Write the common name and the chemical name of the compound $\mathrm{CaSO_4\cdot\frac{1}{2}H_2O}$. Write the method of its preparation. Give chemical equation for the reaction, when water reacts with $\mathrm{CaSO_4\cdot\frac{1}{2}H_2O}$.

The common name of the compound $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$ is Plaster of Paris and its chemical name is Calcium sulphate hemihydrate.

Method of Preparation:

It is prepared by heating gypsum ($\mathrm{CaSO_4 \cdot 2H_2O}$) at $373\text{ K}$ ($100^\circ\text{C}$). On heating, it loses water molecules to form calcium sulphate hemihydrate:

$$\mathrm{CaSO_4 \cdot 2H_2O \xrightarrow{373\text{ K}} CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O}$$

Reaction with Water:

When Plaster of Paris is mixed with water, it hydrates back to gypsum, setting into a hard solid mass:

$$\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O}$$