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Question
Dead burnt plaster is ______.
Options
CaSO4
\[\ce{CaSO4 . 1/2 H2O}\]
CaSO4.H2O
CaSO4.2H2O
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Solution
Dead burnt plaster is CaSO4.
Explanation:
Plaster of Paris is prepared by heating gypsum at 120°C
\[\ce{\underset{Gypsum}{2CaSO4 . 2H2O} -> (\underset{Plaster of Paris}{CaSO4)2 . H2}O + 3H2O}\]
On heating plaster of Paris at 200°C, it forms anhydrous calcium sulphate, i.e., dead plaster which has no setting property as it absorbs water very slowly.
\[\ce{CaSO4 . 1/2 H2O ->[200°C] \underset{Anhydride}{CaSO4} ->[1100°C] CaO + SO3}\]
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