What is a primary battery? Give one example.

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Q. What is a primary battery? Give one example.

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A primary cell is a non-rechargeable, single-use galvanic cell; the dry cell is a common example.

A primary battery (primary cell) is an electrochemical cell in which the redox reaction that generates electrical energy occurs only once. Once the reactants (electrode materials/electrolyte) are used up, the cell becomes dead and cannot be recharged by passing current back through it, because the electrode reactions are not reversible under ordinary conditions — the products formed cannot be converted back to reactants in situ. Such cells are meant to be discarded after use.

Example — Dry cell (Leclanché cell):

Anode: Zn container

Cathode: graphite rod surrounded by MnO2 and carbon …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set 56/5/11 markMCQ
$Q. Match the type of cell given in Column I with their use given in Column II. Column I | Column II i. Lead storage cell | a. Wall clock ii. Mercury cell | b. Apollo Space Programme iii. Dry cell | c. Wrist watch iv. Fuel cell | d. Inverter (A) i-a, ii-b, iii-c, iv-d (B) i-d, ii-c, iii-a, iv-b (C) i-c, ii-d, iii-b, iv-a (D) i-b, ii-a, iii-d, iv-c$
›Reveal solutionSolution
This is a matching problem linking battery types to their real-world applications. The correct pairing is: Lead storage cell → Inverter (d), Mercury cell → Wrist watch (c), Dry cell → Wall clock (a), Fuel cell → Apollo Space Programme (b). So the answer is option (B).

The key here is to connect each cell's chemistry and design to the power requirement and portability of the device it powers. You don't need to memorise random facts — just think about what each battery is good at.

Lead storage cell (i) — This is a heavy, rechargeable battery that delivers a high current. It's used where you need a lot of power and weight isn't a problem, like in car starters or inverters (backup power for homes). So it matches d. Inverter.

Mercury cell (ii) — This is a small, compact button cell with a very stable voltage and long shelf life. It was traditionally used in devices that need a tiny, reliable power source for years — like c. Wrist watch (and hearing aids, calculators). Note: modern watches often use silver oxide or lithium cells, but mercury cells were the classic choice.

Dry cell (iii) — The common Leclanché cell (zinc-carbon) is cheap, moderate in power, and non-rechargeable. It's perfect for low-drain, intermittent-use devices like a a. Wall clock (or a torch, remote control). It's not suited for high-drain or continuous heavy use. …
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CBSE 2025Set ANNUAL1 mark
$Q. Write overall reaction of mercury cell.$
›Reveal solutionSolution

The mercury cell has a zinc-mercury amalgam anode and an HgO/carbon cathode paste in an alkaline (KOH/ZnO) electrolyte; because the electrolyte composition never changes as the cell discharges, it gives a remarkably steady voltage throughout its life.

At the anode (oxidation): Zn(Hg) + 2OH- → ZnO(s) + H2O + 2e-

At the cathode (reduction): HgO(s) + H2O + 2e- → Hg(l) + 2OH-

Adding the two half-reactions (the H2O and OH- cancel):

Zn(Hg) + HgO(s) → ZnO(s) + Hg(l)

…

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CBSE 2023Set TERM21 mark
$Q. In the mercury cell, the anode is made of \dots\dots\dots\dots\dots$
›Reveal solutionSolution
The mercury cell's anode is a zinc-mercury amalgam; the cathode is a paste of HgO and carbon.

The mercury cell (used in hearing aids, watches) is a primary cell with:

Anode: Zinc mercury amalgam (Zn–Hg), i.e. zinc metal amalgamated with mercury

Cathode: a paste of HgO and carbon

Electrolyte: a paste of KOH and ZnO

Electrode reactions:

Anode: $Zn(Hg) + 2OH^- \rightarrow ZnO(s) + H_2O + 2e^-$

Cathode: $HgO + H_2O + 2e^- \rightarrow Hg(l) + 2OH^-$

…
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Q.What is a primary battery? Give one example.

Concept understanding — Commercial Cells and Batteries

Commercial Cells and Batteries

Q.What is a primary battery? Give one example.

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