What chemical is used as the electrolyte in nickel-cadmium dry cells?

The correct answer is potassium hydroxide, which is used as the electrolyte in nickel-cadmium (NiCd) dry cells. In nickel-cadmium batteries, the electrolyte is typically an alkaline solution, with potassium hydroxide being the most common choice. This alkaline electrolyte facilitates the movement of ions between the anode and cathode during the discharge and charge cycles of the battery.

In the context of NiCd batteries, the potassium hydroxide solution plays a crucial role in enabling efficient electrochemical reactions. Nickel oxide and cadmium are the primary active materials in the electrodes, and the alkaline environment provided by potassium hydroxide enhances charge transfer and overall battery performance.

Additionally, considering the other options: sodium chloride (a salt) would not generate the necessary ion transfer in the reactions typical for NiCd batteries, while sulfuric acid is associated with lead-acid batteries, and calcium carbonate would not facilitate the requisite ionic conductivity. The specific chemistry of nickel-cadmium batteries is tailored around potassium hydroxide, which is essential for their functionality and efficiency.