A sulphate salt of group 2 element of the periodic tables is a white soft substance which can be moulded in to different shapes by making its dough. When this compound is left in open for some time, it becomes a solid mass and cannot be used for moulding purposes . Identify the sulphates salt and why does it show such a behaviour ? Give the reaction involved .

Explanation

The substance which is used for making different shapes is Plaster of Paris (POP). Its chemical name is calcium sulphate hemihydrates `(CaSO_(4).(1)/(2)H_(2)O)`. The two-formula unit of `CaSO_(4)` share one molecule of water. As a result, it is soft.
When it is left open for some time, it absorbs moisture from the atmosphere and forms gypsum, which is a hard-solid mass.
On heating gypsum at 373 K, it loses water molecules and becomes calcium sulphate hemihydrate. This is called POP, the substance which doctors use as plaster for supporting fractured bones in the right position. POP is a white powder and on mixing with water, changes to gypsum once again giving a hard-solid mass.
`{:(CaSO_(4).(1)/(2)H_(2)O,rarr,CaSO_(4).2H_(2)O),("[Plaster of Paris (POP)]",,"[Gypsum]"):}`