Methyl orange turns red in a colourless solution. On adding sodium hydroxide slowly, the colour changes to yellow. What process does this indicate?

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.