The Lever Rule


Concepts Shown:

Phase diagrams and the lever rule


Transparencies of the attached sheets.


This is a great way to introduce the Lever Rule concept. I constructed a fictional phase diagram for a Peanut Butter Cup because I felt that the students could relate to it more than they would a phase diagram involving metals. The chocolate in Peanut Butter Cups is actually a solid solution containing some peanut butter. To confirm this simply do a taste test between M & M's vs. Reese's Pieces. This explains why phase Alpha on the diagram is not a line phase. Mostly, all the instructor has to do is go through the four transparencies with the class. They are self explanatory.


The main concern for this presentation is to help students understand how to use the Lever Rule. This is what is used to determine the percentage of phases in a two phase region of a phase diagram. Remarks: The information this diagram displays is most likely not accurate quantitatively. The purpose of its construction was mainly as an aid to introduce the Lever Rule in class. If desirable, when the cross section of a Peanut Butter Cup is shown, the lecturer could bite into a piece to show the class just to be a little humorous and help keep their attention. The instructor could draw the cross section of the Peanut Butter Cup for the first point, and then ask a student to use the results obtained in calculation of the second point to draw a cross section next to the first one, showing the differences in the amounts of each phase at each point across the phase diagram. When using the Lever Rule to solve point two on the diagram, I left out the method involved. The method can be reemphasized by the instructor in his part. Copies of these four sheets could even be handed out to the students to use as a reference when doing homework or studying. [eq].



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