I enjoyed Dr. Cliff's "conceptual diagnostic test" regarding calcium phosphate equilibrium and was relieved that I chose the correct answers (1). Question 2 was the more difficult one, and I hesitated before almost answering it incorrectly. Dr. Cliff asks what would happen if more calcium phosphate is added to a beaker containing calcium, phosphate, and calcium phosphate "at equilibrium." His answer states the question differently: "Only 11% of the students correctly answered the conceptual diagnostic question about the addition of solid calcium phosphate to a saturated [emphasis added] solution of calcium phosphate." The question did not specify that the solution was saturated but that it was "at equilibrium." It is not simply a question about understanding "mass action." One also needs to realize that the solvent is saturated if a solid in the solvent is at equilibrium. That is an additional concept itself and, when not made explicit, might explain why only 11% of the students answered it correctly.


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