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I have read many times that the reason temperature stays constant when heating a substance through a change of state is because most of the joules of energy put into the substance go into the potential energy store of the substance (to help the molecules/atoms move further apart from each other even though they have attractive forces towards each other) whilst barely any of the joules of energy put into the substance go into increasing the kinetic energy of the molecules (i.e. by making them vibrate more). (Of course, when the substance is not being heated through a change of state, a greater proportion of the total energy put in goes into the kinetic energy store of the molecules whilst a smaller proportion (than during a change of state) goes into the potential energy store of the molecules: this is what the specific heat capacity reflects, from my understanding.) But no sources I've found explain the reverse situation: where temperature stays constant whilst cooling a substance through a change of state. So I wanted to check if the argument for this would be the reverse argument, as you would expect: during a change of state, you are still taking a total number of joules out of the substance per second in order for it to freeze/condense, but a very large proportion of those joules taken out comes from the potential energy store of the substance (since the molecules/atoms are quite suddenly getting quite a lot closer together again during the change of state) and only a tiny proportion of those joules taken out come from the vibrational/kinetic store of the molecules of the substance, so since this vibrational level is what we measure as heat, the temperature does not go down much. So is this argument correct, please?

Willow
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    Does this answer your question? Why, exactly, does temperature remain constant during a change in state of matter? – Chemomechanics Apr 26 '22 at 21:28
  • The temperature doesn't quite stay constant; some undercooling is required to provide a driving force for nucleating the condensed liquid or solid state. The situation is as described in the link, with all the temperature differences reversed. – Chemomechanics Apr 26 '22 at 21:30
  • @Chemomechanics, I did look through that before writing this question, but all of the answers except the first one were talking about the situation of a constant temperature during heating a substance through a change of state, not cooling it, and I didn't understand the first answer properly, because I'm only doing GCSE. Besides, I really want to be able to visualise this on a conceptual level, and the first answer seemed to be more about mathematical number crunching. But thanks anyway! – Willow Apr 27 '22 at 10:10
  • The math of the first answer is describing the energy released as molecules form bonds while condensing into a liquid droplet or solid crystal. Initial clusters assemble but fail to grow unless they reach a certain size, which happens explosively at a certain undercooling. Is this what you’re asking about? – Chemomechanics Apr 27 '22 at 14:14
  • That is fascinating, and it does kind of answer my question, because if energy is "released as molecules form bonds while condensing" or freezing, then that energy would be the kind of potential energy I was referring to, because it would be released due to the substance being in a more stable state. So that would seem to suggest that my guess in the question above - "during a change of state, you are still taking a total number of joules out of the substance per second in order for it to freeze/condense, but a very large proportion" (response continued in next comment) – Willow May 01 '22 at 19:08
  • "of those joules taken out comes from the potential energy store of the substance" was probably correct, because if a lot of joules of potential energy are being released during this change of state, then" (response continued in next comment) – Willow May 01 '22 at 19:15
  • most of the energy released per second from that cooling substance would be being released from the potential energy store during those seconds, whilst only a little was being released from the thermal energy store. This would cause the constant temperature during cooling through a change of state. However, the whole reason I spent time writing out the question in the first place was because I hoped for definite verification of my guess, to make sure I was understanding it correctly, which I still haven't got ... But thanks for demystifying the first answer, @Chemomechanics; that has helped. – Willow May 01 '22 at 19:21

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