UPSC CDS 2 2021 GK Paper

Concept:The reactivity order of metals with dilute $\mathrm{HCl}$ follows the metal activity series, where metals above hydrogen react to liberate $\mathrm{H}_2$ gas, and higher reactivity means a faster reaction.Explanation:The metal activity series lists metals in decreasing order of their reducing ability.More reactive metals oxidise easily and displace hydrogen from acids like dilute $\mathrm{HCl}$.Magnesium ($\mathrm{Mg}$) is very reactive and reacts vigorously with dilute $\mathrm{HCl}$ to produce $\mathrm{H}_2$ gas.Aluminium ($\mathrm{Al}$) is less reactive than $\mathrm{Mg}$; it has a protective oxide layer but still reacts readily with the acid.Zinc ($\mathrm{Zn}$) reacts moderately with dilute $\mathrm{HCl}$, evolving hydrogen at a slower rate than $\mathrm{Al}$.Iron ($\mathrm{Fe}$) is the least reactive among the given metals and reacts slowly with dilute $\mathrm{HCl}$.Therefore, the correct descending order of reactivity is: $\mathrm{Mg} > \mathrm{Al} > \mathrm{Zn} > \mathrm{Fe}$.This matches option A.Answer:Option A: $\mathrm{Mg} > \mathrm{Al} > \mathrm{Zn} > \mathrm{Fe}$