Question

Plaster of Paris was first found in Montmartre in Paris in the form of white rocks. Chemically, it can be obtained by heating gypsum carefully to about 373K in a kiln. The temperature must be carefully controlled because at higher temperatures, it gets completely dehydrated to form anhydrous calcium sulphate. It is very commonly used in surgical bandages for setting fractured bones. What is dead burnt plaster?

Answer 1

Dead burnt Plaster:

1. It is a completely dehydrated substance called anhydrous calcium sulphate$\left({\mathrm{CaSO}}_4\right)$.
2. It is obtained by heating gypsum carefully at a high temperature of around 373 K

$$\begin{gathered} \underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}\stackrel{{120}^{\mathrm{o}}}{\to }\underset{\mathrm{Plaster}\mathrm{of}\mathrm{Paris}}{{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}+\underset{\mathrm{Water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)} \\ \underset{\mathrm{Plaster}\mathrm{of}\mathrm{Paris}}{{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{aq}\right)}\stackrel{\mathrm{High}\mathrm{Temperature}}{\to }\underset{\mathrm{Dead}\mathrm{burnt}\mathrm{Plaster}}{{\mathrm{CaSO}}_4\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)} \end{gathered}$$.

Therefore, anhydrous calcium sulphate is a dead burnt plaster.