Name the cell which is used in hearing aids.

The correct answer is Option (1) → Mercury cell

Small devices that draw a low current for a long time, such as hearing aids and watches, need a cell whose voltage stays constant over its life. The mercury cell does this. Its anode is zinc–mercury amalgam, its cathode a paste of HgO and carbon, and its electrolyte a paste of KOH and ZnO:

Anode:

    \( \displaystyle \ce{Zn(Hg) + 2OH^- -> ZnO(s) + H2O + 2e^-} \)

Cathode:

    \( \displaystyle \ce{HgO(s) + H2O + 2e^- -> Hg(l) + 2OH^-} \)

Cell reaction:

    \( \displaystyle \ce{Zn(Hg) + HgO(s) -> ZnO(s) + Hg(l)} \)

No ion appears in the cell reaction, so the composition of the electrolyte does not change during use and the cell gives a constant potential of about 1.35 V. So the cell used in hearing aids is the mercury cell.

NCERT (2026-27 edition):
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