Lead–acid batteries — active material on negative plates In a fully charged lead–acid starting battery, which material forms the active mass on the negative plates?

Introduction / Context:
Understanding electrochemistry of lead–acid batteries helps in maintenance and diagnostics. The active materials on positive and negative plates change during charge and discharge.

Given Data / Assumptions:

• Battery is fully charged and in normal service.
• Negative and positive plate compositions are standard for automotive batteries.
• Electrolyte is aqueous sulphuric acid.

Concept / Approach:
In the charged state, the positive plate active material is lead dioxide (PbO2), while the negative plate is porous or spongy lead (Pb). During discharge, both convert to lead sulphate (PbSO4) as sulphate ions combine with lead from each plate, and water content in electrolyte increases.

Step-by-Step Solution:
Identify charged-state materials.Positive: PbO2; Negative: Pb.Select “spongy lead (Pb)” for the negative plate.

Verification / Alternative check:
Charge reactions confirm conversion pathways between Pb, PbO2, and PbSO4 depending on current direction.

Why Other Options Are Wrong:

• Lead peroxide (PbO2): positive plate material when charged.
• Lead sulphate (PbSO4): discharge product on both plates.
• Sulphuric acid: electrolyte, not plate active mass.

Common Pitfalls:
Assuming both plates use the same material at all times; ignoring that plate chemistry depends on state of charge.

Final Answer:
spongy lead (Pb)