Question

Why should the Plaster of Paris be stored away from the moisture? Explain using balanced reactions.

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 ${\mathrm{CaSO}}_4.1/2{\mathrm{H}}_2\mathrm{O}$, which is also known as Calcium sulfate hemihydrate.

$\underset{\left(\mathrm{Gypsum}\right)}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}\stackrel{373\mathrm{K}}{\to }\underset{(\mathrm{Plaster}\mathrm{of}\mathrm{Paris})}{{\mathrm{CaSO}}_4.1/2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$

• It is white in colour and available in powder form.

Storage of plaster of Paris:

• When the plaster of Paris is exposed to moisture, the Plaster of Paris absorbs the moisture and changes back to gypsum, which is a hard mass in about half an hour with slight expansion in volume.
• $\underset{(\mathrm{Plaster}\mathrm{of}\mathrm{Paris})}{{\mathrm{CaSO}}_4.½{\mathrm{H}}_2\mathrm{O}}+\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\to \underset{\left(\mathrm{Gypsum}\right)}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}}$

Therefore, the Plaster of Paris is stored away from the moisture.