Question

(a) What will happen if heating is not controlled while preparing plaster of Paris?
(b) Write an equation to show the reaction between plaster of Paris and water.

Answer 1

(a) 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.

(b) ${\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\stackrel{}{\to {\mathrm{CaSO}}_4}·2{\mathrm{H}}_2\mathrm{O}$