I can't think of a good title for my post, sorry about that.
I've got a lunch box (called a bento box) Basically it's a plastic box with a plastic lid with a rubber rim around the lid to create an air tight seal with the top is on.
Now when I microwave my lunch (take the top off, stick it in the microwave to heat the contents, then take it back out and put the top on) it creates a vacuum such that sometimes I really need to pry the lid off with a fork.
How does this work? Does it have to do with the lid and base being different temperatures?
My very rudimentary understanding of physics is that the hot gases should be expanding and pushing outwards. I would think that would make it easier to open when the contents were hot.
Best Answer
The sealing effect is caused by pressure differences.
Once you put the lid back on, the warm air in the box cools, reducing the gas pressure inside the box below atmospheric pressure. So the atmospheric pressure outside the box tends to seal the box if there is contact all around the lid to prevent air moving between inside and outside. Water vapour condensing inside the box adds to the effect.
The same sort of thing can happen when you close the door of a refrigerator and the air inside cools, again leading to a partial seal.