Methyl orange is red and phenolphthalein is colourless in a solution. What should be added to neutralise it?
Answer and explanation
Correct answer: Sodium hydroxide solution
Indicator behaviour shows that the solution is acidic: methyl orange is red in a strongly acidic medium, and phenolphthalein remains colourless in acidic and neutral media. To neutralise an acid, a suitable base must be added so that H⁺ and OH⁻ react to form water: H⁺ + OH⁻ → H₂O. Sodium hydroxide solution supplies OH⁻ ions and is therefore the correct choice, option B. Adding more acid would increase acidity, while sodium chloride or water alone does not provide the required neutralising base.
Frequently asked questions
What is the correct answer to this question?
Sodium hydroxide solution
Why is this the correct answer?
Indicator behaviour shows that the solution is acidic: methyl orange is red in a strongly acidic medium, and phenolphthalein remains colourless in acidic and neutral media. To neutralise an acid, a suitable base must be added so that H⁺ and OH⁻ react to form water: H⁺ + OH⁻ → H₂O. Sodium hydroxide solution supplies OH⁻ ions and is therefore the correct choice, option B. Adding more acid would increase acidity, while sodium chloride or water alone does not provide the required neutralising base.
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.
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