Question 34

The Correct Answer is C ### Why Option C (Potassium Hydroxide) is Correct The nickel-cadmium (NiCd) battery is an alkaline secondary battery. The electrolyte used in these batteries must be strongly alkaline to facilitate the reversible oxidation-reduction reactions at the electrodes. The standard electrolyte used is an aqueous solution of **potassium hydroxide (KOH)**, typically in a concentration range of 20% to 30%. The KOH solution serves as the medium for ion transfer (specifically hydroxide ions, $\text{OH}^{-}$) between the nickel oxyhydroxide positive electrode and the cadmium negative electrode during charging and discharging. The electrolyte itself is not consumed or produced during the main reaction; its primary role is ionic conduction. Distilled water is used as the solvent for the KOH. ### Why the Other Options are Incorrect **A) diluted sulfuric acid:** Sulfuric acid ($\text{H}_{2}\text{SO}_{4}$) is a strong acid and is the electrolyte used in lead-acid batteries (acidic environment). It would react destructively with the alkaline materials (nickel and cadmium oxides/hydroxides) used in NiCd batteries, rendering the battery inoperable. **B) lead sulfate:** Lead sulfate ($\text{PbSO}_{4}$) is an insoluble salt that forms on the electrodes of a lead-acid battery during discharge. It is not an electrolyte itself, but rather a reaction product within an acidic electrolyte system. **D) zinc oxide:** Zinc oxide ($\text{ZnO}$) is an insoluble solid compound sometimes used as an electrode material (e.g., in zinc-air batteries) or as an additive, but it is not used as the primary liquid electrolyte (ionic conductor) in a nickel-cadmium battery. The electrolyte must be a highly conductive ionic solution.