Question

Plaster of paris hardens by :

• A. giving $C{O}_2$
• B. changing into carbonate
• C. uniting with water
• D. giving out water

Answer 1

Answer: (C)

uniting with water

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

Plaster of paris absorbs water to form dihydrate, which sets to form a hard unified mass.