Question

A sulphate salt of the 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 solid mass and cannot be used for moulding purposes. Identify the sulphate salt, and why does it show such behaviour? Give the reaction involved.

Answer 1

Calcium:

1. Calcium is a group $2$ element and its sulfate is known as Calcium sulfate or commonly known as plaster of Paris with the chemical formula${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$.
2. Plaster of Paris can be molded into any shape after making its dough.
3. When it is exposed to the air for a while and absorbs moisture from it, due to the presence of a half-water molecule acting as the water of crystallization in plaster of Paris it becomes hard material known as gypsum.
4. Gypsum further cannot be molded into any shape.
5. The reaction involved is $\underset{\mathrm{Plaster}\mathrm{of}\mathrm{paris}}{{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to \underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}}\left(\mathrm{s}\right)$

Therefore, the sulfate salt formed is Calcium sulfate