Answer: CaSO4·0.5H2O (hemihydrate).
- A CaSO4·0.5H2O (hemihydrate)
- B CaSO4·2H2O, the fully hydrated gypsum form
- C CaCO3, common limestone before any calcination
- D Ca(OH)2, ordinary slaked lime used in mortar
Correct answer: A. CaSO4·0.5H2O (hemihydrate)
Explanation: Plaster of Paris is calcium sulfate hemihydrate (CaSO4·0.5H2O), made by heating gypsum.
Each s-block metal ion emits a characteristic flame colour because heating excites electrons to higher energy levels; light is emitted as they fall back, with the colour determined by the specific energy gap.
Concept context
Group 1 (alkali metals) and Group 2 (alkaline earth metals). Learn their reactions with water and air, important compounds like NaOH, Na₂CO₃, and CaCO₃, and anomalous behaviour of lithium and beryllium.