Question

Calcium Sulphate Hemihydrate is also known as _______.

Answer 1

1. Calcium Sulphate Hemihydrate ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$ also known as plaster of Paris is a neutral salt formed by heating the gypsum $\mathrm{CaSO}.2{\mathrm{H}}_2\mathrm{O}$at 373K.
2. The heating is required to remove the water of crystallization from the gypsum. It has the property to absorb 3/2 of water of crystallization when exposed to air and converting into a hard mass.

$$\begin{gathered} \mathrm{CaSO}.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)\stackrel{373\mathrm{K}}{\to }{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)+\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right) \\ \mathrm{Gypsum}\mathrm{Plaster}\mathrm{of}\mathrm{paris} \end{gathered}$$

Hence, Calcium Sulphate Hemihydrate is also known as Plaster of Paris (${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$)