When gypsum is heated under controlled conditions, plaster of Paris is formed. How does the water content change?
Answer and explanation
Correct answer: Some water of crystallisation is removed
The governing concept is controlled dehydration of a hydrated salt. Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. On careful heating, it loses only one and a half molecules of water per formula unit and forms calcium sulfate hemihydrate, CaSO₄·½H₂O, called plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Therefore option B is correct. Excessive heating can remove more water and affect the useful setting property.
Frequently asked questions
What is the correct answer to this question?
Some water of crystallisation is removed
Why is this the correct answer?
The governing concept is controlled dehydration of a hydrated salt. Gypsum is calcium sulfate dihydrate, CaSO₄·2H₂O. On careful heating, it loses only one and a half molecules of water per formula unit and forms calcium sulfate hemihydrate, CaSO₄·½H₂O, called plaster of Paris: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. Therefore option B is correct. Excessive heating can remove more water and affect the useful setting property.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
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