alleryStart learning →Plaster of Paris is obtained by heating gypsum to about 100°C and has the formula
A$CaSO_4\cdot 2H_2O$
B$CaSO_4$
C$CaSO_4\cdot \tfrac{1}{2}H_2O$
D$CaCO_3$
Answer & Solution
Correct answer: C. $CaSO_4\cdot \tfrac{1}{2}H_2O$
1. Gypsum is $CaSO_4\cdot 2H_2O$.
2. Heating it to ~100°C drives off most of the water, leaving **$CaSO_4\cdot\tfrac{1}{2}H_2O$** (Plaster of Paris).
3. On adding water it sets back into a hard gypsum mass, so it is used for plaster casts.
_Source: Karnataka SSLC (KSEEB) Class 10 Science, Ch2 'Acids, Bases and Salts', §2.4_
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