A simple question about boiling water

Discussion in 'Chemistry' started by Magical Realist, Sep 10, 2012.

  1. Emil Valued Senior Member

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  3. KilljoyKlown Whatever Valued Senior Member

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    So you think we might still be here if it never rained anywhere on the planet since it first formed? There might be some life in the oceans but I really doubt any of it would have moved on to the land.
     
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  5. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Trippy´s post 14 "answer" to your question (“vapor pressure”) in no answer at all. - It just gives a more sophisticated name to the fact that both liquids and solids do evaporate. That answer is like the doctor "explaining" why sleeping pills work by saying: "They contain a narcoleptic agent." - He has not explained anything - just like Trippy, he is only restated the facts in more sophisticated terms.

    Here is why warm (not at absolute zero) solid and liquid things evaporated and faster the higher their temperature is:

    Warm molecules always have a spectrum or distribution of kinetic energy, KE and speeds. Some have several times the average KE. If these "hot molecules" are in the interior they will collide with others and give part of the extra KE to other molecules.

    Solids and liquids, but not gases, have nearly constant volume with reasonably well defined average separation between their atoms or molecules. This is because they mutually attract each other but as they get close, a repulsive force arises to stop further approach. In the interior, the long term average (a micro second or so) of the mutual attraction force is zero, but this is not true for a molecule or atom at the interface between the dense state material and the air.

    Those “surface molecules or atoms” have a long term average force towards the interior (the gas above them is not attracting them as strongly, if at all.) They can only break free from this attraction and escape into the gas above the interface when another atom or molecule hits them from below with sufficient KE (and speed directed towards the interface) to over come the downward attraction. These liberating “hot” molecules or atoms are upward moving molecules or atoms with adequate KE to free a bound surface atom or molecule and are more numerous (greater fraction of the total) if the dense mater is hotter. That is why the vapor pressure always increases with temperate.

    That is why both warm solids and liquids evaporate. The answer to your question is in understanding this atomic dynamics, not in re-naming the observed effect “vapor pressure.” (or “narcoleptic agent”).
     
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  7. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Why do you doubt that? * True, some of the movement onto the land was forced by them getting trapped in a cut off pond of evaporating water and that would not be happening, BUT some was to escape being eating by a hungry preditor, or to find food few others were already eating. This would still be happening even with out rain or evaporation.

    As a general rule, life forms evolve to occupy any viable nitch - certainly the shore line of an ocean is a "viable nitch."

    * Again I am only suggesting that give enough time, some advanced life forms would live on land - not that it would have five fingers on each hand etc. and look just like us. We are as we are only by long string of chances, probably never exactly repeated even on a million exact copies of Earth and the solar system. I have often regreted we developed 5 instead of 6 fingers. A base 12 number system is much better. How easy a number system is for mental calculations is determine by the number of exact divisors of the base. Ten has only two (2 and 5) but 12 has twice as many: (2, 3, 4 & 6)

    You know 250 / 5 = 50 easily because 5 is one of the two divisors of the base 10 number system. A base 12 number system is much more convenient. Fortunately our clocks at least split the day into 12 or 24 parts so it is easy to know how many hours are in 1/2, or 1/3 or 1/4 or 1/6 or (with the 24 hour clock) 1/8 of a day is as 8 is a divisior of 24.

    Making each hour have 60 minutes was a stroke of pure genius as 60 has divisiors of 2, 3, 4, 6, 12, 15, 20, & 30 so you easily know how many minutes are in 1/2, or 1/3 or 1/4 or 1/6 or 1/12 or 1/15 or 1/20 or 1/30th of an hour but I don´t wish for 30 fingers on each hand. Perhpas 6 hands with 10 fingers each might be nice, especailly if each of my 6 arms had a local brain to control them like the 8 local brains and octpus has, all under the direction of a central brain, of course.
     
    Last edited by a moderator: Sep 11, 2012
  8. Trippy ALEA IACTA EST Staff Member

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    You're skating on thin ice here Billy T, and you can consider that a warning if you want.

    I also provided a link to a detailed explanation of what vapour pressure was, so I will thank you to withdraw your claims.
     
  9. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    No your link has a great deal of facts about vapour pressure BUT NOT ONE WORD of explanation ABOUT THE MECHANISMS THAT CAUSE IT.

    It is thus only a more complex re-naming of the effect and giving some more facts about it. Sort of like a doctor saying: These red sleeping pills are twice as effective as the green ones as they have twice the narcoleptic agent in them. The blue pills have the narocleptic agent bound in a time release material so take them if you are troubled by waking up in the middle of the night. The pink ones have a buffering agent in them too, so if the red pills are up setting your stomach, take them instead ... I.e. lots more information but zero explanation of how they work - just like your link has zero explanation about the CAUSE of evaporation.


    The above, like your link (and your "explanation" that "vapor pressure" is the cause of evaporation) don´t tell anything abour why the sleeping pills have the properties they do or why vapor pressure has the properties it does. For example you and your link offer zero understanding of why vapor pressure increases with temperature - it just state that it does.

    An explainion of why sleeping pills work would be something like: the narcoleptic agent in them inhibits neural activity in the renticular formation at the top of the brain stem, which by its activity makes the remainer of the brain be in the alert state.

    An explantion of WHY solids and liquids evaporate is like the MECHANISTIC one I gave, not a re-naming of the facts or givening more characteristic of evaporation and data comparing vapor pressure of different substances at differnt temperatures. - that is DATA, not an explanation of why evaporation occurs.

    MacGyver1968 asked WHY, not what is the data about evaporation (its relation to temperature and specific matterials etc.) which all your link gives.
    To quote his exact post 8 words: "What CAUSES water to evaporate at room temperature?"
    To quote his exact post 10 words: "what causes liquid water to turn into vapor"

    SUMMARY: There is nothing for me to "retract." I answered MacGyver1968´s twice asked question about the CAUSE of evaporation and you did not. - You only gave a more sophisticated name to evaporation - "vapor pressure" and, like your link, said nothing about the cause of vapor pressure.
    You did exactly what the doctor did in "explaning" sleeping pills by stating: "Sleeping pills contain a narcoleptic agent."
    I.e. you said "Evaporation is due to "vapor pressure."

    In both cases, giving a new more sophisticated name for the same old rose is NOT an explanation of the CAUSE.
     
    Last edited by a moderator: Sep 11, 2012
  10. Aqueous Id flat Earth skeptic Valued Senior Member

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    Whoa.

    Not to make a mountain out of a molehill (certainly not with you guys who are obviously on top of your game)... but isn't this a relative POV?

    Look at rpenner, who was quick to contribute facts in the manner of (what I might call) a materials scientist.

    Billy T, I could say you haven't explained how the kinetic energy of the molecules was raised to the point of liberating themselves, because you didn't go into the photoelectric effect, radiation, wave propagation, and, ultimately Maxwell's equations. (And then someone else would want to take the explanation to some other reference point.)

    I think this may be a case where you're simply "in violent agreement" with Trippy.

    (Also, I'm not sure the gold coin evaporated. I think it went into liquidity. Sublimation would refer more appropriately to the leisure it could buy.)
     
  11. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    The velocity distribution in a liquid is much more complex than the Maxwell-Boltsman distribution, which describes it for a gas as there are essentially continuous and changing forces between the atoms, not the rare brief collisions that in a gas do result in the Maxwell-Boltsman distribution. For atoms near the surface it more complex still as not even isotropic.

    None the less there is a distribution of speeds so some molecules or atoms are moving faster than others and their "collisions" with "surface atoms" do occasional give some particular surface atom enough energy to over come the net downward attraction from other atoms below the surface atoms - i.e. make it leave the liquid or "evaporate."

    Unless ionizing energy radiation is incident upon the liquid surface we do not need to consider the photo-electric effect. The interior of a liquid or solid is filled with black body radiation, but radiation pressure on a neutral surface atom or molecule can be neglected too even though it probably does exert more but negligible force helping the surface atom escape – evaporate.

    As an aside: a block of glass even when glowing dull red hot is still not a good conductor (or convector) of heat energy from the interior to the surface. It uses radiative transfer of energy mainly to heat your hand held near it and cool the interior.

    I am assuming that there are only the normal thermal spectrum of phonons with in the liquid – I.e. there is no driven plate vibrating to launch other waves but you could certainly increase the rate of evaporation, even throw tiny drops of water out of the surface this way if you wanted to. These normal thermal photons are the collectively organized collisions and the upward bound photons “reflecting” off the surface have transferred upward momentum to the atoms of the surface but I think this momentum is given to many surface atoms and not much to any one. I.e. these “collective atom collisions” do not cause surface atoms to escape nearly as fast as the individual collision of a “hot upward bound” individual atom does. Possibly they do make a small increase it the evaporation rate which is one atom at a time still assuming the photon spectrum is thermal and with no vibrating plate driven by some other energy source.

    Other than the fact the black body radiation is from Maxwell´s E&M equations there is no effect of them on evaporations rate assuming you are not shinning a strong beam of radiation on the surface, locally heating it.

    I.e. and SUMMARY: As we were asked to explain WHY there is some evaporation at temperatures well below boiling (at room temperature, for example) I assumed the liquid was in thermal equilibrium without any of these potentially perturbing things you listed. Then the slow evaporation is due almost entirely to some surface atom (or molecule) gain significantly more energy during a collision with a “hot upwardly bound atom (or molecule) hitting it from below.

    I think I have given you about as complete a explanation of slow evaporation as you are likely to get, because, AFAIK the precise distribution of velocities (that includes direction of travel) of atoms near the surface air interface is not known.

    I.e. That is the dominate mechanism of slow evaporation. Faster evaporation with bubbles forming in the liquid is a somewhat different story; however, these bubble often initially form immediately above a microscopic crack –a void in the bottom of the heated pan so there is here too a liquid gas (or vacuum/liquid) interface at time of microscopic bubble first forms. Obviously as that bubble rises in the liquid it expands with less pressure acting on it and there is an interface between liquid and gas, (water vapor inside the bubble) so the “story” is not greatly different.

    Look at a pan of water just beginning to boil. You will see that there is a stream of tiny bubbles rising from only a few specific spots on the bottom of the pan. If pan was clean (nothing on the bottom to “nucleate” these bubble steams) then there is a microscopic crack in the pan bottom from which the bubble stream arias.
     
  12. Trippy ALEA IACTA EST Staff Member

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    Get off your high horse Billy, righteous indignation does not sit well when it is you that went on the offensive.
    Let's examine it shall we?
    A succinct explanation of what vapour pressure is.

    This is self explanatory - equilibrium vapor pressure gives us information on how quickly a liquid evaporation.

    A description of what vapor pressure is. How easy it is for particles to escape from their condensed phase.

    A description of the relationship between vapor pressure and evaporation rate.

    Combined those three points and my post tell us that:

    "Evaporation (and sublimation) occur because in a closed system the gas phase of a substance and its condensed phases are in thermodynamic equilibrium. A liquid that evaporates quickly has a high vapor pressure and a liquid that evaporates slowly has a low vapor pressure. A liquid that has a high vapor pressure, has a high vapor pressure because it is easy for molecules to escape from the surface of the condensed phase. Likewise, a liquid that has a low vapor pressure, has a low vapor pressure because it is difficult for molecules to escape from the surface of the condensed phase."

    The only remotely salient point not covered in the first paragraph of my link, then, is where the energy to escape the surface comes from and the answer to that, as I recall, has to do with asymetric collisions. But I would expect that if MacGyver1968 wanted more detail of that aspect, then he would ask.

    I'm ignoring the rest of your post, because it's just more of the same.
     
  13. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    You and you link offer a lot of facts, data on vapor pressure and evaporation, but not one word about its cause.

    MacGyver 1968 ask only and specifically about the cause twice (posts 8 &10) so I tried to explain the MECHANISMS that cause vapor pressure and evaporation to exist. Your explain is much like explaining, if not just simple re-naming of the effect, is high level telling how a car moves by saying: "You put the key in here, turn it and press down with foot here." A real MECHNAISTIC explanation would mention combustion of fuel in a chamber which makes pressure on a piston to force it down. And note that the doward linear motion of the piston is converted to rotation via a crankshaft. And this roation is applied via gears and othe linkages to the wheels to move the car forward.

    As MacGyver1968 asked about the CAUSE, I think he wanted a mechanistic explaination, like I gave, not the facts about vapor pressure and evaporation like your link gave, but lets ask him? OK?

    I may be a little super sensitive about what is an "explaination" as I have many years of exchanges with psychologists, etc. even "cognitive scientists," who explain behavior at high level in invented renaming terms with not the slightest mention of mechanistic facts relating to nerves firing. In those fields that is a little more acceptable as we are quite ignorant of what the real causes are, but not so about what causes, the mechnisms, of evaporation. Evaporation is caused by the atomic dynamics near the liquid air interface as I described.
     
  14. Trippy ALEA IACTA EST Staff Member

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    No. Me and my link offered "It's related to how easy it is for molecules to escape the condensed phase" as an explanation, which is perfectly adequate as an explanation, and lets MacGyver 1968 know that there are deeper mechanisms involved. It gives him the opportunity to either ask "Why is it easier for molecules to escape the condensed phase for some liquids?" or "How do molecules escape the condensed phase?" or to say "Cool, thanks."

    This is your assumption. Based on your assumption you derided my explanation and didn't give him the opportunity to formulate his own questions based on his level of prior knowledge. Which to me is just ironic for reasons I'm quite sure you've forgotten.

    Well, I'm glad you've acknowledged it's you that has the problem. Now, can we move on then? Because I really don't care for this conversation, and the only thing that it's motivating me to do is to unsubscribe and/or close the thread.
     
  15. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    Your "explanation" is just a re-naming of the phenomena and your link is just a rich data bank of facts about the phenomena of evaporation (or vapor pressure). Neither provides any understanding of why molecules leave the surface or permits you to make any predictions. Not even the well known fact that rate of evaporation is higher when temperature increases. There is no way to deduce that from your "evaporation is due to vapor pressure" - You must separately know than fact, not derive it from an explained understanding of the process.

    Here, in blue, from your post 14 link is the ONLY mention of the fact that particles (or molecules or atoms) escape from the evaporating material surface:

    “The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid).”
    Compressing that sentence a little to leave out the other name and needless words, it becomes:
    "The equilibrium vapor pressure is … the tendency of particles to escape from the liquid (or a solid).”

    Which now is clearly just re-naming the phenomena by substitution of the three words “equilibrium vapor pressure” for the one word “evaporation” i.e.
    The “evaporation” is… the tendency of particles to escape from the liquid (or a solid). - That is a definition, not an explanation.

    That sentence and the two earlier quoted are is only defining “evaporation” or “vapor pressure” and does not tell what mechanism CAUSES them to escape or allow any predictions about how and why certain factors change the rate of evaporation / vapor pressure (two different names for the same rose).

    A true “explanation” is much more than just re-naming the phenomena as it:
    (1) gives understanding of why the phenomena happens and
    (2) allows one to make predictions about the phenomena.

    Your re-naming “explanation” is exactly like the doctor "explaining" why sleeping pills work by saying: "They contain a narcoleptic agent." Unlike the doctor´s "explanation" your link of post 14 does tell many facts about many materials BUT without telling why they are true. Nor do these "re-namings" permit any predictions as no understanding of the mechanism is given.

    From post 23, here in blue is a true explanation of evaporation:

    Warm molecules always have a spectrum or distribution of kinetic energy, KE and speeds. Some have several times the average KE. Solids and liquids, but not gases, have nearly constant volume with reasonably well defined average separation between their atoms or molecules. This is because they mutually attract each other but as they get close, a repulsive force arises to stop further approach. In the interior, the long term average (a micro second or so) of the mutual attraction force is zero, but this is not true for a molecule or atom at the interface between the dense state material and the air.

    Those “surface molecules or atoms” have a long term average force towards the interior (the gas above them is not attracting them as strongly, if at all.) They can only break free from this attraction and escape into the gas above the interface when another atom or molecule hits them from below with sufficient KE (and speed directed towards the interface) to over come the downward attraction. These liberating “hot” molecules or atoms are upward moving molecules or atoms with adequate KE to free a bound surface atom or molecule and are more numerous (greater fraction of the total) if the dense mater is hotter. That is why the vapor pressure always increases with temperate.


    That gives one an understanding of the process´s mechanisms. Here are some of the predictions which follow from this teaching explaining I gave in blue. (None of which follow or can be deduced from your re-naming “explanation”):

    (1) The evaporation rate will increase if the air pressure above the liquid is decreased and decrease if the air pressure is increased even though the air temperature remains the same as the liquid.
    (2) Colder air above the surface will increase the rate of evaporation if a heat lamp shinning on the surface keeps the surface temperature unchanged.
    (3) Warmer air above surface will decrease the rate even if the liquid is so well stirred (without making surface waves that increase the surface area or that increase is corrected for) that the surface temperature remains the same.
    (4) Changing the atmosphere above the surface from air to helium will not change the evaporation rate if the atmospheric pressure and temperature are unchanged.
    (5) Raising the surface temperature will increase the rate even if the bulk liquid temperature is slightly lowered.
    (6) A sound wave in the liquid driven upwards and reflected at the surface can increase the rate even if the liquid is slightly cooler.
    (7) Adding a tiny amount (completely dissolved) of surface tension reducing agent (a few drops of detergent) will increase the rate of evaporation.
    (8) Why evaporation produces cooling of the liquid.
    (9) Was given in the last two sentences of the blue text above.

    (1), (2) & (3) occur because many if not most atoms that break free from the surface are knocked back down into the liquid by atmospheric atoms, almost* none of which at (or very near) the surface have a velocity component upward to help the “nearly evaporated” atom get away from the surface. I.e. most collision with the air immediately above the surface will transfer some downward momentum to the atom “trying to evaporate” and drive it back into the liquid.

    * In atmospheric gas the molecules are very widely (on atomic scale) separated. (Why it is filling the entire container and much lower in density than the liquid.) This is also true of their separation when they do hit the liquid surface and "bounce" upward. Thus, escaping liquid molecules will almost always be leaving the surface "far between" the bouncing upwards air molecules. However, before the escaping liquid molecule has traveled two mean free paths thru the air, it has a high probability of being knocked downwards towards the liquid again.

    (4) Does not do this as even though helium atoms have much higher speeds, they do not have more kinetic energy on average as that is the definition of “same temperature” as the air. If they also have the same pressure that means they are transferring momentum to the walls of their container (or the liquid surface) at the same rate too.

    (5) Is due to fact that only the upward collisions in the liquid made by an atom just below (~one “mean free path”or less) against an atom at the surface can free it from the surface if and only if (IFF) it transfers the escape energy needed to overcome the net binding forces which are directed down ward into the liquid. The hotter the surface is, the greater fraction of the atoms moving upward with this required energy will be.

    (6) A sound wave is the collective and common motion added to the random thermal motion. So when it reaches the surface (and reflects, which basically doubles the momentum transfer) there is increased energy given to the surface atoms to help them break free from the downward binding forces.

    (7) The “surface tension” is very directly related to the attractive forces between atoms of the liquid and detergent reduces that force, in ways I can not explain (but I could rename as “orbital weakening”). Thus with lower downward binding force, a great fraction of the upward collision on surface atoms do get the energy transfer needed for them to break free.

    (8) Each atom that does get permanently away from the surface (that evaporates) has taken away more than the average kinetic energy in the liquid, so that average is deceased for those remaining – Again, average kinetic energy IS temperature.

    SEE THE DIFFERENCE between "RE-NAMING" and EXPLAINING?

    Explaining gives understanding and that permits predictions, but definitions or re-naming do neither.


    It is an abuse of your power as a moderator to close a thread because someone has corrected or criticized you, especially when others are posting in it. You have done this to avoid replying to me in another thread. Don´t do it again.
     
    Last edited by a moderator: Sep 18, 2012

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