If a solution has pH 11 and phenolphthalein is pink, what will decrease first when acid is added?
Answer and explanation
Correct answer: Effect of hydroxide ions
The governing concept is neutralisation of a basic solution. A pH of 11 and pink phenolphthalein both indicate an alkaline medium. When acid is added, its hydrogen ions react with hydroxide ions to form water: H+ + OH− → H2O. Consequently, the effective hydroxide-ion concentration or basic effect decreases first. Option A is correct; chloride ions and sodium mass are not the primary quantity reduced, and water does not lose its chemical identity.
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What is the correct answer to this question?
Effect of hydroxide ions
Why is this the correct answer?
The governing concept is neutralisation of a basic solution. A pH of 11 and pink phenolphthalein both indicate an alkaline medium. When acid is added, its hydrogen ions react with hydroxide ions to form water: H+ + OH− → H2O. Consequently, the effective hydroxide-ion concentration or basic effect decreases first. Option A is correct; chloride ions and sodium mass are not the primary quantity reduced, and water does not lose its chemical identity.
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.
