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
Id wager its because of pressure. Water is hot, put into sealed container. Water evaporates and produces pressure.
You open the straw thing. The bottom of the straw is IN the water, the pressure forces the water through the straw.
You open the other one. The bottom of it is NOT in the water, so instead the hot steam can escape, without having to push any water aside.
Do they both have vents? Maybe one of the vents is clogged or has a one way valve.
Open the straw when you screw the lid on too. It allows the coffee to fill the straw so that the next time you open it, it won’t have the additional pressure difference of the liquid’s height.
When you increase the temperature of air, the air has to either expand in volume, or increase in air pressure. When you put a lid on your hot coffee, you have also trapped some amount of ambient air in there too. The hot coffee heats up that air. The air can’t expand because it is contained in the cup. So the air has to build in pressure instead., which might push coffee up the straw, or rapidly depressurize when opening the spout. PV=nRT for those a little older than 5.
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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