UPSC NDA I 2021 General Ability Solved Paper

Concept:Identifying which chemical reaction does not produce hydrogen gas involves knowing the conditions under which hydrogen evolves.Explanation:Reaction of zinc metal with dilute sulphuric acid produces hydrogen gas: $Zn + H_2SO_4 \to ZnSO_4 + H_2 \uparrow$ Heating zinc metal with sodium hydroxide solution also yields hydrogen: $2NaOH + Zn \to Na_2ZnO_2 + H_2 \uparrow$ Potassium metal reacts vigorously with water to liberate hydrogen: $2K + 2H_2O \to 2KOH + H_2 \uparrow$ When water is mixed with Plaster of Paris ($\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O}$), it forms gypsum ($\mathrm{CaSO_4 \cdot 2H_2O}$) without any gas evolution: $\mathrm{CaSO_4 \cdot \frac{1}{2}H_2O + \frac{3}{2}H_2O \to CaSO_4 \cdot 2H_2O}$ Thus, only this reaction does not produce hydrogen gas.Answer:The reaction that does not result in evolution of hydrogen gas is mixing water to Plaster of Paris.