Question

At what temperature is gypsum heated to form Plaster of Paris?
(a) 90°C
(b) 100°C
(c) 110°C
(d) 120°C

Answer 1

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Option (B) 100°C

Answer 2

Gypsum is heated at 100°C to form Plaster of Paris. (option b)

What is Gypsum?

• Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate.
• It has the chemical formula CaSO₄·2H₂O.
• It is widely used in fertilizers and as different forms of plaster.

What is Plaster of Paris(PoP)?

• Plaster of Paris is quick-setting gypsum plaster.
• Chemically, it is calcium sulfate hemihydrate and its formula is CaSO₄.$\frac{1}{2}$H₂O.
• It hardens when moistened and allowed to dry. It is used for making molds, plasters for fractured bones, etc.

What happens to Gypsum at 100°C?

• At 100°C, Gypsum loses $\frac{3}{4}$th of its water of crystallization to form Plaster of Paris.
• The equation is:

                       CaSO₄.2H₂O      → CaSO₄.$\frac{1}{2}$H₂O + 1$\frac{1}{2}$ H₂O

• This equation is reversible. We can get gypsum from PoP by adding water to it.
• PoP is shown to have $\frac{1}{2}$ a molecule of water of crystallization because 1 molecule of water is shared between 2 molecules of Calcium Sulphate.

Hence, Gypsum is heated at 100°C to form Plaster of Paris.

Option (b) is correct.