I have two insulated travel mugs. One has a pour spout and one has a straw. They are both leak proof, so in theory, the lids of each form a tight seal. When I open the lid to drink my hot coffee from the thermos with a straw, the coffee explodes out the top. When I open the lid to drink from the thermos with the pour spout, it doesn’t. What gives?
In: Chemistry
This has been answered, so I’ll add an anecdote: don’t use a straw-based water bottle on an airplane. I filled a water bottle on the ground at normal atmospheric pressure, and then opened it to take a drink on the plane. It shot out water for a good half second before I figured out what was going on and got my mouth on the straw. I was forced to take a goodly-sized gulp in order to equalize the pressure.
Same deal as your scenario, except it’s the expansion of the heated air (or, because expansion was constrained, the increase in pressure of the heated air). Once it was exposed to lower-pressure outside air, the liquid was forced out.
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