Question

Electrolyte used in lead acid battery is

• A. Sulphuric acid
• B. Caustic potash
• C. Nitric Acid
• D. Hydrochloric acid

Hint:

The "health" of a lead-acid battery is often checked using a hydrometer to measure the specific gravity of the sulphuric acid. A full charge typically shows a specific gravity of around 1.28.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
A lead-acid battery is a secondary (rechargeable) cell. It requires a liquid medium called an electrolyte to allow the flow of ions between the positive and negative plates during charging and discharging.

Step 2: Detailed Explanation:
In a lead-acid battery, the positive plate is Lead Peroxide ($\text{PbO}_2$) and the negative plate is Spongy Lead ($\text{Pb}$). To facilitate the chemical reaction, they are immersed in a solution of Dilute Sulphuric Acid ($\text{H}_2\text{SO}_4$). During discharge, the acid reacts with the plates to form lead sulphate and water, which is why the specific gravity of the electrolyte drops as the battery loses charge.

Step 3: Final Answer:
The electrolyte used is sulphuric acid.

Answer 2

This question asks which electrolyte is used inside a lead-acid battery. Let's examine each option based on the actual chemistry of this specific battery type.

1. Sulphuric acid: A lead-acid battery consists of a lead dioxide positive plate and a spongy lead negative plate, both immersed in dilute sulphuric acid. During discharge, this acid reacts with both plates to form lead sulphate, and its concentration (and hence specific gravity) drops as the battery discharges, which is why a hydrometer is used to test a lead-acid battery's charge state.
2. Caustic potash: This is potassium hydroxide, an alkaline electrolyte used in nickel-cadmium and nickel-iron alkaline batteries, not in lead-acid batteries, which rely on an acidic rather than alkaline electrolyte.
3. Nitric acid: Nitric acid is a strong oxidizing acid used in various chemical processes and certain primary cells, but it is not compatible with the lead / lead-dioxide plate chemistry of a rechargeable lead-acid battery.
4. Hydrochloric acid: This acid would corrosively attack the lead plates and does not support the reversible lead-sulphate formation and reduction reactions that a rechargeable lead-acid battery depends on for charging and discharging.

Only sulphuric acid matches the chemistry required for the reversible charge-discharge reactions of a lead-acid battery.

Therefore, the correct answer is sulphuric acid.