Making sodium, potassium and ammonium salts
2 marks · HardExplain why methyl orange is a suitable indicator for titrations involving sodium hydroxide and hydrochloric acid. In your answer, mention what the colour change shows about the reaction mixture.
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Also accept: Methyl orange changes from yellow to orange at the endpoint, showing when neutralisation is reached, making it easy to identify when the reaction is complete.; Methyl orange gives an orange colour at the endpoint during titration of sodium hydroxide and hydrochloric acid, indicating just enough acid has been added to neutralise all the alkali.; The colour change signals the approximate endpoint, which allows accurate determination of when neutralisation occurs.
Common mistakes: Only stating colour change without linking to endpoint.; Saying methyl orange is 'exactly neutral at pH 7' (endpoint is approximate).
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Marking points
M1 · 1 mark · States that methyl orange changes colour (from yellow to orange/red) at the endpoint in acid-alkali titrations.
A1 · 1 mark · Explains what the colour change indicates — that neutralisation has occurred/all acid has reacted with all alkali.
Worked solution
Methyl orange is used because it changes from yellow (in alkali) to orange at the endpoint of the titration.
This colour change indicates that enough acid has been added to just neutralise the sodium hydroxide, showing the reaction is complete.