Plaster of paris is represented as

${\mathrm{CaSO}}_4\cdot \frac{1}{2}{\mathrm{H}}_2\mathrm{O}$ is a representation of plaster of paris. It is a hemihydrate calcium sulphate.
It is obtained when gypsum, CaSO₄ ·2H₂O, is heated to 393 K.

$2\left({\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\right)\stackrel{∆}{\to }2\left({\mathrm{CaSO}}_4\right).{\mathrm{H}}_2\mathrm{O}+3{\mathrm{H}}_2\mathrm{O}$