Question

Plaster of Paris is a _______ hydrate of calcium sulphate.

Answer 1

Plaster of Paris:

• Plaster of Paris is obtained by heating Gypsum$({\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O})\mathrm{at}373\mathrm{K}$.
• The formula of Plaster of Paris is $({\mathrm{CaSO}}_4.1/2{\mathrm{H}}_2\mathrm{O})$ , So it is a hemihydrate of Calcium sulfate.

$\underset{(\mathrm{Calcium}\mathrm{sulphate}\mathrm{dihydrate})}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{373\mathrm{K}}{\to }\underset{(\mathrm{Calcium}\mathrm{sulphate}\mathrm{hemihydrate})}{{\mathrm{CaSO}}_4.1/2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$

• It is white in color and available in powder form.
• When water is mixed with Plaster of Paris, it absorbs it and changes back to gypsum, which is a hard mass in about half an hour with slight expansion in volume. This can be used as a plaster cast to fix fractured bones.

$\underset{(\mathrm{Plaster}\mathrm{of}\mathrm{Paris})}{{\mathrm{CaSO}}_4.1/2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+\frac{3}{2}\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\left(\mathrm{Gypsum}\right)}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$

Therefore, Plaster of Paris is a hemi hydrate of Calcium sulfate.