Reading Phase Diagrams Reference Page
Can You Tell an Azeotrope from a Peritectic? Get Your Phase Diagrams Right!

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Intro to Can You Tell an Azeotrope from a Peritectic? Get Your Phase Diagrams Right!

Many chemical processes involve systems where multiple phases can exist simultaneously. Phase diagrams provide a graphical way to understand how temperature, pressure, and composition determine which phases are present.

Learning to read phase diagrams allows engineers to predict phase behavior, determine phase compositions, and calculate the relative amounts of phases in equilibrium mixtures.

The videos on this page introduce the structure of phase diagrams and demonstrate how to interpret them correctly.

Part 1

This video introduces the basic structure of phase diagrams and explains the meaning of phase boundaries, phase regions, and the Gibbs phase rule.

Visuals

Part 2

This video continues with examples of interpreting phase diagrams and demonstrates how the lever rule can be used to determine the relative amounts of phases in equilibrium mixtures.

Visuals

Examples and Definitions

Definitions

Phase Diagram
A graphical representation of the equilibrium relationships between phases of a substance or mixture as a function of temperature, pressure, and composition.
Azeotrope
A mixture that boils at a constant temperature with the vapor having the same composition as the liquid. Because of this, the mixture cannot be separated by simple distillation.
Vapor
The gas phase of a substance below its critical temperature.
Gas
The gas phase of a substance above its critical temperature. In everyday language the terms gas and vapor are often used interchangeably.
Triple Point
The unique combination of temperature and pressure at which three phases coexist in equilibrium.
Lever Rule
A graphical method used to determine the relative amounts of phases present in a two-phase region of a phase diagram.
Eutectic
A reaction in which a liquid transforms into two solid phases upon cooling.
Eutectoid
A reaction in which a solid phase transforms into two different solid phases upon cooling.
Peritectic
A reaction in which a liquid and a solid combine to form a new solid phase upon cooling.
Peritectoid
A reaction in which two solid phases combine to form a new solid phase upon cooling.
Monotectic
A reaction in which a liquid transforms into a solid and a second liquid phase upon cooling.
Syntectic
A reaction in which two liquid phases react to form a solid phase upon cooling.