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One of my Chemistry professors said that the hard mass formed after adding water to Plaster of Paris, cannot be converted back into Plaster of Paris. However, he said that the original Gypsum can be heated to make Plaster of Paris.

What causes the hard mass not to convert back into POP by adding water even though the hard mass and the gypsum share the same chemical formula? If there is a structural change? How is the structure changed here?

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    $\begingroup$ Sounds like nonsense afaik: en.wikipedia.org/wiki/Plaster#Gypsum_plaster. $\endgroup$ – Buck Thorn Jul 31 '19 at 8:08
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    $\begingroup$ If it can be converted once, it can be done again. Are you sure you understood the professor's comments? If the prof seems reasonable, you could easily show this in the lab, using an oven at ~130-180 °C. $\endgroup$ – DrMoishe Pippik Aug 1 '19 at 2:12

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