Question

Give reasons for the following:

Only one-half of the water molecule is shown in the formula of Plaster of Paris.

Answer 1

Plaster of Paris:

• Plaster of Paris is a white powdery chemical substance composed of Hydrated Calcium Sulphate $2{\mathrm{CaSO}}_4.{\mathrm{H}}_2\mathrm{O}$, which is often made by calcining gypsum.
• In other words, Plaster of Paris is often manufactured by heating gypsum at a high temperature.
• ${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$ is the formula for Plaster of Paris.
• When gypsum is heated to 393 degrees Celsius, it loses Water molecules and produces Calcium Sulphate Hemihydrate (${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$).
• Plaster of Paris is the name given to this product. When water is added with dried Plaster of Paris, however, it restructures into gypsum ($2{\mathrm{CaSO}}_4.{\mathrm{H}}_2\mathrm{O}$).

As a result, only one-half of the water molecule is indicated in the Plaster of Paris formula since two formula units of ${\mathrm{CaSO}}_4$ (Calcium sulphate) share one molecule of water during crystallisation.