CAPF AC 2025 Q96: Science & Technology – Chemistry
- (a) On heating gypsum at 373 K, it loses water molecules and becomes CaSO4·H2O.
- (b) On mixing with water, plaster of Paris changes to gypsum.
- (c) Plaster of Paris is used for supporting fractured bones.
- (d) Although gypsum possesses water of crystallization, it is not a liquid at room temperature.
Answer: (a) On heating gypsum at 373 K, it loses water molecules and becomes CaSO4·H2O.
Explanation
Gypsum and plaster of Paris
How to think: the question asks for the statement that is not correct.
Gypsum is CaSO₄·2H₂O. Heating it at 373 K drives off most of its water and gives plaster of Paris, CaSO₄·½H₂O, not CaSO₄·H₂O.
| Statement | Fact | Verdict |
|---|---|---|
| a. At 373 K gypsum loses water and becomes CaSO₄·H₂O | It becomes CaSO₄·½H₂O (half a molecule of water) | Not correct |
| b. Plaster of Paris with water changes to gypsum | It sets into gypsum, a hard mass | Correct |
| c. Plaster of Paris supports fractured bones | Used for plaster casts | Correct |
| d. Gypsum has water of crystallisation but is not a liquid | The water is bound in the crystal, so it is a dry solid | Correct |
Bank key corrected: the stored key was (b); the statement that is not correct is (a).
Answer: (a).
Why each option is right or wrong
- (a) CORRECT. Plaster of Paris is CaSO₄·½H₂O, not CaSO₄·H₂O.
- (b) Incorrect. This is correct: plaster of Paris sets into gypsum with water.
- (c) Incorrect. This is correct: plaster of Paris is used to support fractured bones.
- (d) Incorrect. This is correct: the water in gypsum is bound in the crystal.
Key point
Gypsum CaSO₄·2H₂O, heated at 373 K, gives plaster of Paris CaSO₄·½H₂O. Plaster of Paris + water gives gypsum (sets hard). Uses: casts for fractures, toys, decoration, smoothing walls.
Remember
Gypsum 2H₂O, heat to plaster of Paris ½H₂O, add water to get gypsum.
Common mistakes
- Writing plaster of Paris as CaSO₄·H₂O (one full water molecule).
- Thinking water of crystallisation makes a substance wet.
- Choosing a correct statement because the question says 'not'.
Exam tip
Educated guess: the formula in (a) looks slightly off from the famous 'half water' formula. Plaster of Paris is always written with ½H₂O.
Sources
- NCERT Class X Science, Chapter 2 (Acids, Bases and Salts: plaster of Paris)
- NCERT Class XI Chemistry, Part II, Chapter 10 (The s-Block Elements: gypsum and plaster of Paris)