The following statements describe various properties of a Mercury cell:

(A) It converts energy of combustion into electrical energy

(B) It is rechargeable

(C) The cell reaction involved is 𝐙𝐧(𝐇𝐠) + 𝐇𝐠𝐎(𝐬) ⟶ 𝐙𝐧𝐎(𝐬) + 𝐇𝐠(𝐥)

(D) It is a low current device used in hearing aids

Choose the correct answer from the options given below:

The correct option is (C) and (D) only.

Let us analyze each statement step-by-step to understand why this option is correct:

Statement (A): It converts energy of combustion into electrical energy.
This statement is incorrect. Cells that convert the energy of combustion of fuels (like hydrogen, methane, or methanol) directly into electrical energy are called fuel cells. A Mercury cell, on the other hand, is a primary galvanic cell that converts chemical energy from a spontaneous redox reaction (not combustion) into electrical energy.

Statement (B): It is rechargeable.
This statement is incorrect. A Mercury cell is a primary cell. In primary cells, the electrode reactions occur only once, and after a period of use, the cell becomes dead and cannot be reused or recharged.

Statement (C): The cell reaction involved is $\mathrm{Zn}\left(\mathrm{Hg}\right)+\mathrm{HgO}\left(\mathrm{s}\right)⟶\mathrm{ZnO}\left(\mathrm{s}\right)+\mathrm{Hg}\left(\mathrm{l}\right)$.
This statement is correct. In a Mercury cell, zinc-mercury amalgam is used as the anode, and a paste of mercury(II) oxide ($\mathrm{HgO}$) and carbon acts as the cathode. The individual electrode reactions are:
At Anode: $\mathrm{Zn}\left(\mathrm{Hg}\right)+2{\mathrm{OH}}^{-}⟶\mathrm{ZnO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}$
At Cathode: $\mathrm{HgO}\left(\mathrm{s}\right)+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}⟶\mathrm{Hg}\left(\mathrm{l}\right)+2{\mathrm{OH}}^{-}$
Combining these gives the overall cell reaction:

$\mathrm{Zn}\left(\mathrm{Hg}\right)+\mathrm{HgO}\left(\mathrm{s}\right)⟶\mathrm{ZnO}\left(\mathrm{s}\right)+\mathrm{Hg}\left(\mathrm{l}\right)$

Statement (D): It is a low current device used in hearing aids.
This statement is correct. Mercury cells produce a steady voltage of about $1.35\mathrm{V}$ during their lifetime because the overall reaction does not involve any ions in solution whose concentration can change. Due to their miniature size and steady low-current output, they are widely used in low-power portable devices like hearing aids, watches, and cameras.

Therefore, only statements (C) and (D) are correct, making (C) and (D) only the correct choice.