# Why does the plot of volume and temperature have a kink in a second order phase transition?

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?

• 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. Dec 10 '20 at 19:34
• Thank you Mr Jon Custer. I was interested in the actual mechanism as to why the kink was there. Dec 11 '20 at 4:14
• 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.). Dec 11 '20 at 13:57
• 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 Dec 11 '20 at 14:16
• @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. Dec 11 '20 at 17:01