Gypsum forms plaster of Paris on controlled heating. Why is excessive heating a problem?
Answer and explanation
Correct answer: Too much water may be removed and the useful hemihydrate form may be lost
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. If heating is excessive, still more water is driven off and anhydrous calcium sulphate may form. This material does not rehydrate and set in the desired way, so the useful plaster quality is reduced. Hence option A is correct; the other products are chemically unrelated.
Frequently asked questions
What is the correct answer to this question?
Too much water may be removed and the useful hemihydrate form may be lost
Why is this the correct answer?
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, calcium sulphate hemihydrate, CaSO₄·½H₂O. If heating is excessive, still more water is driven off and anhydrous calcium sulphate may form. This material does not rehydrate and set in the desired way, so the useful plaster quality is reduced. Hence option A is correct; the other products are chemically unrelated.
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.
