Question

 What will happen if heating is not controlled while preparing plaster of Paris?

Answer 1

 If gypsum (or calcium sulphate dihydrate) is heated above 100⁰C (373K) while preparing plaster of Paris, then all its water of crystallisation is eliminated and anhydrous calcium sulphate (CaSO₄), called dead burnt plaster, is formed. The anhydrous calcium sulphate does not set like plaster of Paris on addition of water.