fluid).

Question:I thought the opposite would occur. Please explain the reason/s in a basic manner as I am new to this stuff

Answers:Air flow is only cooling because the atoms in the substance with the highest kinetic energy are removed. A liquid in equilibrium with its surroundings has a certain partial pressure of its gaseous form coexisting with it. Air flowing over the surface removes the gaseous form and, normally, replaces it with air with a lower amount of those molecules. So some of the molecules in the liquid evaporate to replace them. Those molecules with the highest energy (moving fastest) are the most likely to evaporate and so the average energy of the liquid molecules falls -- that is the definition of cooling. Now, the liquid absorbs heat from the surroundings and reestablishes the same average and range of energy in the liquid so more molecules can evaporate. The faster the evaporated molecules are removed, the faster the liquid evaporates. The most obvious case of this is sweating. Sweat on your brow (or wherever) evaporates and cools you. If you stand in front of a fan, the water vapor formed from your sweat is removed, the humidity in your local area goes down, and more sweat evaporates, cooling you more. If the fan is turned up faster then you may become quite cold. But the air blowing across you is not at any lower temperature than that elsewhere -- rather it causes your body to cool more rapidly. If the humidity of the air is really high, the movement of air does not result in much lower water vapor pressure and does not increase the evaporation rate. In that case the air flow is uncomfortable.