When opening the closure of a thermos flask filled with coffee one often “hears” the equalization of the pressure difference that appears to be present between the cavity inside the flask and the surrounding room atmosphere. Recently we discussed this phenomenon when drinking coffee and guessed about the direction of the gas stream accompanying the pressure release. Our hypothesises were tested and we found, that the pressure was enhanced inside the flask. But this only holds when the coffee is fresh. After some hours one observes an inversion: the pressure inside dropped clearly under the environmental value and we wondered: what is going on inside the flask? That question gives the opportunity to discuss some basic thermodynamic concepts on different degrees of difficulty in a motivating manner.
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