Thermodynamics and microwaving food

[berra]

Hi, I just microwaved some food in a plastic container and noticed that it started to buckle inwards. So I wonder what thermodynamic variables were constant? Obviously not the volume V. The number of particles N was probably approximately constant. What more than N was constant?? What ensemble should I use if I want to do computations on this??

 

[SteamKing]

What computations? Use a different type of dish to microwave your food.

 

[berra]

Thermodynamical computations. Gibbs free energy requires temperature and pressure constant, for example. But the temperature is certainly not constant when heating in a microwave.

 

[Chestermiller]

Hi, I just microwaved some food in a plastic container and noticed that it started to buckle inwards. So I wonder what thermodynamic variables were constant? Obviously not the volume V. The number of particles N was probably approximately constant. What more than N was constant?? What ensemble should I use if I want to do computations on this??

If it buckled inwards (and warpage of the plastic was not a problem, see SteamKing response), some mass of water vapor must have escaped from the container during the heating. If it only buckled when the container cooled down, then this is almost certainly what happened. When the container cooled, the remaining water vapor in the container mostly condensed, and this created a partial vacuum in the container. The walls were not structurally stiff enough to support the pressure difference.

Chet

 

[dauto]

Alternatively the plastic container warped with the heat and became permanently warped. If that's the case then
1st: The volume changed because the container changed, particles were most likely lost. N is not conserved.
2nd: The plastic outgassed some chemicals into your food contaminating it. I wouldn't use this kind of container in the micro-wave any more.