Answer: CaSO 4 ·0.5H 2 O (hemihydrate).
- A CaSO<sub>4</sub>·0.5H<sub>2</sub>O (hemihydrate)
- B CaSO<sub>4</sub>·2H<sub>2</sub>O, the fully hydrated gypsum form
- C CaCO<sub>3</sub>, common limestone before any calcination
- D Ca(OH)<sub>2</sub>, ordinary slaked lime used in mortar
Correct answer: A. CaSO<sub>4</sub>·0.5H<sub>2</sub>O (hemihydrate)
Explanation: Plaster of Paris is calcium sulfate hemihydrate (CaSO<sub>4</sub>·0.5H<sub>2</sub>O), 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.