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I have observed a phenomenon that I find hard to understand.

A plastic, melamine like dish is filled with food (spaghetti, chile, soup, etc.) The dish and food is heated in a normal microwave oven for several minutes. When it is pulled out, the food is steaming and the dish is also somewhat hot. The food is consumed over the next couple of minutes and the empty dish is left on the counter. Now in my estimation the slightly hot dish should cool off until it reaches the ambient temperature of the room and then stay at that temperature until possibly used for another meal. However what I have observed is that the dish cools to room temperature, but then keeps on cooling. If the dish is picked up 10 or 15 minutes later, the bottom is close to ice cold.

Can anyone explain what may be happening?

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Options:

  • Your dish is not actually as cold as you think it is: verify with a thermometer that the dish is actually colder than ambient temperature. Your hand is a terrible thermometer because it does not reach thermal equilibrium with the object you are trying to measure, and so it is sensitive to the thermal conductivity of the substance you are touching.
  • Your dish is resting on a surface that is colder than ambient temperature. I'm not sure what your dish is resting on. If you confirm that your dish is in fact colder than ambient temperature, you should check the surface it is on (again, with a thermometer, not your hand). Also check the temperature of the air.
  • Your dish has a refrigerator hidden inside it. I say this partly as a joke, but it is plausible that there could be some endothermic chemical reaction going on. Cooling (below the surrounding temperature) requires work, which means the energy has to come from somewhere. This is why you have to plug in your fridge. For your plate, the only plausible energy source I can think of is chemical, though I've never heard of anything like this for a plastic dish before.
  • Your dish is violating the second law of thermodynamics. This is improbable...
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    $\begingroup$ Another possibility is that the dish is wet and there is some evaporative cooling going on to bring it below room temperature. $\endgroup$
    – James
    Commented Sep 1, 2015 at 17:39
  • $\begingroup$ Thanks to the comments received so far. I'd like to respond to each: The hand temperature is not the issue. This dish is markedly cooler, not just a few degrees and several people have all had the same observation. It feels like a bag of ice had been left in the dish for 10 mins or so $\endgroup$
    – Need 2 no
    Commented Sep 1, 2015 at 18:15
  • $\begingroup$ The dish is not resting on a cold surface. Usually it sits on countertop or sometimes on a pad of note paper. Neither surface changes the experience. $\endgroup$
    – Need 2 no
    Commented Sep 1, 2015 at 18:16
  • $\begingroup$ There is no refrigerator inside :-) It may be some sort of chemical reaction, though like you I can't think of how a dish of this sort could do that. $\endgroup$
    – Need 2 no
    Commented Sep 1, 2015 at 18:23
  • $\begingroup$ I don't think the dish really violates the 2nd law because eventually it does reach ambient room temperature. This is a transitory state that lasts about 10 to 20 mins. $\endgroup$
    – Need 2 no
    Commented Sep 1, 2015 at 18:24

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