194 The Principles of Thermodynamics
facilitated by this, but we will discuss that issue elsewhere, but it is experimentally
true that the melting point of ice is indeed lowered at higher pressure. But there is
something else about water that is unusual too! Below 4
◦
C, water expands upon
cooling! So in fact ice is lighter than water reversing the usual situation that solids
are denser than liquids! Therefore our ’rule’ still works, and since the sign of relative
densities is reversed, so must the slope of the freezing curve!
Example 10.1: Ice melting on a very very cold day
How does snow melt during a bitter winter when temperatures are much below 0
◦
C? It
certainly does not melt by first becoming water. To understand what happens,
refer to the phase ...