Difference Between Phase Diagram and Equilibrium Diagram

Key Difference – Phase Diagram vs Equilibrium Diagram
 

Phase diagrams or equilibrium diagrams are charts or graphs showing the relationship between temperature, pressure and the composition of any system. These diagrams give details on conditions of thermodynamically distinct phases that coexist in equilibrium with each other. A phase is a homogeneous portion of a system that has uniform chemical and physical properties. There are three major phases that a substance can exist: solid, liquid and gas phase. There is no difference between phase diagram and equilibrium diagram.

CONTENTS

1. Overview and Key Difference
2. What is a Phase Diagram
3. What is an Equilibrium Diagram
4. Phase Diagram vs Equilibrium Diagram
5. Summary

What is a Phase Diagram?

A phase diagram is a chart that summarizes the details on conditions of thermodynamically distinct systems that coexist in equilibrium with each other. A phase diagram shows phase boundaries or equilibrium boundaries that separate phases from each other.  The phase diagram is a plot of pressure vs temperature.

Important Components of a Phase Diagram

Some important components in a phase diagram include followings:

• Phase Boundaries (equilibrium boundaries) – the lines in a phase diagram that separate two phases; each phase that is in equilibrium with another phase.
• Triple Point – a point at which equilibrium line intersect. A triple point indicates the conditions of a system that has a substance that can coexist in all three phases of matter (solid, liquid and gas).
• Critical Point – the temperature and pressure at which that substance can behave as a gas and a liquid at the same time; indistinguishable gas and liquid phases occur. Hence, there are no phase boundaries. Critical point is the end point of the curve in the phase
• Fusion Curve (melting or freezing curves) – the boundary line in a phase diagram that shows the conditions for the phase transitions between the gas phase and the liquid phase (blue coloured line in above diagram).
• Vaporization Curve (or condensation curve) – the boundary line in a phase diagram that shows the conditions for the phase transitions between solid phase and liquid phase (green coloured line in above diagram).
• Sublimation Curve – the boundary line in a phase diagram that shows the conditions for the phase transitions between solid phase and gas phase (red coloured line in above phase diagram).

Here, fusion is the melting or freezing that involves the phase transition between gas phase and liquid phase. Vaporization is the conversion of a liquid into vapor phase (gas phase) whereas condensation is the conversion of a vapor into liquid. Sublimation is the conversion of a solid into gas phase directly, without going through a liquid phase.

Types of Phase Diagram

There are few types of phase diagrams.

Unary Phase Diagrams

These are the simplest form of phase diagrams. These diagrams show the three phases of a substance separated from equilibrium boundaries (such as in Figure 1).

Binary Phase Diagrams

A binary phase diagram shows the equilibrium between two substances that exist in the same system. In most cases, the pressure remains the same, and the variables are temperature and the compositions of substances. Here, the two substances can be metals, a metal and a compound or can two compounds.

What is an Equilibrium Diagram?

An equilibrium diagram is a chart that shows the equilibrium between phases of a substance that coexist in a closed system. It is also known as a phase diagram.

What is the Difference Between Phase Diagram and Equilibrium Diagram?

• There is no difference between phase diagram and equilibrium diagram because they are synonyms. Both terms refer to the chart that summarizes the details on conditions of thermodynamically distinct systems that coexist in equilibrium with each other.

Summary – Phase Diagram vs Equilibrium Diagram

A phase is a homogeneous portion of a system that has uniform chemical and physical properties. There are three major phases that any substance can exist: solid phase, liquid phase and gas phase. A phase diagram is a chart that represents the equilibrium exists between different phases that coexist in the same closed system. This diagram is also called an equilibrium diagram because it shows equilibriums.

Reference:

1. “Equilibrium diagram.” The Free Dictionary, Farlex, Available here.
2. Libretexts. “Phase Diagrams.” Chemistry LibreTexts, Libretexts, 6 Feb. 2018, Available here.
3. “Liquids.” Phase Diagrams, chemed.chem. Available here.

Image Courtesy:

1. “Phase-diag2” By Matthieumarechal (CC BY-SA 3.0) via Commons Wikimedia [modified]
2. “Partial Melting Phase Diagram” By Solidification_1er_cristal.svg: Cdangderivative work: Fitzgabbro (talk) – Solidification_1er_cristal.svg (CC BY-SA 3.0) via Commons Wikimedia [modified]