Which one among the following statements regarding gypsum is not correct?
- A.On heating gypsum at 373 K, it loses water molecules and becomes CaSO4 · H2O.
- B.On mixing with water, plaster of Paris changes to gypsum.
- C.Plaster of Paris is used for supporting fractured bones.
- D.Although gypsum possesses water of crystallization, it is not a liquid at room temperature.
Correct answer
Explanation
The correct answer is A. Gypsum heated at 373 K becomes calcium sulphate hemihydrate, CaSO4 · ½H2O, which is plaster of Paris, not CaSO4 · H2O as the statement says.
Gypsum is calcium sulphate dihydrate, CaSO4 · 2H2O, with two molecules of water of crystallisation in each formula unit. At 373 K (100 °C) it loses one and a half molecules of water, leaving half a molecule of water shared by each CaSO4 unit. The product is a white powder called plaster of Paris. Heating it too strongly drives off all the water and gives dead-burnt plaster, which no longer sets.
B is correct because plaster of Paris takes up water again and hardens into gypsum; this is why it sets. C is correct because the hard mass formed this way is used in casts that hold fractured bones in place. D is correct because water of crystallisation is locked inside the crystal, so gypsum stays a dry solid.
Exam tip: gypsum CaSO4 · 2H2O, plaster of Paris CaSO4 · ½H2O, made by heating at 373 K.
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