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enter image description here

The $(b)$ part of the figure shows how thermodynamic variables change in a second order phase transformation. We observe there is a kink in the plot of volume with temperature.

Why is that so?

The plot is from http://faculty.chem.queensu.ca/people/faculty/mombourquette/Chem221/5_PhaseChanges/PhaseThermo.asp

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    $\begingroup$ Because they are second order phase transitions? GO back to the definition of a second order phase transition, such as in en.wikipedia.org/wiki/Phase_transition. $\endgroup$
    – Jon Custer
    Commented Dec 10, 2020 at 19:34
  • $\begingroup$ Thank you Mr Jon Custer. I was interested in the actual mechanism as to why the kink was there. $\endgroup$
    – Kashmiri
    Commented Dec 11, 2020 at 4:14
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    $\begingroup$ An example for a), first order of phase transition, is the one about liquid water $\ce{->}$ gaseous water / steam; you have both a sudden jump in $\Delta{}V/\Delta{}T$ and the energy needed for this transition ends in latent heat. Guess what, not all transitions involve latent heat (i.e., compare with your lecture notes, references at disposition, etc.). $\endgroup$
    – Buttonwood
    Commented Dec 11, 2020 at 13:57
  • $\begingroup$ Thank you buttonworth. I was interested in the second order transition in which there isn't a jump but just a bend in the plot of V with T $\endgroup$
    – Kashmiri
    Commented Dec 11, 2020 at 14:16
  • $\begingroup$ @YasirSadiq - because instead of a jump in volume, there is a jump in the derivative of volume (for example). And for a third order phase transition there would be a jump in the second derivative of volume... That is how the order of a phase transition is defined. $\endgroup$
    – Jon Custer
    Commented Dec 11, 2020 at 17:01

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