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I had a vegetable soup in a crystal tupperware and I put it in the microwave to heat it up. The funny thing is that when I took it out, it was impossible to open it up. The lid is bent towards the inside, as if the air has escaped the tupperware somehow.

Can anyone tell me why this might have happened?

Thanks

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1 Answer

up vote 1 down vote accepted

When heated, air expands. Now when you take the tupperware out of the microwave and it cools down, the air inside cools down again and contracts, and 'sucks' the lid inwards.

Actually, that's not quite right - it's more precise to say the air outside presses the lid shut. The bodies of air outside and inside seek to equalize their pressure, and normally the tupperware would just shrink / contract until it's inside pressure is equal to the room pressure. But since the container is somewhat rigid, it can't shrink enough and there remains a pressure difference. This causes a force acting on the lid, keeping it shut.

So:

cooling down => volume shrinks

cooling down, but forcing the volume to be constant => pressure goes down

inequal pressure => force towards the lower pressure

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Very good, but one rather big mistake: this "action" comes from condensation of vapour, its similar to the working of the atmospheric steam engines of Newcomen or Renoir. When heating close to boiling point, the vapor pressure becomes about one atmosphere and the air is replaced entirely by vapor. After cooling the vapor pressure falls again, this resulting in a much bigger pressure difference than the one You get from expanding/contracting air, as You meant. Since about 110 Years this method is used to preserve food in Germany/France in glasses. de.wikipedia.org/wiki/Einkochen –  Georg Sep 22 '11 at 16:35
    
Fascinating... but you would probably get the suction also without the vapor effect. What the Einkochen gives you - besides a bigger pressure difference - is an almost oxygen-free environment. (Btw. German technical terms FTW!) –  jdm Sep 22 '11 at 18:41
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protected by Qmechanic Feb 15 '13 at 1:42

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