Answer: Rubidium and caesium.
- A Lithium and sodium
- B Sodium and potassium
- C Rubidium and caesium
- D Lithium and potassium
Correct answer: C. Rubidium and caesium
Explanation: Rubidium and caesium react explosively even with traces of air and moisture, so they are kept in sealed glass tubes rather than merely under kerosene.
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.