Question

In the hardening stage of plaster of paris, the compound formed is :

• A. $CaS{O}_4$
• B. orthorhombic $CaS{O}_4.2H_{2}O$
• C. $CaS{O}_4.H_{2}O$
• D. monoclinic $CaS{O}_4.2H_{2}O$

Answer 1

The correct option is D monoclinic $CaS{O}_4.2H_{2}O$

$\underset{\text{plaster of paris}}{CaS{O}_4.1/2H_{2}O}\underset{\text{setting}}{\overset{H_{2}O}{\to }}\underset{\begin{array}{c}\text{gypsum}\\ \text{orthorhombic}\\ \text{dihydrate}\end{array}}{CaS{O}_4.2H_{2}O}\underset{\text{hardening}}{\overset{}{\leftarrow }}\underset{\begin{array}{c}\text{gypsum}\\ \text{monoclinic dihydrate}\end{array}}{CaS{O}_4.2H_{2}O}$

The plaster of paris absorbs water to form orthorhombic calcium sulfate dihydrate which sets to form a hard mass containing monoclinic $CaS{O}_4.2H_{2}O$.