Case Study:

Every morning, millions of vehicles start within seconds, even after standing overnight. One important reason behind this smooth starting system is the car battery. A standard car battery contains sulphuric acid solution and lead plates arranged inside separate cells. When the ignition key is turned on, chemical reactions begin immediately and electrical energy is produced to start the engine.

While sulphuric acid is the powerhouse here, you can master the basics of identifying different acids and bases to better understand their unique chemical behaviors.

A mechanic observed that an old battery in a car was giving only 10.8 V instead of the normal 12 V. On testing, he found that the sulphuric acid concentration had decreased from 35% to nearly 20%. At the same time, a white coating had formed on the battery plates. Surprisingly, even after repeated use, the acid inside the battery was not completely “used up” instantly. Instead, the battery slowly lost efficiency over months of operation.

The mechanic explained that sulphuric acid acts as an electrolyte. It allows ions to move between the electrodes so that electric current can flow through the external circuit. During discharge, lead sulphate is formed on the plates, and the acid concentration decreases. However, when the battery is recharged by the vehicle’s alternator, many of these reactions reverse, restoring the acid concentration again.

This reversible chemical process is the reason car batteries can work repeatedly for long periods instead of getting exhausted after one use. The case highlights the important role of the Chemical Properties of Acids and Electrolytes in everyday technology.

The chemistry of batteries might seem complex, but these same principles apply to items in your kitchen. Check out how acids and bases like lemon juice and baking soda work at home.

Case - Based Questions

 MCQ 

Q1. Why does sulphuric acid help a car battery produce electricity continuously?
(a) It increases the battery temperature permanently
(b) It acts as an electrolyte and allows ion movement
(c) It converts all metals into salts instantly
(d) It stops chemical reactions inside the battery

Q2. The decrease in acid concentration from 35% to 20% mainly indicates that:
(a) The battery is becoming fully charged
(b) Sulphuric acid is participating in chemical reactions
(c) Water alone is producing electricity
(d) The lead plates are melting completely

 Assertion–Reason 

Q3. Assertion (A): A car battery can be used repeatedly for months.
Reason (R): The chemical reactions inside the battery are reversible during recharging.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true

 Application-Based  

Q4. A driver leaves the headlights on overnight. In the morning, the car fails to start. Based on the case study, explain what likely happened inside the battery.

Q5. During winter, mechanics often advise checking the battery acid concentration. Why is maintaining proper acid concentration important for battery performance?

 Data/Logic-Based 

Q6. A fully charged battery has an acid concentration of 35% and produces 12 V. Another battery shows 24% acid concentration and produces 10.5 V. What logical conclusion can be drawn?
(a) Lower acid concentration reduces the battery’s ability to produce electricity
(b) Voltage increases when acid concentration decreases
(c) Acid concentration has no role in battery functioning
(d) The battery plates have become stronger

 Application + Reasoning 

Q7. A student says that the acid in a battery should disappear completely after one use because it takes part in chemical reactions. Using the case study, explain why this statement is incorrect.

ANSWER KEY WITH EXPLANATION

A1. (b) It acts as an electrolyte and allows ion movement
Sulphuric acid contains ions that help conduct electricity inside the battery. According to NCERT, acids in aqueous solutions conduct electricity because of ion formation.

A2. (b) Sulphuric acid is participating in chemical reactions
During discharge, sulphuric acid reacts with lead plates to form lead sulphate, decreasing acid concentration gradually.

A3. (a) Both A and R are true, and R is the correct explanation of A
Rechargeable batteries work because many chemical reactions inside them are reversible. Recharging restores the reactants partially.

A4. Leaving the headlights on caused continuous discharge of the battery. More lead sulphate formed on the plates, and the acid concentration decreased, reducing the battery’s ability to supply current.

A5. Proper acid concentration ensures efficient ion movement and stable voltage production. Low acid concentration reduces the effectiveness of the electrolyte and weakens battery performance.

A6. (a) Lower acid concentration reduces the battery’s ability to produce electricity
The data shows that lower acid concentration corresponds to lower voltage output, proving the role of electrolytes in electrical conduction.

A7. The acid does participate in reactions, but many reactions are reversible during charging. The alternator restores much of the acid concentration, so the battery can be reused multiple times.

HOTS EXTENSION QUESTIONS

Q1. Predict what may happen if distilled water is never added to a car battery for a long time. How would it affect ion movement and battery efficiency?

Q2. Suppose scientists replace sulphuric acid with a weak acid in car batteries. Predict how this change may affect conductivity, voltage output, and overall battery performance.

 Looking to ace your boards?  We've compiled the top 10 must-know exam questions on acids and salts to give you an extra edge in your preparation.