Answer: Calgon (sodium hexametaphosphate) or HCl treatment.
- A Calgon (sodium hexametaphosphate) or HCl treatment
- B Concentrated NaOH solution flushed through the boiler
- C Additional hard water poured in to dissolve existing scale
- D Simply boiling the water further without any added reagent
Correct answer: A. Calgon (sodium hexametaphosphate) or HCl treatment
Explanation: Boiler scale (mainly CaSO<sub>4</sub> and CaCO<sub>3</sub>) can be dissolved using hydrochloric acid or prevented by sequestering agents like Calgon.
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.