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I have two energy drink cans on my desk next to each other. One is unopened, full of liquid. The other is opened and mostly consumed. They have both been in this location on my desk for several days. The unopened can feels cool to the touch. The opened can feels warmer to the touch, except for the portion of the bottom of the can where there is still liquid--that feels cool to the touch too.

I read an answer that said water stays cooler than air due to evaporation. But what about the unopened can? Why does the surface of that can feel cooler to the touch than does the portion of the opened can that doesn't contain liquid?

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The most straightforward explanation is that the liquid, being a better thermal conductor than air (by a factor of about 20), with a greater volumetric heat capacity (by a factor of about 3000), is drawing heat from your finger faster, and stays near its original temperature longer. (Thank you to Puk for pointing out the latter factor.)

Modeling this multilayer heat transfer process isn't trivial, but we can refer to Fourier's law and conclude that for any given temperature difference between your fingertip and inanimate objects at room temperature, the heat flux over a given distance increases with increasing thermal conductivity. In addition, as mediated by the heat capacity, a certain volume of water heats up much less than air for a given flux. The container material is the same in both cases, so we look to the material properties of the medium directly behind it.

The same effect occurs with heating; putting your hand in a 100°C oven isn't immediately hazardous, but contact with 100°C water would burn you.

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    $\begingroup$ There is also a massive difference in volumetric heat capacity between air and water, which could be an even more important factor in this case. $\endgroup$
    – Puk
    Commented Dec 14, 2023 at 5:12
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    $\begingroup$ Great point, definitely worth editing to add. $\endgroup$
    – Chemomechanics
    Commented Dec 14, 2023 at 5:58
  • $\begingroup$ The illustration of the hand momentarily in a hot oven versus in hot water vividly illustrates the point. Thank you! $\endgroup$
    – loopernow
    Commented Dec 14, 2023 at 14:57

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