Why are Melting Point and Freezing Point the Same?

Most recent answer: 09/10/2013

Q:
How come melting point and freezing point are same?
- cath (age 16)
A:

That simple question has a deep answer. At any temperature and pressure, things settle in to the form that has the lowest "free-energy". Another way to think of it is that they settle down to whatever has the most entropy, counting the entropy of their neighbors. That basically means they pick the form which represents the most possible quantum states, under the circumstances. 

Now at some temperature and pressure the free energy of the liquid can either be greater than, equal to, or less than the free energy of the solid. If it's less than, the equilibrium state is liquid. If it's greater than, the equilibrium state is solid. Only at just the right point for the two free energies to be equal can you either get the liquid or the solid or a mixture. That's the melting point and the freezing point.

In practice, things can be a little more complicated. When you start to cool the liquid it can be very slow to freeze even when it's colder than the true freezing point. The reason is that it's hard to get a little solid cuyrstal big enough to get the process started. You can get what's called a super-cooled liquid. (see ) So it can look like the freezing point is lower than the melting point. In more careful measurements, using more time or using things to help get the process started, you see that there's just one true melting/freezing point.

Mike W.


(published on 09/10/2013)