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Can boiling and evaporation occur simultaneously?

  1. Jun 22, 2013 #1
    I understand the differences between evaporation and boiling as stated in secondary school textbooks.

    But I would like to know, can evaporation occur at or above the boiling point?

    I think that, since evaporation is the escaping of high energy molecules, then when the water is boiling, there may also be some molecules whose kinetic energy is higher than others, thus they escape faster. It's not like all molecules vaporize at the same time, after all.

    Or evaporation is just a word specially used "for vaporisation below the boiling point"?

    Thank you.
     
  2. jcsd
  3. Jun 22, 2013 #2

    SteamKing

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  4. Jun 22, 2013 #3
    What was stated in your text books that you found easy to understand?
    Do you now have doubts about what you read?
     
  5. Jun 23, 2013 #4

    sophiecentaur

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    I can sympathise with your confusion and it could be just down to the 'classifying' problem. What we actually observe is due to a number of factors. Water is always evaporating (losing molecules) at any temperature. There is a positive 'vapour pressure'. If the surrounding air is not 'saturated' then there will be a net loss of water because more molecules are lost than returning. If new, 'dry' air is passing over the surface, then the evaporation rate is higher because fewer molecules get back onto the surface.
    Boiling takes place when the vapour pressure is greater than the atmospheric pressure plus any hydrostatic pressure at the bottom of the container (or the position of the heating element). Bubbles can form around an element when enough vapour is produced fast enough. That's boiling - but I suppose you could just call it very rapid evaporation - with no balancing condensation.
    There's another issue, here, which comes into play. Evaporation / boiling takes energy and all the water won't boil at once - unless there is a fantastic rate of power supplied. This 'latent heat of vaporisation' is what limits the rate of boiling. And, of course, the boiling takes place where the heat is being supplied - usually at the bottom. You can get 'explosive' boiling if the energy is supplied quick enough.
     
  6. Jun 23, 2013 #5
    Plus the capillary pressure on the bubbles forming.
    It still wouldn´t be at once. As the temperature of water increases, bubbles will get easier to nucleate.
    The rate of power supply does not limit the rate of boiling. Boiling liquid can rapidly cool towards its boiling point, even though the heat supply was slow and remains slow.
    Where heat is being supplied is the hottest part of liquid, due to heat conduction being limited by temperature gradients. So it is the most likely place to boil. But it does not need to be the place where the liquid can boil. For one, the liquid might have higher boiling poing there, for example due to higher pressure. For another, the part where bubbles are nucleated might be elsewhere than where heat is supplied.
    Or if the energy builds up above boiling point due to lack of bubble nucleatrion sites. Explosive boiling is a known hazard with test tubes, microwave ovens etc.

    And evaporation can easily happen well above boiling point. If a vessel contains overheated liquid which is unable to form bubbles due to lack of nucleation sites in the liquid and bottom, it still can evaporate from free surface to air.
     
  7. Jun 23, 2013 #6
    'Explosive' boiling is a function of surface tension! The pressure in tiny bubbles is very large and bubbles will not form unless there are nucleation sites....dirt, scratches etc, these are places where bubbles in the form of trapped gas already exist.
    Anti bump granules are sometimes added to liquids to act as nucleation sites.
    In a similar way, but nothing to do with evaporation, a clean glass of lager produces very few bubbles....drop a sugar cube in and see the release of trapped (dissolved) gas once nucleation sites are available. Then drink the lager.
     
  8. Jun 24, 2013 #7

    sophiecentaur

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    Yup. That's all good stuff when dealing at the next level up. Trouble is, there's no end to these explanations and I wanted to keep it as simple as possible.
     
  9. Jun 24, 2013 #8
    I'm sorry if I'm hijacking this thread, but I think my question is relevant here: Why is atmospheric pressure an important consideration in the case of boiling? Water vapor specifically, is lighter than air, so by Archimedes principle, it should be able to escape liquid water regardless of the air pressure?

    I'm sure I'm missing a very important detail here. Please excuse my ignorance.
     
  10. Jun 24, 2013 #9

    russ_watters

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    It can: that's what evaporation is. But boiling involves bubbles of water vapor inside a liquid. The pressure inside those bubbles is determined by atmospheric pressure and depth.
     
  11. Jun 30, 2013 #10
    So,

    1. Evaporation does not involve bubbles inside the liquid; the molecules just simply escape from the surface.
    2. Boiling must involve the formation of bubbles.

    Are these both correct?
     
  12. Jun 30, 2013 #11

    russ_watters

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    Yes. Those are the definitions.
     
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