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Evaporation is a cooling process. What cools and what warms?

  1. Nov 14, 2011 #1
    Evaporation is a cooling process. What cools and what warms? Also, condensation is a war,ng process. What cools and what warms?

    I understand that during evaporation the water molecules of water slow down therefore cooling down. But does this mean that the air warms up? The opposite goes for condensation, the object upon which the vapor condenses is warmed. So whatt's cooled? The air vapors?

    Thanks.
     
  2. jcsd
  3. Nov 14, 2011 #2

    ehild

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    Re: Evaporation/condensation

    Evaporation means that molecules of the liquid leave the liquid phase and become part of the gas phase above the liquid. Only those molecules can escape from the liquid which have enough energy. You know that the temperature of the substance is proportional to the average KE of its molecules. Loosing molecules with high energy would decrease the average energy of the remaining molecules, so the liquid losses heat when evaporating. The energy of the escaped molecules is transferred as heat to the gas phase above the liquid.


    ehild
     
  4. Nov 14, 2011 #3

    PeterO

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    Re: Evaporation/condensation

    The temperature of water is a measure of the average kinetic energy of the particles.
    In any sample of water, some molecules will have above average energy, others below average.
    If a water molecule is lucky to have enough energy, it can exist as a gas molecule, rather than just part of the liquid. If it happens to be near the surface, it may even escape the liquid. If there is a breeze blowing, it may get swept away, while still a gas molecule, and thus not return, with its energy, to the body of liquid.

    Note that this means one of the more energetic molecules in the original sample has left with its energy, meaning the average energy of the molecules left behind is lower - the water is cooler [remember temperature is a measure of the average KE of the particles.

    Imagine doing this with money in a group of people. The average wealth of a group of 20 people is $1000. People exchange amounts of money between them selves - but the average wealth remains the same. Suddenly, one person notices that they are currently holding $5000, so decides to leave the group and take that money with them.
    The remaining 19 people now have a collective wealth of $15000, so an average wealth of of just under $790. The average wealth of the group reduced when one member of the group evaporated.


    We could cover condensation in the same way.
    The average energy of the water molecules is sufficient for them to be a gas. Some have high energy, some have low energy, but the average is what ever it is.

    At some time, there may be a group of atoms which individually only have sufficient energy to be a liquid, so they fall to the bottom of the container as a water droplet. The average energyof the remaining water molecules in the gas is now higher .... .... etc.
     
  5. Nov 15, 2011 #4
    Re: Evaporation/condensation

    It all makes perfect sense now.

    This is the first time I post anything on any forum (I know weird), but the replies you gave were great! Thank you again.
     
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