First slide

Alkaline earth metals

Question

Dead burnt plaster is

Easy

A

CaSO₄
B

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

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

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

Solution

Plaster of Paris is prepared by heating gypsum at 120°C
$\underset{Gypsum}{2 {CaSO}_{4} \cdot 2 H_{2} O ⟶} ⟶ \underset{Plaster of Paris}{{({CaSO}_{4})}_{2} \cdot \frac{1}{2} H_{2} O} + 3 H_{2} O$
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.
${CaSO}_{4} \cdot \frac{1}{2} H_{2} O \overset{200^{\circ} C}{⟶} \underset{Anhydride}{{CaSO}_{4}} \overset{1100^{\circ} C}{⟶} CaO + {SO}_{3}$

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