Why may excessive heating of gypsum reduce the useful setting property of plaster of Paris?
Answer and explanation
Correct answer: Too much water of crystallisation may be removed
Gypsum is calcium sulphate dihydrate, CaSO₄·2H₂O. Controlled heating removes part of its water of crystallisation and produces plaster of Paris, CaSO₄·½H₂O, which sets after mixing with water. Excessive heating can remove nearly all the water and form over-burnt anhydrous calcium sulphate, which hydrates very slowly and therefore loses useful setting ability. Thus option A is correct; the other options describe impossible products.
Frequently asked questions
What is the correct answer to this question?
Too much water of crystallisation may be removed
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, CaSO₄·½H₂O, which sets after mixing with water. Excessive heating can remove nearly all the water and form over-burnt anhydrous calcium sulphate, which hydrates very slowly and therefore loses useful setting ability. Thus option A is correct; the other options describe impossible products.
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.
