UPSC NDA II 2025 General Ability Paper

Concept:Water of crystallization is the fixed number of water molecules bonded to a salt in its crystal. Plaster of Paris has formula $CaSO_{4}\cdot 0.5H_{2}O$, not $CaSO_{4}\cdot 2H_{2}O$. Washing soda removes permanent hardness. Venus has thick sulphuric acid clouds.Explanation:Option A is correct: Water of crystallization is indeed the fixed number of water molecules per formula unit of a salt.Option B is incorrect: Plaster of Paris is $CaSO_{4}\cdot 0.5H_{2}O$ (calcium sulphate hemihydrate), not $CaSO_{4}\cdot 2H_{2}O$ (gypsum).Option C is correct: Washing soda ($Na_{2}CO_{3}\cdot 10H_{2}O$) precipitates calcium and magnesium ions as insoluble carbonates, removing permanent hardness.Option D is correct: Venus’s atmosphere is mostly carbon dioxide with thick clouds of sulphuric acid.Therefore, the statement that is NOT correct is Option B.Answer:Option B