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Condensation on a cold glass during a hot day

  1. Aug 21, 2012 #1
    Hello all.

    Consider the case whereby one brings out a cold drink on a hot day. Why do water droplets form on the surface of the cold glass?

    I would think that when water molecules in the gaseous state collide with the glass surface, they transfer some energy into the glass. Therefore by the surface of the glass, water molecules lose enough energy to form bonds with other molecules (they couldn't before since they had too much energy), eventually resulting in a droplet.

    Here is where I'm a little lost... why wouldn't condensation, then, occur if both the air and the glass were the same temperature as the glass in the former example?

    Basically, lets assume that the water molecule has energy X after interacting with the glass in the example above.

    Why wouldn't a gas of water molecules at temperature T of energy X simply condense on any surface, also of temperature T, since their energy is low enough to coagulate with other water molecules?
    Last edited: Aug 21, 2012
  2. jcsd
  3. Aug 21, 2012 #2
    Suppose the glass and the atmosphere were the same temperature. I will refer to the material of the glass as molecules, even though the nature of the glass solid may be a bit more complex.
    A molecule of water vapor collides with molecules of gas. Kinetic energy from the water molecules is passed on to the glass "molecules". Then, the water molecule may stick to a glass "molecule". Then, the motion of a glass molecule knocks the water molecule back into the atmosphere.
    If the glass is cold, the molecules of glass aren't moving as fast. They can't knock as many molecules of water off the glass. Molecules of water that stick to the glass remain stuck. They accumulate on the glass. Eventually, a liquid water film grows on the glass.
  4. Aug 21, 2012 #3


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    Water vapour in the air will condense if the air is cooled to below that humidity level's saturation temperature. Your explanation sounds okay, but requires that the molecules make contact with the glass. However, if you observe a cold flask sitting on a table, visible frost can be falling around it as air merely in the vicinity of the flask feels the chill.
  5. Aug 21, 2012 #4


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    My first thought is there needs to be a difference in termperature, else condensation isnt going to occur. In the atmosphere when talking in terms of meteorology we call this the Dewpoint ... from wiki...
    ... or as clouds in the atmosphere.

    I would imagine the same principle occurrs with your cold glass example .... if the vater vapor and the glass is the same or close in temperature, then the dewpoint wont be reached and condensation will not occur.
    On the other hand, as you, say, slowly cool down the glass, it will reach that dewpoint and condensation will eventually occur.

    That wiki link has lots of wonderful formula that I personally are not even going to try and comprehend ;)

  6. Aug 21, 2012 #5


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    Note that room temperature is room temperature, in the summer or winter. Condensation occurs in the summer and not the winter due to the difference in humidity, not the difference in temperature.
    So yeah - it can't be the collision itself that matters if the difference between the two cases is temperature. So what happens is that the glass, being cool, cools the air and water vapor in its direct vicinity. And when the pressure of water vapor in the air (aka absolute humidity) is higher, it can squeeze the water vapor out of the air easier when the water vapor's energy is low enough....which happens when it comes near a cold glass.
    You kinda answered your own question with the self-contradiction: The water molecule's energy isn't low enough to condense if what it hits has the same temperature as it does.
  7. Aug 24, 2012 #6
    That's precisely the explanation I gave; but if that was all that was happening, we would expect condensation between wet air of any temperature and a 'cold glass'.

    So the two ingredients are 1) temperature difference and 2) humidity difference.

    What's the physical explanation for how the water vapor 'squeezes out' water when the vapor is cooler, i.e. lower energy? This is the source of my confusion, really. Clearly the process can't be solely dependent on the energy of the water molecules in the cool air for the same reasons as in my OP.

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