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V asked in Science & MathematicsChemistry · 1 decade ago

when you close the hot water with a lid do you see a water droplets under the lid? WHY?

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  • 1 decade ago
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    V,

    When you have water, and especially hot water, some amount of the water is going to be in the vapor phase (steam). When the steam contacts the lid, which is often cooler than the steam, the steam condenses back to water (which is why you get the droplets under the lid).

    Once this starts to happen, equilibrium gets set up where the water molecules leave the surface of the liquids (both under the lid and in the main liquid in the cup). The side which remains warmer will lose water and the one that cools more (usually the lid) will get more water condensation (droplets) -- that's why the droplets on the lid tend to get bigger with time.

    Hope that helped!

  • 1 decade ago

    I think you mean "when you put a lid on a vessel containing hot water, do you see water droplets on the lid, and why? " Well, of course you do as you know if you've ever put a lid on hot water. I suppose "hot" is defined for this purpose as hot enough to create the droplets. The water is hot, so the vapor pressure of the water is higher than the vapor pressure of water would be at the surrounding room temperature. The sides of the vessel may or may not have droplets, but the vessel is usually at a temperature that is about the same as the water. The lid you put on, however, is usually at room temperature and so it's colder than the water. Therefore water vapor at the higher vapor pressure of the hot solution condenses on the cool lid. This warms the lid so that the process eventually stops. You can stop it from happening at all if you heat the lid before you put it on. Even then, when the air of the room cools the lid again the droplets may start to form.

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