Question

Plaster of Paris hardens by

• A. Giving off ${\mathrm{CO}}_2$
• B. Changing into ${\mathrm{CaCO}}_3$
• C. Giving out water
• D. Utilizing water

Answer 1

The correct option is D

Utilizing water

Explanation of correct option.

Option (D) Utilizing water

• Plaster of Paris is a salt of calcium metal which is also called calcium sulfate hemihydrate.
• The chemical formula of Plaster of Paris is ${\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}$.
• It is formed by the intense heating of gypsum which is also a salt of calcium.
• When we add water to the Plaster of Paris it becomes soft and forms a paste. This paste becomes hardened after some time because of the formation of strong hydrogen bonding between molecules of Plaster of Paris and water.

Explanation of incorrect options:

Option (A) Giving off ${\mathrm{CO}}_2$

• The equation can be written as,

$\underset{\mathrm{Plaster}\mathrm{of}\mathrm{paris}}{{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}}+\underset{\mathrm{Water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}}\to \underset{\mathrm{Gypsm}}{{\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{Hard}\right)}$

• The reaction does not give out ${\mathrm{CO}}_2$.
• Hence, incorrect option.

Option (B) Changing into ${\mathrm{CO}}_3$

• The equation can be written as,

$\underset{\mathrm{Plaster}\mathrm{of}\mathrm{paris}}{{\mathrm{CaSO}}_4·\frac{1}{2}{\mathrm{H}}_2\mathrm{O}}+\underset{\mathrm{Water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}}\to \underset{\mathrm{Gypsm}}{{\mathrm{CaSO}}_4·2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{Hard}\right)}$

• This reaction does not give ${\mathrm{CO}}_3$.
• Hence, incorrect option.

Option (C) Giving out water.

• Since water is utilized in the process of hardening of Plaster of Paris, no water is given out.
• Hence, incorrect option.

Therefore, option (D) is the correct answer.