The overall process of reaching phase equilibrium and the concepts associated with it can be very complex. Here’s where the laws of thermodynamics come into play. It allows one to understand and predict the behaviour of substances in different phases interacting with one another. This post is a review of phase equilibrium from related sections 5.1, 6.1 and 6.2 of the Physical Chemistrybook.

What is a phase?

Let us begin by defining a phase. It is a homogeneous portion of matter that has uniform physical and chemical characteristics (everyday examples are ice and water, syrup and sugar).
Elements and compounds can move from one phase to another when specific physical parameters are present, such as temperature, pressure and composition. When energy is added (e.g. by increasing temperature), a change in state results:

solids –> liquids –> gases –>plasmas

Elements and compounds can move from one phase to another but still be the same substance.
Here is a simple example. You can see steam (vapour) over a boiling pot of water. When steam condenses it is transformed into water. By placing the water into a freezer, it is transformed it into an ice particle. Therefore no matter what phase it was in, it was always water and it had the same chemical properties.

Phase Equilibrium

Any change in temperature, pressure and composition causes an increase in free energy and away from equilibrium, i.e. forcing a move to another phase. The Gibb’s Phase Rule is a useful tool used to define the number of phases and or degrees of phase changes that can be found in a system at equilibrium. It can be defined algebraically as:

F = C + 2 – P

F is the number of degrees of freedom, or variance,
C denotes components, where the value 2 stands for temperature and pressure and
P is the number of phases.

A Phase Diagram

A phase diagram is one in which the relationships between the phases of a substance can be summarized. We can see important properties such as melting point, boiling point, transition points and triple points of a substance on a phase diagram.
Every point on the phase diagram (see above) represents a state of the system as it describes temperature and pressure values. The lines on the phase diagram divide into regions – solid, liquid or gas.

The phase diagram of sulphur (a one component system)

Sulphur is a one component system and it can exist in two solid forms: rhombic and monoclinic. The rhombic form is stable at ordinary temperatures while the monoclinic form is stable at higher temperatures.

Substances that can exist in more than one crystalline form are said to display the phenomenon of polymorphism. There are two types of polymorphism, enantiotropy and monotropy.
Enantiotropy is a more common form of polymorphism – exhibited by sulphur, tin, ammonium nitrate, carbon tetrachloride among other substances.
Enantiotropy is the crystalline modifications of the same substance in which one form is stable above a definite temperature and the other stable below it, so that the forms can change reversibly one into the other.
Invariant states are represented by points on phase diagrams. Systems with one degree of freedom, F = 1, are univariant and those with two degrees of freedom, F = 2, are bivariant.
For a one-component system such as sulphur, the coexistence of three phases in equilibrium corresponds to an invariant state since

F = C – P + 2 = 1 – 3 + 2 = 0

This state is called a triple point; the point where it corresponds to the coexistence of solid, liquid and vapour phases at a particular temperature and particular pressure.
In the sulphur system four possible triple points exists for the four phases: rhombic sulphur (solid), monoclinic sulphur (solid), sulphur (liquid) and sulphur (vapour).