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
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.
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