Page 210 - Physical chemistry understanding our chemical world

5

Phase equilibria

Introduction

Phase equilibrium describes the way phases (such as solid, liquid and/or gas) co-exist
at some temperatures and pressure, but interchange at others.
First, the criteria for phase equilibria are discussed in terms of single-component
systems. Then, when the ground rules are in place, multi-component systems are
discussed in terms of partition, distillation and mixing.
The chapter also outlines the criteria for equilibrium in terms of the Gibbs function
and chemical potential, together with the criteria for spontaneity.

5.1 Energetic introduction to phase equilibria

Why does an ice cube melt in the mouth?

Introduction to phase equilibria

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The temperature of the mouth is about 37 C, so an overly simple explanation of
why ice melts in the mouth is to say that the mouth is warmer than the transition
temperature T (melt) . And, being warmer, the mouth supplies energy to the immobilized
water molecules, thereby allowing them to break free from those bonds that hold them
rigid. In this process, solid H 2 O turns to liquid H 2 O – the ice melts.
Incidentally, this argument also explains why the mouth feels cold after the ice has
melted, since the energy necessary to melt the ice comes entirely from the mouth. In
consequence, the mouth has less energy after the melting than before; this statement
is wholly in accord with the zeroth law of thermodynamics, since heat energy travels
from the hot mouth to the cold ice. Furthermore, if the mouth is considered as an
adiabatic chamber (see p. 89), then the only way for the energy to be found for
melting is for the temperature of the mouth to fall.