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Hot air in a closed container

  1. Sep 3, 2009 #1
    I have a plastic Gatorade type bottle that I reuse. Between uses, I fill it with hot water and shake, and then empty it. Once I empty it, if I put the lid on and wait, the bottle eventually deforms and sucks in on itself. Then when I remove the lid it returns to normal. Why does it deform?
    Is it the same reason my red plastic gasoline container deforms when its left out in the summer heat all day? Does hot air take up less volume than cool air?
  2. jcsd
  3. Sep 3, 2009 #2
    Well my explanation won't contain any type of formulas or even a good example. But as far as I know anything hot expands and anything cold contracts except in an event of a change of state for example liquid to solid or vise versa. So given that there is no change of state and that the air in the bottle is heated by the hot water that was previously there and the warm plastic, this air will expand farther than cold air. Some of the air is actually forced out of the bottle due to this expansion and therefore when you close the bottle with the hot air still in there, as it cools it takes up less space forming a sort of small vacuum effect on the bottle. That would be the layman explanation from a layman though.

    As far as the gasoline container, I wouldn't think that would be the same case however as you do not want to pressurize gasoline by having it completely sealed. There is always venting on gasoline cans to prevent vapors from building up. It's probably more than likely a deformation of the plastic due to the sun.
  4. Sep 3, 2009 #3

    About the gas can, when I remove the spout, air rushes in and the container returns to normal shape, just like the plastic bottle. Apparently this gas can does not have a vent. Is that dangerous, a gas can with no venting?
  5. Sep 4, 2009 #4
    Well, doesn't sound like it is vented well. Perhaps this could explain it though. Although it sounds contradictory that the plastic gas can would actually contract due to the heat of the sun when I mentioned before that things expand. But what happens is as the sun heats up the plastic, the molecules in the walls of the gas container expand making it more flexible and elastic, this weaker bond allows the gas container to deform.

    It actually seems kind of strange that the gas container would contract in the heat itself as if it is not vented correctly the warmer air and gasoline should expand due to heat. Perhaps it is that it is bulging on the sides and therefore deforming in height so while the gasoline can appears to be smaller and contracted, the force of the expanded vapors are actually pushing against the side walls of the container more. When you vent the container maybe then that hot air escapes and the bulge on the sides of the container are removed allowing the container to return to it's normal shape. Might need some clarification on this from someone more knowledgeable.

    As far as dangerous, perhaps under extreme heat for long periods of time the pressure could build up enough for the container to fail in some way. I guess I shouldn't say there are always venting caps on gasoline containers, although there probably should be, there are some of containers out there that do not vent. I wouldn't worry about it too much unless perhaps you are somewhere in the Arizona desert and leave a fairly full, non-vented, container of gasoline out in the hot, 115 degree heat for the day.
    Last edited: Sep 4, 2009
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