Ncert Exercises & Solutions

Dead burnt plaster is-

1. ${CaSO}_{4}$

2. ${CaSO}_{4} \cdot \frac{1}{2} H_{2} O$

3. ${CaSO}_{4} \cdot H_{2} O$

4. ${CaSO}_{4} \cdot 2 H_{2} O$

Hint: Dead burnt plaster is anhydrous calcium sulphate CaSO₄

Plaster of Paris is prepared by heating gypsum at

120 ° C

\underset{Gypsum}{2 {CaSO}_{4} \cdot 2 H_{2} O} \to \underset{Plaster of Paris}{{({CaSO}_{4})}_{2} \cdot \frac{1}{2} H_{2} O} + 3 H_{2} O

On heating plaster of Paris at

200 ° C

, if forms anhydrous calcium sulphate i.e., dead plaster which has no setting property as it absorbs water very slowly.

{CaSO}_{4} \cdot \frac{1}{2} H_{2} O \overset{200 ° C}{\to} \underset{Anhydride}{{CaSO}_{4}} \overset{1100 ° C}{\to} CaO + {SO}_{3}