Talk:Evaporation

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Wrong

This article is wrong when it says evaporation is when liquid becomes gasous without being heated to boiling point. I'll fix it--124.186.101.229 08:42, 14 November 2007 (UTC)

Ouch. My students happened to find this article and now I have to fight the confusion and misconceptions it has caused. This article is dead wrong when it says: "When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas." That is not how it works. Firstly, vapour pressure is neither a force nor an energy. Temperature in gases is (see kinetic gas theory) an indirect measure of the average velocity of the molecules and similar (but more complicated) arguments can be made for liquids. When you increase the temperature you increase the average velocity, shifting the entire velocity distribution to higher values. What this means is that when you increase the temperature, you increase the fraction of molecules that have a kinetic energy that is large enough to escape from the liquid and go into gas phase. This is the reason why the vapour pressure increases when the temperature increases. Note: this process keeps happening even when the partial pressure of the substance has increased such that it equals the vapour pressure. (The article calls this "saturated" and says that no evaporation happens then). What is true, at steady-state at least, is that there is no net evaporation. Evaporation and condensation are two processes that happen simultaneously. I'm unfortunately not an expert in physical chemistry, but I have read a few research articles on estimating volatility from physical characteristics and the actual process, on a molecular level, is (at least from my perspective) rather complicated. I would suggest the following change: To escape from the liquid to the gas phase, a molecule must have enough kinetic energy to escape the intermolecular forces within the liquid. In a liquid of a certain temperature, the individual molecules have different kinetic energy and, to simplify, the temperarture is a measure of the average kinetic energy. When the temperature increases, this distribution shifts towards higher kinetic energy and the fraction of molecules with high enough kinetic energy to escape the liquid increases. This is why the vapour pressure increases with increasing temperature. At steady-state the partial pressure of the substance in the gas phase above the liquid equals the vapour pressure and the net evaporation rate is zero (i.e. the evaporation rate equals the condensation rate) I would suggest that someone with a solid knowledge of physical chemistry look up proper references. (It might be noted that the partial pressure close to a liquid surface often can be assumed to equal the vapour pressure and that the rate of evaporation will be determined by the film thickness, i.e. how thick the film is where the concentration, n_A/V=P_A/RT changes, see e.g. Stefan diffusion… Saittam (talk) 11:29, 17 December 2020 (UTC)

Evaporation

Has anyone ever fixed "evaporation is when liquid becomes gaseous without being heated to boiling point"? Apparently not. I'll fix it. Dan Hickman (talk) 23:55, 11 December 2007 (UTC) what is the answer then? What is evaporation? "Evaporation is a spontaneous process in which liquid when kept open in atmosphere, gets converted into it's vapour at a temperature below it's boiling point".

     As evaporation is an endothermic process, it brings about coolness. However, to understand endothermic nature of evaporation, we must understand, "the changes taking place at molecular level" during the process of evaporation. 

Understanding concept of evaporation & it's consequences, taking example of water:

        In liquid state, molecules of water are joined with each other through a weak chemical bond called, "Hydrogen bond". This weak force, resulted through hydrogen bonding, holds the molecule of water together and retards them escaping into the surrounding. 
      Further, in liquid state, the molecules of water are always in random motion called, "Brownian motion", which imparts some kinetic energy to each molecule of water. However, due to continuous inter moleular collision taking place among the molecules of water, some of them are moving very fast while some very slow. Due to this reason each of them does not posses same amount of kinetic energy. It is due to this reason that a more realistic scientific term, "average kinetic energy" is used for this. 
       At any temperature, there always exists a fraction of molecules of water possesing sufficiently high kinetic energy to escape into surrounding by breaking some of the hydrogen bonds. This fraction of molecules of water, which continuously escape into the surrounding, is called, "evaporation"! At lower temperature, fraction of such molecules of water possesing sufficiently high kinetic energy is small, causing slow evaporation; but at higher temperature, fraction of such molecules becomes large, causing speedy evaporation. 
       Here, the important point is, "energy is needed to maintain the fraction water molecules undergoing evaporation". As this energy is continuously absorbed from the surrounding, the surrounding becomes cooler.  — Preceding unsigned comment added by Chemiyog (talk • contribs) 07:57, 8 October 2014 (UTC) 

Diffusion vs. convection

Water evaporation is a special case, but an important one. In still air, over still water, a saturated vapor layer would develop immediately above the water. But humid air is less dense than dry air, so this vapor layer would be buoyant and drive convection. An important detail to understand in the case would be, does diffusion or convection dominate in determining the long-term evaporation rate of the water? -69.87.199.175 (talk) 15:08, 28 February 2008 (UTC)

Wrong. Humid air is denser than dry air.

Gravuritas (talk) 12:29, 26 December 2013 (UTC)

Sorry, the original statement that humid air is less dense than dry air is correct. It is because water molecules have an average molecular mass of 18, whereas N2 and O2 -- the two major constituents of air -- have molecular masses of 28 and 32 respectively. At a given pressure, more humid air will be less massive and less dense. My favorite example is hitting a baseball: It goes farther in humid conditions. See, e.g. http://www.exploratorium.edu/baseball/howfar3.html

Notwithstanding, your intuition is correct to the extent that vapor layer of humid air will not convect. Why? Evaporation is endothermic which generally makes the vapor layer cooler (and denser) than the air above it. Swlenz (talk) 20:35, 12 January 2015 (UTC)

My mistake- thank you for pointing it out.

Gravuritas (talk) 08:22, 13 January 2015 (UTC)

I suggest that the article has a section on the special case of water evaporation in air, linked to psychrometrics which doesn't seem to cover evaporation or condensation in detail. This would clarify that the rest of the article was about evaporation in general, so future edits can confine water evaporation to this section and general principles to the rest. Natty Stott (talk) 20:38, 20 November 2013 (UTC)

Needs rephrase

"On average, the molecules do not have enough energy to escape from the liquid, or else the liquid would turn into vapor quickly." -- this is stupid Needs to be rephrased, as in various cases the liquid does turn into vapor more or less "quickly". -- Writtenonsand (talk) 16:03, 24 July 2008 (UTC)

Done (to my few, two eyes). That segment looked in need of edit to me too, I interpreted it as though they were trying to paint a picture of "what evaporation is in the real world, in contrast to boiling" but they didn't describe "the molecules" enough, and painting a picture doesn't work with too much abstraction. Now it goes "the molecules in a glass of water," which I think paints the picture they were going for a lot better. I also tossed in the word & link to "boil," which was left out and I believe the intended contrast the original author meant to make, that is, I don't think it was mere coincidence that the logical phrase "unless it is boiling" is the 100% perfect catch to the statement... although if the glass of water is very near boiling point, some may precive it's rate of evaporation as "quick" but hopefully it is an easy thing to read past. I'm glad it didn't just delete it, it's a pretty important distinction to make. —Preceding unsigned comment added by 96.18.60.113 (talk) 08:04, 5 December 2008 (UTC)

"On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid." isn't much better. What does "on average" mean? S C Cheese (talk) 16:35, 15 May 2021 (UTC)

Needs reworking

Re: "Three key parts to evaporation are heat, humidity and air movement." 1) Humidity is a key part of the (important) evaporation of water but not of evaporation in general? 2) Surface area is a key part to evaporation? Bill B —Preceding unsigned comment added by 216.69.46.3 (talk) 20:33, 8 September 2008 (UTC)

Basically agree with Bill B. When there is a gas-liquid interface with two different components (e. g. air and water), the gaseous part will accept the evaporated liquid only until relative humidity reaches 100 %; then condensation will compensate evaporation; in case of external heat source, the change in temperature may modify the gas-to-liquid ratio in the saturated mix, but nothing more. On the other hand, if the gas-liquid system has a single component (e. g. boiling water), the above simply does not make sense; you may evaporate as long as you feed enough energy. Re point 2 - no, surface area is NOT a key point; it might be influent - sorry for the brutal simplification - since a larger area makes evaporation easier, but the key points is the energy input to the system; in my understanding, also humidity and air movement are not key factors in the change of physical state; they are to be taken into account only as long as you refer to natural evaporation from water reservoirs (lakes, sea, etc.).--Ub (talk) 17:09, 9 September 2008 (UTC)

Looking better into the article. The problem is in chapter 2 : Factors influencing the rate of evaporation. The previous part is quite correctly dealing with evaporation; this one concerns psychrometry. Therefore, the considerations by Bill B and myself are in the wrong place. The idea is to look at the main article (psychrometry) and to arrange this one accordingly. Hope to find the time. --Ub (talk) 18:46, 10 September 2008 (UTC)

Why has somebody replaced humidity with atmospheric pressure here? Did they mean the vapour pressure of water in the air? In general is evaporation mainly influenced by the partial pressure of the gas evaporating or does the presence of other gas significantly slow it down? Natty Stott (talk) 17:35, 16 November 2013 (UTC)

At 0.1 bar water evaporates considerably faster, to the extent that at 50 °C it actually boils, even in dry air. Your reasoning about partial pressures is fine for a mixture of ideal gases but not when there are phase changes. Vaughan Pratt (talk) 03:08, 19 November 2013 (UTC)

Energy costs of form change

Link is gone! DGerman (talk) 00:20, 31 July 2012 (UTC)

evaporation time

this article needs to explain why, after running a dishwasher, plastic or tupperware dishes are commonly still wet long after smooth glass or china dishes are dry. —Preceding unsigned comment added by 70.131.106.2 (talk) 00:34, 30 June 2010 (UTC)

Remember - heat is the driving force for evaporation. The glass (silica is involved) can "absorb" alot more heat and then evaporate the moisture "quicker" than the other materials mentioned. Dok Uranium (talk) 05:13, 27 March 2011 (UTC)

Factors affecting evaporation rate

Isn't viscosity of the liquid a factor whereby there is less evaporation from more viscous liquids? - perhaps more so than density? Is there an expert who can advise on this? Eeno11 (talk) 15:14, 6 September 2011 (UTC)

No it doesn't, at least in a simple picture. Do you have an example in mind? Density doesn't have any effect either. So, some work needed... AlanParkerFrance (talk) 10:45, 13 November 2012 (UTC)

There are effects from viscosity but as AlanParkerFrance said, not in a simple picture. As a simple example, a thick layer of a viscous 10% polymer solution in a solvent may lose solvent by evaporation at its surface, faster than the solution is mixed by diffusion etc. This leads to an increase in solids content at the surface, an increase in viscosity, a decrease in mixing etc until the solution has formed a solid or near-solid 'skin' at the surface while the lower layers are still liquid. The evaporation of these lower layers is now extremely low. But I wouldn't have thought this degree of complication would be helpful to include in the article.Gravuritas (talk) 12:39, 26 December 2013 (UTC)

June 19, 2019 - I removed this from the Factors affecting rate section: ";Concentration of other substances in the air: If the air is already saturated with other substances, it can have a lower capacity for the substance evaporating." Carax (talk) 00:14, 20 June 2019 (UTC)

Help needed

We need an engineer to work on this. I can supply some input from having worked with evaporators in the pulp and paper industry, but I don't really have time to do it now. Also I need someone to check my work. Thanks.Phmoreno (talk) 20:50, 29 April 2013 (UTC)

Mechanism of evaporative cooling

Is the article's account of evaporative cooling complete? Certainly the statistical account, that the vapor molecules leaving the liquid are the faster ones, must be part of the explanation. But for it to be the whole explanation, the average translational energy of the departing molecules would have to equal that of the air into which they escape. (The vibrational and rotational energy need not be the same since water vapor is triatomic and bent while oxygen and nitrogen are only diatomic, but that only makes the cooling effect even greater.) This seems highly unlikely, even for air well below 0 °C.

More likely, surely, is that on entry to air the molecules are initially well below average in translational energy and are speeded up to average by collision with the other air molecules, and moreover additional rotational and vibrational energy is absorbed. This will reduce the temperature of the skin layer of air, and then it's a good question how that cooling effect on the skin layer is divided between the liquid below and the air above.

Is this in fact the case, and if so is there a source for it? If there is it should go in the article. If not it sounds like a nice little project for a physics student to investigate, especially the question of how the skin layer cooling is apportioned between liquid and air, presumably as a function of the temperatures of the respective phases. Vaughan Pratt (talk) 18:43, 28 October 2013 (UTC)

Meanwhile I found some relevant discussion at Talk:Evaporative_cooler#Physical_principles_section_needs_work. Although agreeing with the above it doesn't offer any usable sources (the BEC link suggested there is not ideal since this is not about Bose-Einstein condensates). Vaughan Pratt (talk) 20:16, 28 October 2013 (UTC)

Where's the History?

It seems to me that this article could have been edited less arrogantly and destructively. Is it necessary to say, "This article is wrong", or, "this is stupid"?

Due to the way this Talk page was edited, I'm not sure who asked, "what is the answer then?", or whether the question was ever answered. However, the fact that the question was asked after two people said they would "fix it", suggests destructive editing.

Hopefully, obsolete explanations will not be included in the, "wrong", and, "stupid" categories; they are history, and should be included in the History section. Ideally, a History section about a chemistry subject would go back at least as far as Alchemy, and perhaps back to ancient applications of the subject. In the case of Evaporation that would probably include early stills and porous ice trays and jugs such as ancient Egyptians and others used to chill water, and Mesoamerican descendants continue to use. Downstrike (talk) 13:33, 15 November 2013 (UTC)

Are you asking about the history of this talk page or the history of evaporation? Your comments seem to be addressing both. For the former, use the history tab to locate specific examples of "destructive editing" of this talk page (I took a look and was unable to find any). For the latter, see Evaporative cooler and distillation, both of which have long history sections. Evaporation itself presumably began not too long after the Big Bang, what would you like to see about its history in this article? Vaughan Pratt (talk) 17:02, 15 November 2013 (UTC)

Semi-protected edit request on 24 February 2016

in "Factors influencing the rate of evaporation" the "Concentration of other substances in the air" should not have effect, the correct factor should be "Concentration of other substances in the liquid". 139.78.154.131 (talk) 18:34, 24 February 2016 (UTC)

Not done: as you have not cited reliable sources to back up your request, without which no information should be added to, or changed in, any article. - Arjayay (talk) 19:14, 24 February 2016 (UTC)

I take back the thing about "Concentration of other substances in the air" should be deleted. However, "Concentration of other substances in the liquid" should be added into the factors. The whole reference and explanation could be found in wiki page "Colligative properties" — Preceding unsigned comment added by 139.78.252.129 (talk) 18:30, 25 February 2016 (UTC)

Precipitationshed in the lead?

The final sentence of the lead seems to me to be a bit odd. "The tracking of evaporation from its source on the surface of the earth, through the atmosphere as vapor or clouds, and to its fate as precipitation closes the atmospheric water cycle, and embodies the concept of the precipitationshed."

First off, it's a bit of an awkward sentence to make sense of.

Second, and more important - why is precipicationshed there? It's a bit of a niche concept, and consensus on Talk:Precipitationshed seems to be that it's not really a word. Why is it even mentioned in the lead paragraph of the article on evaporation?

I think this sentence could be tidied up grammatically, and should drop the mention of the precipitationshed (while perhaps retaining reference to the water cycle). Girth Summit (talk) 07:08, 31 October 2016 (UTC)

I've gone ahead and removed this sentence. I thought about rewording it to make it a bit more readable, while keeping the essence of its description of the rest of the water cycle; however, it occurs to me that this is an article about evaporation, and so probably does not need the rest of the water cycle to be described - there is a link to that water cycle at the start of the paragraph.Girth Summit (talk) 03:12, 5 November 2016 (UTC)

Semi-protected edit request on 12 June 2020

223.235.143.135 (talk) 08:59, 12 June 2020 (UTC)
vipul kumar shah

 Not done: it's not clear what changes you want to be made. Please mention the specific changes in a "change X to Y" format and provide a reliable source if appropriate. TheImaCow (talk) 10:48, 12 June 2020 (UTC)