Question

Why are casts made from Plaster of Paris sharp?

Answer 1

1. Chemical name of Plaster of Paris: Calcium sulfate hemihydrate.
2. Chemical formula of Plaster of Paris: ${\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$
3. Reason:

• When Plaster of Paris (POP) is mixed with a slight amount of water, it quickly sets into a hard porous mass (Gypsum) and this process is called the setting of Plaster of Paris.
• Equation for the setting process: $$\begin{gathered} {\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)+\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to {\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right) \\ \mathrm{Plaster}\mathrm{of}\mathrm{Paris}\mathrm{Water}\mathrm{Gypsum} \end{gathered}$$
• During the process of setting, the volume of Plaster of Paris expands and this property helps to fill the casts with a sharp impression.