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Phase Diagrams

Phase Diagrams

Author: Jennifer Roushar
  1. Define triple point and critical point.

  2. Demonstrate how to extract valuable information from a phase diagram.

This packet should help a learner seeking to understand how to graphically represent phase changes.

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How to Interpret a Phase Diagram

Source: Jennifer Roushar; Images from

What Does Triple Point and Critical Point Mean?

On a phase diagram, you will see the labels triple point and critical point.  The triple point is the point where all three states of matter (solid, liquid, and gas) can be found together.  Only at this temperature and pressure will you find all 3 states. 


The critical point represents the temperature at which if exceeded the substance will always be a gas no matter what the pressure is.