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Phase Diagrams: Critical Point, Triple Point and Phase Equilibrium Boundaries

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  • 0:05 Phase Diagram
  • 1:48 Triple Point
  • 2:16 Critical Point
  • 2:47 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn what a phase diagram is and how to interpret phase diagrams. Discover how to use a phase diagram to locate the phase equilibrium boundaries. Also, learn what a triple point and critical point are and how to locate them on a phase diagram.

Phase Diagram

Look on the back of a package of cake mix. See where it says 'high-altitude directions?' Ever wonder why it has this section? It is because things cook differently at higher altitudes. The pressure is less the higher you are on Earth, so the chemicals, such as the baking soda in cake, behave differently. We can predict how substances will behave at different pressures and temperatures with a phase diagram.

A phase diagram is a graph of the physical state of a substance (solid, liquid or gas) and the temperature and pressure of the substance. Phase diagrams are unique to every different substance.

Let's start by looking at a phase diagram and exploring everything that's on it. The x-axis of the graph shows temperature. As temperature increases, most substances change from solid to liquid and then to gas. The y-axis shows pressure.

Example of a phase diagram
Phase Diagram Example

Look at the three solid lines on the graph. These are the phase equilibrium lines. They are the lines on a phase diagram where two phases are in equilibrium. One line shows the equilibrium point between solid and liquid. One shows the equilibrium point between liquid and gas. The third shows the equilibrium point between solid and gas. Yes - solid and gas. Some substances can change directly from solid to gas and gas to solid without going through the liquid phase.

Look at the solid equilibrium line. If you move to the left of this line, either by increasing pressure or lowering temperature, the substance turns solid. If you move to the right, the substance is liquid until it hits the liquid/gas equilibrium line.

Triple Point

The three phase equilibrium lines meet at one point. This triple point is the point where the temperature and pressure conditions are right for all three states (solid, liquid and gas) to exist together at equilibrium. The triple point is like the peak of a pyramid. If you are standing on the top of the pyramid, no matter which direction you step, you are stepping onto a different side. No matter where you move from a triple point, you are entering a different phase.

You enter a different phase regardless of where you move from the triple point.
Phase Diagram Triple Point

Critical Point

The three phases of a substance can sometimes be indistinguishable from each other. Look at the line on this graph separating liquid from gas:

At the critical point, the gas and liquid states are identical.
line separating liquid from gas

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