In my science book it is written that as the pressure increases the melting point of a solid decreases but i think that it is wrong as the pessure will make the molecules stick together instead of flowing like liquid thus increasing the melting point right? So my book is wrong right?

  • $\begingroup$ What does it mean that this point "decreases" ? $\endgroup$ – Tom-Tom Apr 27 '15 at 8:49
  • $\begingroup$ A comment about the title: "as the pressure increases the melting point of a solid decreases" does not compel the relation to be one of inverse proportionality. $\endgroup$ – AV23 Apr 27 '15 at 13:11

First of all, it's not universally true. Particularly with solid phases that only appear at high pressures.

But the reason why it does often occur, e.g. with ice, is that as you increase the pressure, you force the atoms closer together which causes the enthalpy of the system to increase. Increasing the pressure of a liquid phase, on the other hand, does not produce the same enthalpy rise. This is hopefully intuitive: since the atoms are not rigid, they are better able to respond to pressure-induced changes in the local structure.

Consequently, the free energy of the solid phase increases more than the liquid phase, which shifts the transition point (where the free energies are equal) towards the solid phase.

  • $\begingroup$ I did not understand can you explain what i asked i asked about solid ice $\endgroup$ – Bhavesh Apr 27 '15 at 9:28
  • $\begingroup$ @Bhavesh My answer applies to ice. Which part of it did you not understand? A shortened version: Pressure increases the enthalpy of ice more than it does water. (In other words, your book is correct, which you can see if you look up the phase diagram of water). $\endgroup$ – lemon Apr 27 '15 at 10:13
  • $\begingroup$ A different way to phrase it: The full Gibbs free energy formulation includes $PV$. Since a first order phase transition requires a change in volume, there is a $P \Delta V$ factor. Thus, depending on the sign of $\Delta V$, higher pressures may either help or hinder the phase transition. $\endgroup$ – Jon Custer Apr 27 '15 at 15:36

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