A student investigates the reaction between dilute nitric acid and sodium hydroxide solution. She adds a few drops of universal indicator to the sodium hydroxide solution before slowly adding dilute nitric acid from a burette. Describe what the student would observe as the acid is added, and explain why neutralisation has occurred when the indicator shows a green colour.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Neutralisation is the reaction in which an acid and a base react together to form a salt and water. Universal indicator shows a green colour at pH 7.
Model answer (4 marks)
The indicator colour changes from purple/blue/violet in the alkaline solution to green as the acid is added.
When the colour turns green the solution is neutral (pH ≈ 7), meaning the acid and base are in stoichiometric proportion.
The H⁺ ions from the nitric acid react with the OH⁻ ions from the sodium hydroxide.
The reaction produces sodium nitrate (NaNO₃) and water.
When the colour turns green the solution is neutral (pH ≈ 7), meaning the acid and base are in stoichiometric proportion.
The H⁺ ions from the nitric acid react with the OH⁻ ions from the sodium hydroxide.
The reaction produces sodium nitrate (NaNO₃) and water.
Examiner tips
- Use the exact colour sequence (purple/blue/violet → green).
- State that green indicates pH 7 and stoichiometric reaction.
- Mention the ionic reaction H⁺ + OH⁻ → H₂O.
- Identify the salt formed (NaNO₃).
Common mistakes
- Confusing the initial colour as green; it should start purple/blue/violet.
- Saying the reaction is complete before the colour changes.
Mark scheme (4 marks)
- The indicator colour changes from purple/blue/violet (in the alkaline sodium hydroxide) towards green as the acid is added
- When the indicator is green, the solution is neutral (pH 7), meaning the acid and alkali have reacted in exactly the right proportions
- The hydrogen ions (H⁺) from the nitric acid have reacted with the hydroxide ions (OH⁻) from the sodium hydroxide
- The products of the neutralisation reaction are a salt (sodium nitrate) and water
Key terms in this question
neutralisation · universal indicator
Related
- All Pearson Edexcel International GCSE Chemistry (4CH1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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