MCQ
Dead burnt plaster is :
- ✓$\text{CaSO}_4$
- B$\text{CaSO}_4.\frac{1}{2}\text{H}_2\text{O}$
- C$\text{CaSO}_4.\text{H}_2\text{O}$
- D$\text{CaSO}_4.2\text{H}_2\text{O}$
✓
Answer
Correct option: A.
$\text{CaSO}_4$
Plaster of paris is prepared b heating gypsum at $120^{\circ}C.$
$2\text{CaSO}_4.2\text{H}_2\text{O}\xrightarrow{\ \ \ \ }(\text{CaSO}_4)_2.\text{H}_2\text{O}+3\text{H}_2\text{O}\\\ \ \ \ \ \text{Gypsum}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Plaster of Paris}$
On heating plaster of paris at $200^{\circ}C$, it forms anhydrous calcium sulphate i.e., dead plaster which has no setting property as it absorbs water very slowly.
$\text{CaSO}_4.\frac{1}{2}\text{H}_2\text{O}\xrightarrow{200^\circ\text{C}}\text{CaSO}_4\xrightarrow{1100^\circ\text{C}}\text{CaO}+\text{SO}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Anhydride}$
$2\text{CaSO}_4.2\text{H}_2\text{O}\xrightarrow{\ \ \ \ }(\text{CaSO}_4)_2.\text{H}_2\text{O}+3\text{H}_2\text{O}\\\ \ \ \ \ \text{Gypsum}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Plaster of Paris}$
On heating plaster of paris at $200^{\circ}C$, it forms anhydrous calcium sulphate i.e., dead plaster which has no setting property as it absorbs water very slowly.
$\text{CaSO}_4.\frac{1}{2}\text{H}_2\text{O}\xrightarrow{200^\circ\text{C}}\text{CaSO}_4\xrightarrow{1100^\circ\text{C}}\text{CaO}+\text{SO}_3\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Anhydride}$
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