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.

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Solution

## Calcium: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${\mathrm{CaSO}}_{4}.\frac{1}{2}{\mathrm{H}}_{2}\mathrm{O}$. Plaster of Paris can be molded into any shape after making its dough.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.Gypsum further cannot be molded into any shape.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

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