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I was reading the following basic question about temperature change during phase change (During phase change in matter, why doesn't the temperature change?), and the answers provided clearly explained the concept. However, in everyday life, for instant a pot of frozen soup, this does not always prove true. Part of the soup in the pot can be boiling, while there are parts that are still frozen. They cannot possibly all be the same or nearly the same temperature. How can this be explained thermodynamically?

suse
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    You are not at equilibrium so cannot use normal thermodynamics. Thermal diffusion is slow compared to the rate of heating is giving rise to what you observe, usually there is melting round outsides of the pan and solid in the middle. – porphyrin Oct 28 '21 at 07:29
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    Without even mentioning thermodynamics, doesn't this look even obvious to you? – Alchimista Oct 28 '21 at 09:08

2 Answers2

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The system you describe is not at thermodynamic equilibrium. The system must be spatially uniform only if it is at thermodynamic equilibrium.

Chet Miller
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I was reading the following basic question about temperature change during phase change (During phase change in matter, why doesn't the temperature change?), and the answers provided clearly explained the concept.

One of the answers in the link spells out the condition for the temperature not changing: "Mix water and ice in a container. Keep stirring the mix while the ice is melting." Ideally, the heat transfer is slow compared to the mixing so that the temperature is identical throughout the sample. When you try to find the melting or boiling point of a substance, that is your goal.

However, in everyday life, for instant a pot of frozen soup, this does not always prove true. Part of the soup in the pot can be boiling, while there are parts that are still frozen.

Yes, your observation is correct. This is not an ideal experiment but an everyday observation. When you want to eat warm or hot soup, you are not trying to determine the melting point of water (it is not pure water anyway). So you just crank up the heat to get your soup hot as fast as possible without burning any of it.

They cannot possibly all be the same or nearly the same temperature. How can this be explained thermodynamically?

Different parts of a sample can have different temperatures. In those cases, the system will never be at equilibrium. Most of the well-defined thermodynamics are about equilibrium states, so there is no contradiction of theory and observation here. Even when you melt pure ice very slowly, it is not an equilibrium process; however, you can describe it partially through equilibrium thermodynamics because you are very close to equilibrium in that case. When you boil soup while there are still chunks of ice in it, you are not.

Karsten
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