Why does pH increase when hydroxide ions are added to an acidic solution?
Answer and explanation
Correct answer: They combine with hydrogen ions to form water.
pH is related inversely to hydrogen-ion concentration: when hydrogen-ion concentration decreases, pH rises. An acidic solution contains excess H⁺ ions. Added hydroxide ions, OH⁻, react with them according to H⁺ + OH⁻ → H₂O. This neutralisation removes free hydrogen ions from the solution, so its acidity falls and its pH increases. The other choices describe neither the relevant reaction nor the reason for the pH change.
Frequently asked questions
What is the correct answer to this question?
They combine with hydrogen ions to form water.
Why is this the correct answer?
pH is related inversely to hydrogen-ion concentration: when hydrogen-ion concentration decreases, pH rises. An acidic solution contains excess H⁺ ions. Added hydroxide ions, OH⁻, react with them according to H⁺ + OH⁻ → H₂O. This neutralisation removes free hydrogen ions from the solution, so its acidity falls and its pH increases. The other choices describe neither the relevant reaction nor the reason for the pH change.
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.
