CHAP. 7: PHASE EQUILIBRIA [CONTENTS] 172
7.1.3 Phase transition.
The term phase transition applies to a process during which a certain amount of substance
changes from one phase to another. Phase transitions may be reversible or irreversible. (For
the classification of phase transitions into first-order and second-order transitions see7.2.1).
Example
Give an example of an irreversible and reversible phase transitions.Solution
Ice melting in water at 40◦C and 101.325 kPa represents an irreversible phase transition. It is
typical of irreversible phase transitions that a slight change of conditions does not change the
direction of the process in any way. In our case a slight change of the initial temperature of water
(e.g. from 40◦C to 39.9◦C) or pressure (e.g. from 101.325 kPa to 101 k Pa) would not initiate
the freezing of water.Ice melting at 0◦C and 101.325 kPa may serve as an example of a reversible phase transition.
In this case the slightest lowering of temperature or pressure would replace melting with freezing.
Typical phase transitions in one-component systems are shown in the following table:
Type of phase transition Name of the process
Liquid → gas (vapour) vaporization, boiling
Gas (vapour) → liquid condensation
Solid → liquid melting, fusion
Liquid → solid freezing
Solid → gas sublimation
Gas → solid (vapour) deposition
Crystalline form → another crystalline form transformation of the
crystalline form
All these types of phase transition exist in multicomponent systems as well. In these systems,
however, such processes may occur which would be impossible in pure substances. For example,
the gas→liquid phase transition also involves, besides condensation, the dissolution of the gas
in the liquid, the solid→liquid phase transition also involves, besides melting, the dissolution