Task 4 FAQs

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Task 4: Fluoride Treatment and Removal

Q1: We have not been able to get our calcium sulfate to dissolve. What should we do?

A1: The task statement says that the test solution will contain 1.7 grams of calcium sulfate per liter of solution.  According to the literature, the solubility of calcium sulfate in water at 20C is 0.2 grams/100 ml of water (solubilities are always expressed as per 100 ml of water).  This translates to 2 grams/liter, which is more than the task calls for, so one would expect to be able to dissolve 1.7 grams per liter.  So I don’t know why you are having trouble getting it to dissolve.

One issue might be applying heat.  For many compounds, solubility decreases with increasing temperature, and calcium sulfate is one of them.  (It seems counter-intuitive, but that’s the way it is.)  Its solubility at 30C drops to 0.16 grams/100 ml.  You might be able to get it to dissolve by cooling the solution, say, in a refrigerator, and see what happens.  Letting it sit at room temperature may not be cool enough.
We here at WERC will do some preliminary tests and let you know what we find.

Please check back regularly for more FAQs.