Methyl orange turns red in a colourless solution. On adding sodium hydroxide slowly, the colour changes to yellow. What process does this indicate?
Answer and explanation
Correct answer: Neutralisation of an acid by a base
Methyl orange is red in an acidic medium and yellow in a neutral or alkaline medium. The original colour therefore shows that the colourless solution is acidic. When sodium hydroxide, a base, is added gradually, hydroxide ions react with hydrogen ions and reduce the acidity: acid + base → salt + water. The indicator change consequently signals neutralisation, not salt decomposition, melting, or evaporation.
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What is the correct answer to this question?
Neutralisation of an acid by a base
Why is this the correct answer?
Methyl orange is red in an acidic medium and yellow in a neutral or alkaline medium. The original colour therefore shows that the colourless solution is acidic. When sodium hydroxide, a base, is added gradually, hydroxide ions react with hydrogen ions and reduce the acidity: acid + base → salt + water. The indicator change consequently signals neutralisation, not salt decomposition, melting, or evaporation.
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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