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?