Blue litmus turns red in a solution. Then sodium hydroxide is added drop by drop. What happens near the neutralisation point?
Answer and explanation
Correct answer: Acidity decreases and pH approaches 7
The governing concept is acid-base neutralisation. Blue litmus turning red first shows that the original solution is acidic. Sodium hydroxide is a soluble base, so its hydroxide ions react with hydrogen ions from the acid to form water: H⁺ + OH⁻ → H₂O. As sodium hydroxide is added, acidity decreases and the pH moves toward 7 near the equivalence point. The other choices contradict this reaction. Thus option A is correct.
Frequently asked questions
What is the correct answer to this question?
Acidity decreases and pH approaches 7
Why is this the correct answer?
The governing concept is acid-base neutralisation. Blue litmus turning red first shows that the original solution is acidic. Sodium hydroxide is a soluble base, so its hydroxide ions react with hydrogen ions from the acid to form water: H⁺ + OH⁻ → H₂O. As sodium hydroxide is added, acidity decreases and the pH moves toward 7 near the equivalence point. The other choices contradict this reaction. Thus option A is correct.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
