Phase Changes

The way in which substances change phase is totally fascinating.

A collection of water molecules are capable of smashing the steel hull of an ocean liner if their temperature is 272 K. If their temperature is increased by less than 1% to 274 K they pose no danger to the ship at all. When ice melts some of its properties change dramatically even though the temperature changes by only a tiny amount. The phenomenon of ‘sudden change’ of properties is the most general, and the most striking, characteristic of phase changes. (Image is the Sinking of the Titanic taken from Wikipedia)

In the preceding chapters we have examined the properties of gases, solids and liquids, and seen how it is possible to understand their behaviour in terms of the interactions between the atoms and molecules of which they are made. The division into just three phases is natural, since in our experience solids, liquids and gases behave strikingly differently.

In order to understand phase transitions in anything more than a qualitative sense it is necessary to grasp the concept of Free Energy. Curiously, calculations of Free Energy are routine for first year students of Chemistry, but are frequently only introduced in the later years of Physics courses. I don’t know why. The concept is enormously powerful.

In Chapter 10 we see how the concept of Free Energy and allows us to understand the key features of phase transitions, in particular: the idea of the ‘order’ of a phase transition; the concept of nucleation in enabling transitions; and phase diagrams that show the physical regimes in which different phases exist.

I urge you again to try to retain your sense of wonder as you watch an ice cube met or water boil.  

  • Chapter 10 contains an exposition of the way Free energy .
  • Chapter 11 contains data on the phase transitions and looks at how we can understand the data using the ideas described Chapter 10.