Generated by claude-sonnet-4-6 · 2026-06-15 06:43 · grounding rag
Model Answer
(a) Plaster of Paris is calcium sulphate hemihydrate.
Formula: $\text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O}$
(b) It is prepared by heating gypsum ($\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$) at 373 K:
$$\text{CaSO}_4 \cdot 2\text{H}_2\text{O} \xrightarrow{373\text{ K}} \text{CaSO}_4 \cdot \frac{1}{2}\text{H}_2\text{O} + \frac{3}{2}\text{H}_2\text{O}$$
(c) Two uses of Plaster of Paris:
- Used by doctors for setting fractured bones.
- Used for making toys, decorative materials, and smooth surfaces in construction.
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Explanation
- (a) Always write both the name and formula; the formula with ½ water of crystallisation is essential.
- (b) The key detail is the temperature 373 K (not boiling point). Writing gypsum's formula correctly on the left earns marks.
- (c) Any two valid uses are accepted. Medical (bone-setting) and decorative/construction are the standard NCERT examples to memorise.