(b) Phase transition
The degree of dispersal of matter and energy changes when a substance freezes or boils
as a result of changes in the order with which the molecules pack together and the
extent to which the energy is localized or dispersed. Therefore, we should expect the
transition to be accompanied by a change in entropy. For example, when a substance
vaporizes, a compact condensed phase changes into a widely dispersed gas and we can
expect the entropy of the substance to increase considerably. The entropy of a solid
also increases when it melts to a liquid and when that liquid turns into a gas.
Consider a system and its surroundings at the normal transition temperature, Ttrs,
the temperature at which two phases are in equilibrium at 1 atm. This temperature is
0°C (273 K) for ice in equilibrium with liquid water at 1 atm, and 100°C (373 K) for
liquid water in equilibrium with its vapour at 1 atm. At the transition temperature,
any transfer of energy as heat between the system and its surroundings is reversible
because the two phases in the system are in equilibrium. Because at constant pressure
q = ΔtrsH, the change in molar entropy of the system is3