Question

At what temperature is gypsum heated to form Plaster of Paris?

• A. 363 K
• B. 373 K
• C. 273 K
• D. 353 K

Hint:

Remember:
- Heating Gypsum at exactly \(100^\circ\text{C}\) (\(373\text{ K}\)) \(\to\) Plaster of Paris [\(CaSO_4 \cdot \frac{1}{2}H_2O\)].
- Heating above \(100^\circ\text{C}\) (\(\gt 373\text{ K}\)) \(\to\) Dead Burnt Plaster [\(CaSO_4\)].
This temperature boundary is critical.

Answer 1

The Correct Option is B

Step 1: Write gypsum's formula.
Gypsum is calcium sulphate with two water molecules attached, $CaSO_4 \cdot 2H_2O$.
Step 2: Recall the mild heating step.
Heating gypsum gently drives off three quarters of its water, turning it into Plaster of Paris, $CaSO_4 \cdot \tfrac12 H_2O$. This controlled heating happens at the boiling point of water, $100^\circ C$.
Step 3: Convert to Kelvin.
\[ T(K) = 100 + 273 = 373\ K \] Going past this temperature would drive off all the water instead, giving dead burnt plaster, which cannot set with water anymore, so the temperature has to be kept right at this point.
Step 4: Conclude.
Gypsum is heated at 373 K to form Plaster of Paris. \[ \boxed{373\ K} \]