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: Understanding the Question:
The question asks for the specific temperature (in Kelvin) required to heat gypsum to convert it into Plaster of Paris (POP).

Step 2: Key Formula or Approach:
The chemical formula of Gypsum is \(CaSO_4 \cdot 2H_2O\). Upon controlled heating, it loses a portion of its water of crystallization to form Plaster of Paris (\(CaSO_4 \cdot \frac{1}{2}H_2O\)).
The chemical equation is:
\[ CaSO_4 \cdot 2H_2O\text{(s)} \xrightarrow{\Delta} CaSO_4 \cdot \frac{1}{2}H_2O\text{(s)} + 1.5H_2O\text{(g)} \]

Step 3: Detailed Explanation:

• Gypsum is calcium sulphate dihydrate.

• When gypsum is carefully heated to a temperature of \(100^\circ\text{C}\), it loses three-fourths of its water of crystallization to form calcium sulphate hemihydrate, which is Plaster of Paris.

• To convert the temperature from Celsius to Kelvin:
\[ T\text{(K)} = T(^\circ\text{C}) + 273.15 = 100 + 273 = 373\text{ K} \]

• This temperature must be controlled precisely. If gypsum is heated above \(373\text{ K}\) (or \(100^\circ\text{C}\)), it loses all of its water of crystallization to form anhydrous calcium sulphate (\(CaSO_4\)):
\[ CaSO_4 \cdot 2H_2O \xrightarrow{\gt 373\text{ K}} CaSO_4 + 2H_2O \] - This anhydrous form is called "dead burnt plaster," which does not set or harden when mixed with water, unlike Plaster of Paris.

• Therefore, the optimal heating temperature is \(373\text{ K}\).


Step 4: Final Answer:
Gypsum is heated at 373 K to form Plaster of Paris.