Answer: Na 2 CO 3 ·10H 2 O.
- A Na<sub>2</sub>CO<sub>3</sub>·10H<sub>2</sub>O
- B NaHCO<sub>3</sub>
- C NaOH
- D Na<sub>2</sub>SO<sub>4</sub>·10H<sub>2</sub>O
Correct answer: A. Na<sub>2</sub>CO<sub>3</sub>·10H<sub>2</sub>O
Explanation: Washing soda is sodium carbonate decahydrate (Na<sub>2</sub>CO<sub>3</sub>·10H<sub>2</sub>O), used in water softening and cleaning.
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.