Question

A sulphate salt of group 2 element of the priodic table is a white, soft substance, which can be moulded into 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 molding purposes. Identify the sulphate salt and why doies it show such a behaviour? Give the reaction involved

Answer 1

The sulphate salt of group-2 element which is white soft substance and used for making different shapes by making its dough is CaSO₄ hemi hydrate, it is commonly known as Plaster of Paris and its molecular formula is CaSO₄.1/2 H₂O.
When it is left some time in air it absorbs moisture and form a new product called gypsum which is hard in nature and can not be used for moulding purpose.The chemical reaction involve is as follows:
CaSO₄.1/2 H₂O + 1$\frac{1}{2}$ H₂O $\to$CaSO₄.2H₂O
gypsum