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B Why does water evaporate when it's less than 212 F outside?

  1. Jan 12, 2019 #1
    When one boils a pot of water on the stove, the water starts to boil into steam at 212 degrees Fahrenheit. The stove provides the heat both to make the water 212 degrees, and the stove provides the heat to change the state of the liquid water to vapor.

    However, we all know that water (or any other liquid, for that matter) evaporates when it is cold outside, as long as the temperature is 33 degrees or higher.


    I know that it takes a relatively large amount of heat just to change the state of liquid water to vapor without increasing the temperature of water at all. If there is a puddle of 50 degrees Fahrenheit liquid water on the ground outside, and if the temperature outside is 50 degrees Fahrenheit, any increase in temperature would just increase the temperature of the liquid water, not cause a change of state of the liquid water to vapor. Therefore, when it is 50 degrees Fahrenheit outside, why does liquid water evaporate?
     
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  3. Jan 12, 2019 #2

    Drakkith

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    From wiki:

    Does that answer your question?
     
  4. Jan 12, 2019 #3
    No.

    From your source: "Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase when it reaches its boiling point"

    If evaporation occurs as a liquid reaches its boiling point (212F for water), then why does water evaporate at 50 degrees Fahrenheit?
     
  5. Jan 12, 2019 #4

    Drakkith

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    It doesn't occur only at the boiling point. It can occur at any temperature above freezing. Further down in the wiki article:

    I believe the first sentence in the article is simply mistaken. Evaporation does not required the liquid as a whole to reach its boiling point.
     
  6. Jan 12, 2019 #5

    sophiecentaur

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    Evaporation occurs only on the surface. Boiling can cause a change of state anywhere in the liquid. (bubbles)
    The liquid on the surface has what is called a Vapour Pressure. Dalton's Law of Partial Pressures says that the pressure in a mixture of gases is equal to the sum of the partial pressures of each of its constituents. If you have a fixed volume of air (no circulation to the atmosphere) over a water surface, the air pressure will be equal to the pressures of the Oxygen, Nitrogen etc. individually and also the pressure of the gaseous water vapour. Water vapour will leave the surface until equilibrium is reached and the vapour pressure will balance the pressure causing molecules to leave. If you refresh the air over the surface, vapour will keep being lost by evaporation and the temperature will drop due to the lost KE of the escaped molecules. Clothes dry quicker on a warm windy day - higher temperature and constantly refreshed air (with lower partial pressure of water) over the surface.
    Vapour pressure is a function of temperature and it is the same as Atmospheric Pressure at 100°C (so called boiling point). At 100°C (212°F) the internal pressure will cause a faster loss of molecules and all the liquid can be converted to vapour even under the surface as bubbles - as long as you keep supplying heat.
    If you reduce the pressure of the air over the surface (vacuum pump or up a mountain), boiling will occur at a lower temperature.
     
  7. Jan 12, 2019 #6

    russ_watters

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    I'd call it poorly written. The key is that the temperature of any substance is related to the average kinetic energy of the substance. Some molecules have higher kinetic energy and some lower. Those molecules with a kinetic energy equal to vaporization for 212F water/steam will evaporate in liquid water that is overall below 212F. The energy balance bears this out as the energy loss from evaporation per unit mass is higher at lower temperatures, to include the energy required to raise the temperature to 212F. A steam table will show this.
     
  8. Jan 12, 2019 #7
    Who says that ice doesn’t evaporate? Water, including ice, will evaporate if the partial pressure of water vapor in the atmosphere above the condensed phase is less than the equilibrium vapor pressure of water at the temperature of the condensed phase.
     
  9. Jan 12, 2019 #8

    phinds

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    Technically, it doesn't. It sublimates.
     
  10. Jan 12, 2019 #9
    Semantics. I call it evaporation. Potatoes - potahtoes.
     
  11. Jan 12, 2019 #10

    phinds

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    An odd point of view here on PF to say that technical terms don't matter and that it's ok to give them your own definition.
     
  12. Jan 12, 2019 #11
    Would calling it by the correct name change the mechanistic description I provided?
     
  13. Jan 12, 2019 #12

    phinds

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    Naturally not, but I still think it's better when we all use the agreed on words to describe phenomena.
     
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