Calcium sulphate hemihydrate.
It is prepared by heating gypsum at \(373 \mathrm{~K}\).
\(\underset{\text { Gypsum }}{\mathrm{CaSO}_{4} \cdot 2 \mathrm{H}_{2} \mathrm{O}} \stackrel{373 \mathrm{~K}}{\longrightarrow} \mathrm{CaSO}_{4} \cdot \frac{1}{2} \mathrm{H}_{2} \mathrm{O}+\frac{3}{2} \mathrm{H}_{2} \mathrm{O}\)
Plaster of Paris
(i) It is used to prepare chalks.
(ii) It is used to make casts and moulds
