Question

Plaster of Paris is obtained by:

• A. adding water to calcium sulphate.
• B. adding sulphuric acid to calcium hydroxide.
• C. heating gypsum to a very low temperature.
• D. heating gypsum to ${120}^{\circ }C$.

Answer 1

The correct option is D heating gypsum to ${120}^{\circ }C$.

The chemical name of plaster of Paris is calcium sulphate hemihydrate$({\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O})$. It is obtained by heating gypsum at ${120}^{\circ }\text{C}$. The reaction involved is:

${\text{CaSO}}_4.2{\text{H}}_2\text{O(s)}\stackrel{{120}^{\circ }\text{C}}{\to }{\text{CaSO}}_4.\frac{1}{2}{\text{H}}_2\text{O(s)}+\frac{3}{2}{\text{H}}_2\text{O(l)}\text{Gypsum}\text{Plaster of paris}$