Question

Assertion: Plaster of paris is stored in a moisture proof container.

Reason: Plaster of paris set into a hard mass on wetting with water to form anhydrous Calcium sulphate.

• A. Both A and R are true and R is the correct explanation of A.
• B. Both A and R are true and R is not the correct explanation of A.
• C. A is true but R is false.
• D. A is false but R is true.​

Answer 1

The correct option is A

Both A and R are true and R is the correct explanation of A.

Explanation for Correct Option:

Assertion:-

• Plaster of Paris is a white powdery chemical compound that is Hydrated Calcium sulphate$({\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O})$ that is usually obtained from calcining gypsum.
• It reacts with water present in the moisture and gets hard.
• So it should be stored in a moisture-proof container.
• Thus assertion is true.

Explanation for reason:-

• Plaster of Paris when left exposed to moisture and dry, it absorbs the water from it and forms gypsum.
• This gypsum is a hard material and has a chemical formula $({\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O})$
• The complete reaction is:

$\underset{\mathrm{plaster}\mathrm{of}\mathrm{Paris}}{{\mathrm{CaSO}}_4.\frac{1}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}+\underset{\mathrm{Water}}{\frac{3}{2}{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\to \underset{\mathrm{Gypsum}}{{\mathrm{CaSO}}_4.2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)}$

• This is the reason why plaster of paris should be kept in a moisture-proof container to prevent it from getting hard.
• Thus reason is also true and is the correct explanation for the assertion.

Therefore, option (A) Both A and R are true and R is the correct explanation of A, is the correct answer.