Answer: Na2CO3·10H2O.
- A Na2CO3·10H2O
- B NaHCO3
- C NaOH
- D Na2SO4·10H2O
Correct answer: A. Na2CO3·10H2O
Explanation: Washing soda is sodium carbonate decahydrate (Na2CO3·10H2O), 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.