Question

Why is Plaster of Paris written as $\left({\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\right)$? How is it possible to have half a water molecule attached to CaSO₄?

Answer 1

1. The chemical name of Plaster of Paris is calcium Sulphate hemihydrate$\left({\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\right)$.
2. It is prepared when gypsum or calcium sulphate dihydrate (CaSO₄) is heated to a temperature of 100°C, it loses three-fourths of its water of crystallization and produces a Plaster of Paris.

Chemical reaction:

${\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)\stackrel{∆}{\to }{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

1. It is not possible to have half a water molecule attached to CaSO₄ because half a molecule of water means that two molecules of calcium sulphate donate one molecule of water making it half molecule of water as per CaSO₄.
2. This is the reason for plaster of Paris is written as calcium sulphate hemihydrate and it is not possible to have half molecule of water attached to calcium sulphate (CaSO₄).