Question

Plaster of Paris hardens by

• A. giving off CO₂
• B. changing into CaCO₃
• C. combining with water.
• D. giving out water.

Answer 1

The correct option is C

combining with water.

The explanation for the correct option (C):

• Plaster of Paris (${\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$) appears as a white powder.
• When mixed with Water (${\mathrm{H}}_2\mathrm{O}$), Plaster of Paris takes on a unique property of hardening into a solid mass. As a result, when Water is added to Plaster of Paris, it hardens in roughly half an hour.
• When Plaster of Paris combines with Water, it forms Gypsum (${\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}$) crystals that harden to form a hard, solid mass.
• The reaction of Plaster of Paris with Water is:

$$\begin{gathered} {\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}+1\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O} \\ \mathrm{Plaster}\mathrm{of}\mathrm{paris}\mathrm{Water}\mathrm{Gypsum} \\ (\mathrm{Hard}\mathrm{mass}) \end{gathered}$$

Final Answer: Hence, option (C) is correct. Plaster of Paris hardens by combining with water.