Cold Water

  1. Hello ,

    I was always wondering about this phenomenon :

    when i take out a cold bottle from the refrigerator , after a few seconds (lets say at room temperature) the bottle is surrounded by water.

    the question is - why is it? where did the water come from ?
    is it should happen with any cold body ?

    (feel free to speak high phycis )

    thank u !
     
  2. jcsd
  3. Nugatory

    Staff: Mentor

    There's some amount of water vapor floating around in the room air. The air next to the cold bottle is cooled by the bottle, and when it cools some of that water vapor condenses onto the bottle.

    Any cold object will do the trick, but a bottle of water is particularly good at keeping its surface cool (because as the surface warms, water circulation brings more cold water from deep inside the bottle to the edge) so you may get a more pronounced effect with a bottle of water than, for example, an apple.
     
    1 person likes this.
  4. HallsofIvy

    HallsofIvy 40,310
    Staff Emeritus
    Science Advisor

    Note that the amount of water that can be held in the air increases with the temperature of the air. Since the air is, as Nugatory says, cooled by the bottle the amount of water that it can hold is reduced. If the humidity (amount of water already in the air) it high the water will condense out of the air.
     
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  5. sophiecentaur

    sophiecentaur 13,404
    Science Advisor
    Gold Member

    That expression is a bit misleading but it's commonly used. In fact, it's all a matter of partial pressures. Air isn't a sponge (although that's what it seems). The maximum proportion of water vapour in the air is totally due to the vapour pressure at the ambient temperature. You would get exactly the same amount of water vapour in a container if the container were evacuated and then the pressure allowed to increase as water evaporates off a water surface. The equilibrium situation would always involve 'saturated' air (there- I've done it myself!)
     
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  6. so I have another question - what makes the water act like a gas ? I know that the evaporation temp. at room pressure is ~ 100 C , so if the temperture is < 100 and the pressure is ~ 1 atm why the water are gas?

    thank you
     
  7. sophiecentaur

    sophiecentaur 13,404
    Science Advisor
    Gold Member

    There is still a pressure of vapour from the water surface at temperatures less than 100C. The reason that boiling occurs is that the vapour pressure at 100C is equal to atmospheric. Up on a mountain top, water will boil rapidly in the low 90s (Hopeless for cooking) but, in a pressure cooker, the water will boil at a higher temperature.
    Below boiling point, there is a constant exchange of molecules of and onto the surface. Don't confuse the equilibrium situation (in closed box after a long time) with what happens when there is a constant supply of fresh air over the surface.
     
  8. so lets say I take a glass of water and put it in a closed box ( vacccum)- there will be no vapours?
     
  9. sophiecentaur

    sophiecentaur 13,404
    Science Advisor
    Gold Member

    There will be vapour with a pressure appropriate to the temperature in the box. The time taken to reach equilibrium will depend on the volume of the box and the temperature.
     
  10. Perhaps think of "the temperature of the water" as representing the average velocity of the water molecules. There will always be some that are moving faster then average and some will be fast enough to escape the surface forming water vapour. If the box is sealed the vapour pressure rises until there is an equal flow back the other way and an equilibrium is reached.
     
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