Answer: Hydrogen.
- A Hydrogen
- B Oxygen
- C Nitrogen
- D Chlorine
Correct answer: A. Hydrogen
Explanation: 2Na + 2H<sub>2</sub>O → 2NaOH + H<sub>2</sub>. The hydrogen gas evolved can ignite due to the heat produced.
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.