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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, 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 Encyclopedia

alcohol

alcohol any of a class of organic compounds with the general formula R-OH, where R represents an alkyl group made up of carbon and hydrogen in various proportions and -OH represents one or more hydroxyl groups . In common usage the term alcohol usually refers to ethanol . The class of alcohols also includes methanol ; the amyl, butyl, and propyl alcohols; the glycols ; and glycerol . An alcohol is generally classified by the number of hydroxyl groups in its molecule. An alcohol that has one hydroxyl group is called monohydric; monohydric alcohols include methanol, ethanol, and isopropanol . Glycols have two hydroxyl groups in their molecules and so are dihydric. Glycerol, with three hydroxyl groups, is trihydric. The monohydric alcohols are further classified as primary, secondary, or tertiary according to the number of carbon atoms bonded to the carbon atom to which the hydroxyl group is bonded. Many of the properties and reactions characteristic of alcohols are due to the electron charge distribution in the C-O-H portion of the molecule (see chemical bond ). Chemical reactions involving the hydroxyl group in an alcohol molecule include: those in which the hydroxyl group is replaced as a whole, e.g., reaction of ethanol with hydrogen iodide to form ethyl iodide and water; those in which only the hydrogen in the hydroxyl group is replaced, e.g., the reaction of ethanol with sodium, an active metal, to form sodium ethoxide and hydrogen; and those in which the carbon-oxygen bond becomes a double bond to form an aldehyde or ketone depending on whether it is a primary or secondary alcohol. Alcohols are generally less volatile, have higher melting points, and are more soluble in water than the corresponding hydrocarbons (in which the -OH group is replaced with hydrogen). For example, at room temperature methanol is a liquid, while methane is a gas.


From Yahoo Answers

Question:Relating to the atomic structure of alcohol or the chemical properties of it.

Answers:Actually, alcohol has a fairly high boiling temperature for a compound of its mass (46 AMU). Compare propane -42 C / 231 K ethylamine 17 C / 290 K acetaldehyde 26 C / 299 K formic acid 101 C / 374 K The difference is which forces keep the liquid from evaporating. For propane, we have van der Waals forces only. Ethylamine and acetaldehyde both have a polar and an unpolar end. (Dimethylether similarly has a polar centre with the two unpolar rests sticking away at an angle). What we don't have (prominently, in the case of ethyl amine) are hydrogen bonds. Ethanol and formic acid have hydrogen bonds. Formic acid lacks the unpolar rest, so has a slightly higher boiling point than ethanol. If you compare to water, remember that water is an extremely high boiling compound for its mass. Methane boils at 112 K, ammonia at 195 K, water at 373 K. Again, the degree of hydrogen bonds determines the boiling temperature.

Question:3. The temperature of evaporation is much higher for water than alcohol. Without knowing more about the chemistry of alcohol, which of the follwing is the most logical chemical explanation for this phenomenon? a. Ionic bonds form between alcohol molecules. These are the weakest type of bond and are easier to break than the hydrogen bonds between water molecules. b. fewer hydrogen bonds form between alcohol molecules. As a result, less heat is needed for alcohol molecules to break away from solution and enter the air. c. Alcohol has a higher surface tension than water. this means that alcohol molecules can easily break away from other alcohol molecules and evaporate at a lower temp. d. Alcohol molecules are more cohesive than water molecules. this means that as alcohol molecules evaporate, they pull other alcohol molecules into the air along with them. 4. Photosynthesis requires many steps to make glucose. As a result of the synthesis process, a. all carbons from the six carbon dioxide atoms are found in glucose b. water is synthesized by plant from H2 and O2. c. More atoms are present at the beginning than at the end. 5. Which of the following will contain more heat but has a lower temp? a. an olymic-sized heated indoor swimming pool b. the water used in a dishwasher c. the boiling water in a pot for noodles d. a gas-powered lawnmower engine after it has been used for an hour.

Answers:3. B 4. A (other two are ridiculous...water must be GIVEN to plants, they don't produce it and c breaks Conservation of matter law) 5. most heat will be that that has the most volume, in this case

Question:I need to evaporate an 86% alcohol solution without using a temperature higher then like 50 degrees F. I'm going to spread it out on a cookie sheet to see if that will help the evaporation. Please tell me an estimated time that i should expect and any tips to make it evaporate faster please. Thanks!

Answers:Spreading it out should help. I would also say, keep the temperature as high as possible so for you, 50 degrees F. I might try using a surface that is heavier, because as the alcohol evaporates it will take heat with it, so the more heat near it the better, and a heavier surface will be able to give away more heat while remaining close to that 50 degree max. (Kind of like a crock pot would hold more heat than a really thin aluminum pot, without being a higher temperature.) Also, if you keep the air moving, more alcohol can be pulled away from the surface faster, so it will evaporate faster. If you have the equipment, you could also lower the air's pressure above the alcohol, though I'm not sure that I've heard of anything that does that above a cookie sheet sized object. Maybe just try it on a low atmospheric pressure type of day, though that will probably not have a huge effect. Sorry, I don't know how to estimate the evaporation time though. Maybe you could time it a few times under different conditions... Hope that helps a little.

Question:I have a 3 US Fluid Ounce sample of Badardi 151 (75.5% alcohol by volume), mixed with some other stuff (a whole other can o' worms). The short question is: How long will it take this volume of alcohol to evaporate (at least 50% of the alcohol needs to evaporate). The test is being conducted in glass honey jar, which has an opening diameter of approximately 1.25". While I understand increasing surface area will speed up the rate of evaporation, I'm weary of contaminating my experiment with particulates and debris from the air. I had considered heating up the solution, in order to speed up the rate of evaporation, but given the flammability of such high concentrations of ethyl alcohol, I'd rather avoid an open flame. Does anyone have any suggestions? Does anyone have a time frame for evaporation? Would 72 hours in a dry, warm, room be sufficient? Thanks

Answers:Sorry, I don't have a timeframe. Your experiment has a lot of variables which you'll need to control for. As you mentioned, the temperature of the solution and the solution surface area will influence the results. Keeping the solution in a draft will also speed evaporation (think of how fast isopropyl rubbing alcohol evaporates when you blow on it after applying it on a cut), as well as allowing the air above the solution to reach saturation (like 100% humidity, but for alcohol). How will you measure that half of the alcohol has evaporated? Keep in mind that water will evaporate along with the ethanol, but at a different rate.