Why is controlled heating of gypsum necessary?
Answer and explanation
Correct answer: Because only part of water of crystallisation must be removed
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses only one and a half molecules of water and forms plaster of Paris, CaSO₄·½H₂O: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. This hemihydrate mixes with water and sets into a hard solid. Excessive heating can remove all the water of crystallisation and produce anhydrous calcium sulphate, which does not set properly. Therefore option A is correct.
Frequently asked questions
What is the correct answer to this question?
Because only part of water of crystallisation must be removed
Why is this the correct answer?
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. On controlled heating, it loses only one and a half molecules of water and forms plaster of Paris, CaSO₄·½H₂O: CaSO₄·2H₂O → CaSO₄·½H₂O + 1½H₂O. This hemihydrate mixes with water and sets into a hard solid. Excessive heating can remove all the water of crystallisation and produce anhydrous calcium sulphate, which does not set properly. Therefore option A is correct.
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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