Question

Why doesn't Plaster of Paris produce heat when mixed in water, while gypsum does?

Answer 1

When water is added to plaster of paris, it absorbs water and is converted to gypsum and this reaction is an exothermic reaction. Whereas gypsum is already having its water of crystallisation and when added to water, it starts dissolving forming its aqueous solution, which involves less energy of hydration.
CaSO_​4. 1/2 H₂O + 3/2 H₂O -------> CaSO₄. 2 H₂O
CaSO₄.2 H₂O + H₂O -----> CaSO_4(aq)