A lead storage battery uses a spongy lead anode and a lead dioxide cathode immersed in sulphuric acid, and during discharge both electrodes are converted into the same insoluble product. At the anode, lead is oxidised: Pb + SO_4^2- → PbSO_4 + 2e^-. At the cathode, lead dioxide is reduced: PbO_2 + 4H^+ + SO_4^2- + 2e^- → PbSO_4 + 2H_2O. Thus lead sulphate, PbSO_4, deposits on both plates as the battery discharges, and the sulphuric acid is consumed, lowering its density. Lead oxide is the cathode material before discharge, not a product. Sulphuric acid is a reactant that is used up, not formed. Spongy lead is the anode before discharge. On recharging, these reactions reverse and regenerate the original materials. This is the NCERT description of the lead-acid cell. Working through the logic step by step, rather than memorising the result, makes it clear why lead sulphate governs the behaviour seen here. Understanding batteries in this way ties directly into the wider study of redox reactions and electrochemistry, where the same reasoning recurs across many problems. Plausibility check: both half-reactions independently yield PbSO_4, and the measurable fall in acid density during discharge confirms acid consumption, supporting the chosen answer.