Whenever I wash my thermos, I put hot water and then some soap in; then I seal the one end with my hand or use the lid. After shaking it up, if I slowly remove the lid or my hand, it expels a little air. Why is that? Does it have something to do with increased surface area of soapy water? Or is it the fact that the air is heated by the water, even though the water must surely cool slightly?
[Physics] Why does pressure in a thermos increase after shaking up hot water and soap
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Best Answer
When you pour the hot water in, the air inside the thermos is still quite cold (ambient temperature, approx.)
But then when you shake it up the cold air is heated by the hot liquid. Gases expand considerably when heated, approximately acc. the Ideal Gas Law:
$$pV=nRT$$
This causes a modest (and harmless) pressure increase in the flask, which is what you experience.