$A$ sulphate salt of a Group $2$ element of the Periodic Table is a white,soft substance,which can be moulded into different shapes by making its dough. When this compound is left in the open for some time,it becomes a hard solid mass and cannot be used for moulding purposes. Identify the sulphate salt and explain why it shows such behavior. Provide the chemical reaction involved.

(A) The substance described is Plaster of Paris. Its chemical name is calcium sulphate hemihydrate $(CaSO_4 \cdot \frac{1}{2} H_2O)$.
It is a soft powder that,when mixed with water,forms a dough that can be moulded into various shapes. When left in the open,it absorbs moisture from the atmosphere and undergoes a rehydration process to form gypsum $(CaSO_4 \cdot 2H_2O)$,which is a hard solid mass.
The chemical reaction is:
$CaSO_4 \cdot \frac{1}{2} H_2O + 1\frac{1}{2} H_2O \rightarrow CaSO_4 \cdot 2H_2O$
(Plaster of Paris) + (Water) $\rightarrow$ (Gypsum)